What makes Covid-19 so dangerous for some?

More than a million people worldwide are known to have died after testing positive for the coronavirus.

While early research suggests the infection is mild in four out of five cases, a subset of patients endure life-threatening complications.

Everything from pneumonia and punctured lungs to cardiac arrests and sepsis has been reported among critically-ill Covid patients.

READ MORE:Smokers quit but 'high-risk' drinking increased during lockdown

No vaccine and limited treatment options mean wearing face coverings, maintaining social distancing and regular hand washing remains our best defence.

With the coronavirus unheard of this time last year, many are wondering what makes the infection so dangerous for some.

Digital generated image of macro view of the corona virus from the 2020.

Asymptomatic cases do not know to isolate

Anyone in the UK who comes down with the coronavirus' tell-tale fever, cough, or loss of taste or smell must self-isolate immediately.

While it may seem a simple instruction, issues arise when people with the infection develop no symptoms.

The rate of asymptomatic cases has been debated throughout the outbreak.

On 3 March, the World Health Organization's director-general Dr Tedros Adhanom Ghebreyesus said: "Evidence from China is only 1% of reported cases do not have symptoms and most of those cases develop symptoms within two days".

Scientists from the University of Hong Kong later claimed 12.1% of patients do not develop a fever.

According to Professor Paul Lehner from the University of Cambridge "up to 75% are asymptomatic".

It has also been suggested most people who appear to be asymptomatic actually have "really mild disease".

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Among those who notice their symptoms, some research implies patients are at their most infectious before they feel ill.

"Viral infectivity peaks 24 hours before you become unwell," said Professor Lehner.

Through no fault of their own, these patients do not know to self-isolate.

"You're maximally transmitting the virus while feeling well," said Professor Lehner. "You can be at the races, at the pub, singing in a choir."

The circulating coronavirus is one of seven strains of a virus class that are known to infect humans.

It is said to be most genetically similar to severe acute respiratory syndrome (Sars), which killed 774 people during its 2002/3 outbreak, but has had no reported cases since 2004.

The circulating coronavirus is "different from Sars, where you became maximally infected five days down the line", according to Lehner.

The issue of asymptomatic transmission has been debated. Other experts have argued those who do not cough or sneeze – which are both symptoms and the main routes of transmission – would be expected to spread the coronavirus to fewer people.

Coronavirus may linger in tissues

A coronavirus patient is generally thought to have overcome the infection if a throat and nose swab comes back negative.

It has been suggested, however, pockets of the virus may persist in other parts of the body.

While the individual is not thought to be infectious, the virus may linger in places like the gut, potentially causing diarrhoea.

This is one explanation for the phenomenon long COVID, when a former coronavirus patient endures complications after testing negative for the infection.

READ MORE:The risk factors for long COVID

Blood clots have also been reported among seriously-ill patients.

Speaking of a study he was involved with, Professor MauroGiacca from King's College London said: "We found thrombosis [the formation of a clot in a blood vessel] in 79% of patients who died of COVID-19 [the disease caused by the coronavirus].

"Most had [the] virus still present in the lungs. [There's a] persistence of virus-infected cells."

Cytokine storms

A person's immune system is usually able to overcome the coronavirus with no treatment being required.

It can over-react to an infection, however, causing immune cells to mistakenly attack healthy tissue.

Cytokines are proteins released by the immune system to help fight an infection.

Although generally a positive step towards recovery, some patients overproduce cytokines, leading to a "storm".

This can result in other immune cells and haemoglobin, which transports oxygen around the body, being destroyed.

Cytokines' release is regulated by an arm of immunity called the complement system.

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"Complement is a double-edged sword," said Professor Paul Morgan from Cardiff University. "It protects and can cause harm to self if it's over-activated or not switched off once the damage has been dealt with.

"It happens in many diseases; sepsis is an example.

"It [also] happens in severe COVID. In [a patient's] blood we see activation of products of the complement [system] in very large amounts.

"[The complement system] activates cells to produce cytokines and damages the lining of blood vessels."

Damaged blood vessel walls can cause blood pressure to plummet, leading to shock and organ failure, namely of the lungs.

Dr Roberto Caricchio from Temple University in Philadelphia previously told BBC Future between 20% and 30%, a "sizeable fraction", of the severe coronavirus patients he has treated showed signs of a cytokine storm.

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Coronavirus impacts Britain
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Coronavirus impacts Britain
People queue for vaccines outside Cardiff and Vale Therapy Centre in Wales, which is in Level four of its coronavirus restrictions, the highest level available, due to the surge in coronavirus cases. Picture date: Tuesday January 19, 2021. (Photo by Ben Birchall/PA Images via Getty Images)
GLASGOW, SCOTLAND - JANUARY 19: Members of the public are seen in the city center on January 19, 2021 in Glasgow, Scotland. The First Minister has announced that Scotland's Coronavirus lockdown will be extended until at least the middle of February, with most school children to continue learning from home. (Photo by Jeff J Mitchell/Getty Images)
NEWCASTLE-UNDER-LYME, ENGLAND - JANUARY 15: A volunteer from the Tri Service Support Centre prepares a food parcel on January 15, 2021 in Newcastle-under-Lyme, England. Early in 2020 the armed forces charity SSAFA, in collaboration with the Tri Service Support Centre in Newcastle-under-Lyme, created a bespoke foodbank for military veterans living in the the town. Every Saturday tinned goods, fruit, vegetables, dried food and snacks donated by local supermarkets via FareShare, a charity determined not to let food go to waste, is bagged up by volunteers and distributed. This service came into its own at the start of the Covid-19 pandemic when the service was extended to any person or family in need as well as veterans and they have since handed out over 100,000 food parcels. (Photo by Nathan Stirk/Getty Images)
An elderly person in a wheelchair attends a Covid-19 vaccination centre in Wembley, northwest London, on January 19, 2021. - Britain on January 18 extended its coronavirus vaccination campaign to people over the age of 70, as new, tougher restrictions for all arrivals to the country came into force. (Photo by JUSTIN TALLIS / AFP) (Photo by JUSTIN TALLIS/AFP via Getty Images)
Student EMT Ruth Corscadden (left) and Paramedic Daniel McCollam wearing full PPE during their shift for the Northern Ireland Ambulance Service covering the Northern Trust's Hospitals. Picture date: Monday January 18, 2021. (Photo by Liam McBurney/PA Images via Getty Images)
A woman waits at a bus stop with a billboard showing a government message to observe the coronavirus restrictions in St Helen's, northwest England on January 18, 2021. - Britain on January 18 extended its coronavirus vaccination campaign to people over the age of 70, and new tougher restrictions for all arrivals to the country came into force. Since the innoculation campaign began on December 8, more than 3.8 million people have received a first dose of vaccine against the virus that has infected 3.4 million people in the UK and claimed more than 89,000 lives. (Photo by Oli SCARFF / AFP) (Photo by OLI SCARFF/AFP via Getty Images)
Members of the Air Ambulance, wearing face coverings, leave the helipad after arriving by helicopter at Frimley Park Hospital, in Frimley, west of London on January 18, 2021. - Britain on January 18 extended its coronavirus vaccination campaign to people over the age of 70, and new tougher restrictions for all arrivals to the country came into force. Since the innoculation campaign began on December 8, more than 3.8 million people have received a first dose of vaccine against the virus that has infected 3.4 million people in the UK and claimed more than 89,000 lives. (Photo by Adrian DENNIS / AFP) (Photo by ADRIAN DENNIS/AFP via Getty Images)
Britain's Prime Minister Boris Johnson visits the quality control laboratory at Oxford BioMedica where batches of the Oxford/Astrazeneca Covid-19 vaccine are tested as part of the manufacturing process in Oxford on January 18, 2021. (Photo by Heathcliff O'Malley / POOL / AFP) (Photo by HEATHCLIFF O'MALLEY/POOL/AFP via Getty Images)
BOSTON, ENGLAND - JANUARY 18: An Astrazeneca Covid-19 vaccine is prepared at the Princess Royal Sports Arena on January 18, 2021 in Boston, England. Ten new mass vaccination centres will start administering covid-19 vaccines in England this week, joining seven existing "hubs," as well as the hospitals and GP practices enlisted in the nationwide effort to give 15 million people a first dose by February 15. (Photo by Darren Staples/Getty Images)
A person walks past a sign telling people to wear a face mask in Nottingham during England's third national lockdown to curb the spread of coronavirus. Picture date: Monday January 18, 2021. (Photo by Zac Goodwin/PA Images via Getty Images)
People walk along the pier in Bournemouth, Dorset, during England's third national lockdown to curb the spread of coronavirus. (Photo by Steve Parsons/PA Images via Getty Images)
People on the sea front in Brighton during England's third national lockdown to curb the spread of coronavirus. (Photo by Gareth Fuller/PA Images via Getty Images)
Police on patrol in Hyde Park, London, during England's third national lockdown to curb the spread of coronavirus. (Photo by Jonathan Brady/PA Images via Getty Images)
Back lights of driving cars are seen on a long shutter speed as they pass huge advertising boards that thank the National Health Service, or NHS in London, Saturday, Jan. 16, 2021 during England's third national lockdown since the coronavirus outbreak began. The U.K. is under an indefinite national lockdown to curb the spread of the new variant, with nonessential shops, gyms and hairdressers closed, most people working from home and schools largely offering remote learning. (AP Photo/Frank Augstein)
A man walks past an NHS Mural in Loughborough, Leicestershire, during England's third national lockdown to curb the spread of coronavirus. (Photo by Zac Goodwin/PA Images via Getty Images)
Young men in protective face masks are seen by the Thames River, London Eye as the UK's government introduced strict Coronavirus restrictions earlier this month due to sharp increase in numbers of Covid-19 cases in the UK - London, England on January 15, 2021. Under the new regulations exercising and going to work are exceptions for Stay at Home policy. (Photo by Dominika Zarzycka/NurPhoto via Getty Images)
The Pfizer coronavirus vaccine is prepared by a health worker at Salisbury Cathedral, Wiltshire, before being administered to members of the public. (Photo by Steve Parsons/PA Images via Getty Images)
Nurses hold their hands up to signify a free bay, as members of the public receive the Oxford/AstraZeneca coronavirus vaccine at Lichfield Cathedral, Staffordshire. (Photo by Jacob King/PA Images via Getty Images)
Members of the public at Lichfield Cathedral, Staffordshire, to receive the Oxford/AstraZeneca coronavirus vaccine. (Photo by Jacob King/PA Images via Getty Images)
LONDON, UNITED KINGDOM - JANUARY 14: National Health Service (NHS) staff demanding pay raise stage a protest outside Downing Street in London, England on January 14, 2021. As coronavirus death toll reaches over 100.000, health system in UK is reportedly under extraordinary pressure. (Photo by Tayfun Salci/Anadolu Agency via Getty Images)
Shuttered shops are seen along deserted streets in the City of London on January 15, 2021, during the third coronavirus lockdown. - Britain's economy slumped 2.6 percent in November on coronavirus restrictions, official data showed January 15, 2021, stoking fears that the current virus lockdown could spark a double-dip recession. (Photo by Tolga Akmen / AFP) (Photo by TOLGA AKMEN/AFP via Getty Images)
Inside one of the storage units at the overflow mortuary at Breakspear Crematorium in Ruislip, London which will provide an additional 20% in capacity for public mortuaries in London, helping to relieve pressure on hospitals and council-run morgues. (Photo by Jonathan Brady/PA Images via Getty Images)
A public coronavirus information sign is surrounded by snow in Auchendinny, Midlothian. Heavy snow and freezing rain is set to batter the UK this week, with warnings issued over potential power cuts and travel delays. (Photo by Jane Barlow/PA Images via Getty Images)
A man wearing a face mask walks past an advertising board in London, Thursday, Jan. 14, 2021 during England's third national lockdown to curb the spread of coronavirus. (AP Photo/Kirsty Wigglesworth)
Only a few people use the escalator at a tube station in London, Thursday, Jan. 14, 2021 during England's third national lockdown to curb the spread of coronavirus. (AP Photo/Kirsty Wigglesworth)
A jogger exercises on the banks of the river Thames with the backdrop of Tower Bridge in London, Thursday, Jan. 14, 2021 during England's third national lockdown to curb the spread of coronavirus. (AP Photo/Kirsty Wigglesworth)
A Deliveroo cycle courier rides past COVID-19 information boards in Liverpool, on January 13, 2021, as non-essential retailers keep their shops closed, due to England's third lockdown. - Britain's interior minister on Tuesday warned that anyone flouting coronavirus lockdown rules would face action from the police, as the government vowed to step up enforcement measures to cut surging infection rates that risk overwhelming health services. (Photo by Paul ELLIS / AFP) (Photo by PAUL ELLIS/AFP via Getty Images)
Lee Ullha receives an injection of a Covid-19 vaccine at the Depaul UK homeless shelter in Oldham, Greater Manchester. (Photo by Peter Byrne/PA Images via Getty Images)
Shoppers in Brixton, south London, during England's third national lockdown to curb the spread of coronavirus. (Photo by John Walton/PA Images via Getty Images)
A Union flag flies from a building on Ludgate Hill in central London as Britain is in its third lockdown due to the coronavirus on January 12, 2021. - People who flout coronavirus lockdown rules are putting lives at risk, the British government said today, as cases surge to record highs and rumours swirl of potentially tougher restrictions. (Photo by JUSTIN TALLIS / AFP) (Photo by JUSTIN TALLIS/AFP via Getty Images)
Members of the public arrive to receive their injection of a Covid-19 vaccine at the NHS vaccine centre that has been set up at the Millennium Point centre in Birmingham. (Photo by Jacob King/PA Images via Getty Images)
A shopper wearing a face mask enters a Sainsbury's store in south east London. Supermarkets are being called on to revert to more stringent in-store coronavirus measures with members of the public urged to respect regulations when out grocery shopping. (Photo by Dominic Lipinski/PA Images via Getty Images)
An empty carriage on the Jubilee Line underground train during the morning rush hour in London, as England's third national lockdown to curb the spread of coronavirus continues. (Photo by Victoria Jones/PA Images via Getty Images)
Commuters on the concourse at Waterloo Station during the morning rush hour in London, as England's third national lockdown to curb the spread of coronavirus continues. (Photo by Victoria Jones/PA Images via Getty Images)
People walk across Millennium bridge in central London as Britain is in its third lockdown due to the coronavirus on January 12, 2021. - People who flout coronavirus lockdown rules are putting lives at risk, the British government said today, as cases surge to record highs and rumours swirl of potentially tougher restrictions. (Photo by JUSTIN TALLIS / AFP) (Photo by JUSTIN TALLIS/AFP via Getty Images)
Rita Passey receives her injection of the Oxford AstraZeneca vaccine at the new Millennium Point NHS Covid-19 Vaccination Centre on 11th January 2021 in Birmingham, United Kingdom. This coronavirus vaccination centre which is one of the first in the UK with the aim to vaccinate 15 million people by mid-February will administer 600 vaccines on it's opening day, rising to 1200 on the following days and approximately 2500 per day as of the second week. Customers all receive a letter and can come to the vaccine centre from a 45 mile radius. Once in the vaccine lane they are asked health related questions and asked to give consent to being vaccinated after being advised of any possible side effects. (photo by Mike Kemp/In Pictures via Getty Images)
Police officers arrest a member of the public at an anti lockdown protest held by The Scotland Against Lockdown group outside the Scottish Parliament in Edinburgh, Scotland on January 11, 2021. (Photo by Andy Buchanan / AFP) (Photo by ANDY BUCHANAN/AFP via Getty Images)
Paramedics and hospital staff attend to a patient outside the Royal London Hospital in London. A "major incident" was declared by Mayor of London Sadiq Khan last week warning that the spread of coronavirus threatens to "overwhelm" the capital's hospitals. (Photo by Yui Mok/PA Images via Getty Images)
BIRMINGHAM, ENGLAND - JANUARY 11: People queue outside the mass NHS Covid-19 vaccine centre that has been set up at the Millennium Point centre in Birmingham on January 11, 2021 in Birmingham, England. The location is one of several mass vaccination centres in England to open to the public this week. The UK aims to vaccinate 15 million people by mid-February. (Photo by Christopher Furlong/Getty Images)
BRISTOL, ENGLAND - JANUARY 11: A general view of the mass vaccination centre at Ashton Gate Stadium on January 11, 2021 in Bristol, England. The location is one of several mass vaccination centres in England to open to the public this week. The UK aims to vaccinate 15 million people by mid-February. (Photo by Matthew Horwood/Getty Images)
Member of staff erect signs outside the NHS vaccine centre that has been set up at the Life Science Centre International Centre for Life in Newcastle. The centre is one of the seven mass vaccination centres which now opened to the general public as the government continues to ramp up the vaccination programme against Covid-19. (Photo by Owen Humphreys/PA Images via Getty Images)
CROSBY, ENGLAND - JANUARY 10: The Tottenham Hotspur bench watch the match in-front of surrounding houses during the FA Cup Third Round match between Marine and Tottenham Hotspur at Rossett Park on January 10, 2021 in Crosby, England. Sporting stadiums around England remain under strict restrictions due to the Coronavirus Pandemic as Government social distancing laws prohibit fans inside venues resulting in games being played behind closed doors. (Photo by Tottenham Hotspur FC/Tottenham Hotspur FC via Getty Images)
LEWES, ENGLAND - JANUARY 10: A general view on a frosty cold morning of Convent Field as a member of the public plays football on his own at an empty football pitch on January 10, 2021 in Lewes, East Sussex. (Photo by Naomi Baker/Getty Images)
Paramedics tidy up an ambulance wearing PPE outside the Royal London hospital in London on January 10, 2021 as surging cases of the novel coronavirus are placing health services under increasing pressure. - Medical chiefs in England raced to boost treatment capacity on January 10 as a surge in coronavirus cases risked overwhelming hospitals, even as the government stepped up its mass inoculation campaign. A leaked briefing suggested that London's hospitals were on the brink of running out of beds, endangering the lives of patients needing critical care. (Photo by DANIEL LEAL-OLIVAS / AFP) (Photo by DANIEL LEAL-OLIVAS/AFP via Getty Images)
CROSBY, ENGLAND - JANUARY 10: Jose Mourinho, Manager of Tottenham Hotspur speaks to the media as he is interviewed pitch side after the FA Cup Third Round match between Marine and Tottenham Hotspur at Rossett Park on January 10, 2021 in Crosby, England. Sporting stadiums around England remain under strict restrictions due to the Coronavirus Pandemic as Government social distancing laws prohibit fans inside venues resulting in games being played behind closed doors. (Photo by Clive Brunskill/Getty Images)
LONDON, ENGLAND - JANUARY 10: A pedestrian wearing a mask walk past a Kensington and Chelsea community notice displayed outside Town Hall in Kensington, where symptom-free coronavirus tests are available on January 10, 2021 in London, England. England is expanding rapid "lateral flow" testing across all 317 local authorities, prioritising people who cannot work from home. (Photo by Hollie Adams/Getty Images)
LONDON, ENGLAND - JANUARY 09: An air Ambulance arrives at The Royal London Hospital in London, U.K., on Saturday, Jan. 9, 2021. London Mayor Sadiq Khan declared a major incident in the U.K. capital on Friday and warned the state-run National Health Service is at risk of being overwhelmed by a surge in coronavirus cases. (Photo by Kate Green/Anadolu Agency via Getty Images)
LONDON, ENGLAND - JANUARY 09: Paramedics push a patient from an ambulance into The Royal London Hospital in London, U.K., on Saturday, Jan. 9, 2021. London Mayor Sadiq Khan declared a major incident in the U.K. capital on Friday and warned the state-run National Health Service is at risk of being overwhelmed by a surge in coronavirus cases. (Photo by Kate Green/Anadolu Agency via Getty Images)
A young man wearing a face mask waits beside government messaging telling people to stay at home, displayed at a bus stop in Whitechapel in London, England, on January 9, 2021. England began a third national lockdown on Tuesday in a bid to stem coronavirus transmission across the country. In the capital, Mayor of London Sadiq Khan yesterday declared a 'major incident' for the city over coronavirus pressures, warning that hospitals could soon be overwhelmed with patients with covid-19. (Photo by David Cliff/NurPhoto via Getty Images)
LONDON, ENGLAND - JANUARY 9: Police detain several anti lockdown protestors in Brockwell park Brixton whom had initially gathered near Windrush square before heading into the park on January 9, 2021 in London, England. Chief Medical Officer Chris Whitty has filmed an advert for HM Government warning that people should stay home as the COVID-19 virus is spreading across the country. Despite these warnings the protest group StandUpX are demonstrating against coronavirus lockdown measures and vaccines. ( Photo by Guy Smallman/Getty Images)
Ambulances outside the Royal London Hospital, after Mayor of London Sadiq Khan declared a "major incident" as the spread of coronavirus threatens to "overwhelm" the capital's hospitals. (Photo by Aaron Chown/PA Images via Getty Images)
HYDE, ENGLAND - JANUARY 08: A woman sits in her car as she is administered the Pfizer/BioNTech coronavirus vaccine at a drive-thru COVID-19 vaccination centre at Hyde Leisure Centre on January 08, 2021 in Hyde, England. The coronavirus drive-through vaccine centre is believed to be the first in the world. (Photo by Christopher Furlong/Getty Images)
BRISBANE, AUSTRALIA - JANUARY 09: Elodie, a young resident, wears a mask as she rides her bike, on January 09, 2021 in Brisbane, Australia. According to reports, Queensland has recorded no new cases of coronavirus overnight as Brisbane residents remain under lockdown. Greater Brisbane is under a three-day lockdown until 6 pm on Monday, January 12 after a cleaner working at one of the city's quarantine hotels tested positive for a new variant of the Covid-19 strain that originated in the UK. (Photo by Jono Searle/Getty Images)
A couple wear face coverings as they stroll past illuminated trees on the Southbank in London, Friday, Jan. 8, 2021. Britain's Prime Minister Boris Johnson has ordered a new national lockdown for England which means people will only be able to leave their homes for limited reasons, with measures expected to stay in place until mid-February. (AP Photo/Frank Augstein)
A man wearing a face covering walks past the London Eye in London, Friday, Jan. 8, 2021. Britain's Prime Minister Boris Johnson has ordered a new national lockdown for England which means people will only be able to leave their homes for limited reasons, with measures expected to stay in place until mid-February. (AP Photo/Frank Augstein)
Ellen Prosser, known as Nell, who is 100 years old, receives the Oxford/AstraZeneca COVID-19 vaccine from Dr Nikki Kanani at the Sunrise Care Home in London, as the government continues to ramp up the vaccination program. (Kirsty O'Connor/Pool Photo via AP)
An advert shows the word 'Hope' at an unusually quiet Piccadilly Circus, in the normally busy tourist and sightseeing area of London, Wednesday, Jan. 6, 2021. Britain's Prime Minister Boris Johnson has ordered a new national lockdown for England which means people will only be able to leave their homes for limited reasons, with measures expected to stay in place until mid-February. (AP Photo/Kirsty Wigglesworth)
People outside the Chelsea and Westminster Hospital, London, Thursday Jan. 7, 2021, join in applause during the nationwide Clap for Heroes event to recognise and support NHS workers and carers fighting the coronavirus pandemic in the latest lockdown. (Yui Mok/PA via AP)
The London Eye on the bank of the River Thames is lit with blue lights to show support for the NHS in London, Thursday, Jan. 7, 2021. Buildings across the country are being lit with blue lights to thank the work by NHS staff in caring for people during the COVID-19 pandemic. (AP Photo/Kirsty Wigglesworth)
A woman wearing a face mask, walks across Westminster Bridge past the Houses of Parliament in London, Friday, Jan. 8, 2021. Britain's Prime Minister Boris Johnson has ordered a new national lockdown for England which means people will only be able to leave their homes for limited reasons, with measures expected to stay in place until mid-February. (AP Photo/Frank Augstein)
Police officers detain a protestor during an anti-COVID-19 lockdown demonstration outside the Houses of Parliament in Westminster, central London on January 6, 2021. - Britain toughened its coronavirus restrictions on Tuesday, with England and Scotland going into lockdown and shutting schools, as surging cases have added to fears of a new virus variant. (Photo by Tolga Akmen / AFP) (Photo by TOLGA AKMEN/AFP via Getty Images)
Police officers attempt to detain a protestor and his dog during an anti-COVID-19 lockdown demonstration outside the Houses of Parliament in Westminster, central London on January 6, 2021. - Britain toughened its coronavirus restrictions on Tuesday, with England and Scotland going into lockdown and shutting schools, as surging cases have added to fears of a new virus variant. (Photo by Tolga Akmen / AFP) (Photo by TOLGA AKMEN/AFP via Getty Images)
People walk by coronavirus-themed graffiti on Meriden Street in Birmingham. Prime Minister Boris Johnson ordered a new national lockdown for England which means people will only be able to leave their homes for limited reasons, with measures expected to stay in place until mid-February. (Photo by Jacob King/PA Images via Getty Images)
A few shoppers at Bullring Open Market in Birmingham, England, Wednesday Jan. 6, 2021, as food stalls are permitted to remain open during the latest lockdown restrictions. Britain's Prime Minister Boris Johnson has ordered a new national lockdown for England which means people will only be able to leave their homes for limited reasons, with measures expected to stay in place until mid-February. (Jacob King/PA via AP)
Workers sit amongst empty market stalls in Leeds, northern England on January 6, 2021, on the second day of Britain's national lockdown to combat the spread of COVID-19. - Britain toughened its coronavirus restrictions on Tuesday, with England and Scotland going into lockdown and shutting schools, as surging cases have added to fears of a new virus variant. (Photo by Oli SCARFF / AFP) (Photo by OLI SCARFF/AFP via Getty Images)
A woman runs past some graffiti in Edinburgh, the morning after stricter COVID-19 lockdown measures came into force for mainland Scotland, Tuesday Jan. 5, 2021. (Andrew Milligan/PA via AP)
MANCHESTER, ENGLAND - JANUARY 04: Evie the bulldog looks out of a front window as children Eryn, Ewen, Gwen, Emily and Olivia watch Prime Minister Boris Johnson make a televised announcement on January 04, 2021 in Manchester, England. The Prime Minister announced tougher Covid-19 measures following a sharp increase of cases, driven in part by a new variant of the virus that officials say is more infectious. (Photo by Anthony Devlin/Getty Images)
A family in Knutsford, Cheshire, watch Prime Minister Boris Johnson making a televised address to the nation from 10 Downing Street, London, setting out new emergency measures to control the spread of coronavirus in England. (Photo by Martin Rickett/PA Images via Getty Images)
LIVERPOOL, UNITED KINGDOM - JANUARY 05: Ambulances line up as a government sponsored electronic sign gives out coronavirus pandemic information to visitors and staff at the Aintree University Hospital on January 05, 2021 in Liverpool, United Kingdom. British Prime Minister Johnson made a national television address on Monday evening announcing England is to enter its third coronavirus lockdown within a year. On Monday the UK recorded more than 50,000 new confirmed Covid cases for the seventh day in a row. (Photo by Christopher Furlong/Getty Images)
A quiet Piccadilly Circus, London, after Prime Minister Boris Johnson ordered a new national lockdown for England which means people will only be able to leave their homes for limited reasons, with measures expected to stay in place until mid-February. (Photo by Yui Mok/PA Images via Getty Images)
IMAGE PIXELATED BY PA PICTURE DESK Paramedics transfer a patient from an ambulance into Southend University hospital in Essex, as Prime Minister Boris Johnson ordered a new national lockdown for England which means people will only be able to leave their homes for limited reasons, with measures expected to stay in place until mid-February. (Photo by Joe Giddens/PA Images via Getty Images)
LONDON, ENGLAND - JANUARY 05: A sign warning of the rising cases of Coronavirus on January 05, 2021 in London, England. British Prime Minister made a national television address on Monday evening announcing England is to enter its third coronavirus lockdown of the year. On Monday the UK recorded more than 50,000 new confirmed Covid cases for the seventh day in a row. (Photo by Dan Kitwood/Getty Images)
A man walks by an almost deserted Victoria Square in Birmingham during the first day of the new national lockdown. Prime Minister Boris Johnson ordered a new national lockdown for England which means people will only be able to leave their homes for limited reasons, with measures expected to stay in place until mid-February. (Photo by Jacob King/PA Images via Getty Images)
Britain's Prime Minister Boris Johnson gestures, during a visit to view the vaccination programme at the Chase Farm Hospital in north London, Monday Jan. 4, 2021, part of the Royal Free London NHS Foundation Trust. Johnson warned Sunday that more onerous lockdown restrictions in England are likely in the coming weeks as the country reels from a coronavirus variant that has pushed infection rates to their highest recorded levels. (Stefan Rousseau/Pool Photo via AP)
82-year-old Brian Pinker receives the Oxford University/AstraZeneca COVID-19 vaccine from nurse Sam Foster at the Churchill Hospital in Oxford, England, Monday, Jan. 4, 2021. Pinker, a retired maintenance manager received the first injection of the new vaccine developed by between Oxford University and drug giant AstraZeneca. (Steve Parsons/Pool Photo via AP)
EDINBURGH, SCOTLAND - JANUARY 04: Members of the public walk through a deserted Edinburgh City Centre on January 4, 2021 in Edinburgh, Scotland. The Scottish Parliament will be recalled this afternoon to consider further coronavirus measure in an attempt to tackle the rapid increase in cases. (Photo by Jeff J Mitchell/Getty Images)
Practice Sister Tina Sutton (L) hands over an information leaflet after administering a dose of the AstraZeneca/Oxford Covid-19 vaccine to Derek Davies Games at the Pontcae Medical Practice in Merthyr Tydfil in south Wales on January 4, 2021. - Britain on Monday began rolling out the Oxford-AstraZeneca coronavirus vaccine, a possible game-changer in fighting the disease worldwide. (Photo by Geoff Caddick / AFP) (Photo by GEOFF CADDICK/AFP via Getty Images)
Naomi Carpenter, a 20-year-old sports rehab student at Hull University, takes a swab for a lateral flow Covid-19 test at the campus sports facilities on January 4, 2021, as students return to the university. - Britain -- one of the worst-affected countries by the global health crisis, with more than 75,000 deaths -- is pinning its hopes on the jab as the mainstay of its mass vaccination drive. (Photo by Paul ELLIS / AFP) (Photo by PAUL ELLIS/AFP via Getty Images)
John Murphy CEO of Oasis Community Learning unboxes gloves at Oasis Academy Coulsdon in Coulsdon, south London. A mass testing programme is being executed in schools across England to safely accept pupils back into the classroom. (Photo by Aaron Chown/PA Images via Getty Images)
Pupils arrive at Manor Park School and Nursery in Knutsford, Cheshire, as schools across England return after the Christmas break. (Photo by Martin Rickett/PA Images via Getty Images)
A view of a poster informing on Coronavirus safety measures outside Floreat Wandsworth Primary School in Wandsworth as primary schools across London and the south east of England will remain closed until January 18 due to high coronavirus infection rates, on 03 January 2021 in London, England. (Photo by WIktor Szymanowicz/NurPhoto via Getty Images)
Paramedics wheel a patient into the Royal London Hospital in east London on January 3, 2021. - The British prime minister said on January 3 he was "reconciled" to the prospect of tougher restrictions to combat spiralling coronavirus cases. Health Services are under increasing pressure after record levels of daily lab-confirmed cases of Covid-19 has led to more patients being treated in hospital in England than during the initial peak of the outbreak in April. (Photo by JUSTIN TALLIS / AFP) (Photo by JUSTIN TALLIS/AFP via Getty Images)
Ambulances are seen outside the Emergency Department of St Thomas' Hospital in London on January 3, 2021. - The British prime minister said on January 3 he was "reconciled" to the prospect of tougher restrictions to combat spiralling coronavirus cases. Health Services are under increasing pressure after record levels of daily lab-confirmed cases of Covid-19 has led to more patients being treated in hospital in England than during the initial peak of the outbreak in April. (Photo by JUSTIN TALLIS / AFP) (Photo by JUSTIN TALLIS/AFP via Getty Images)
Ambulances parked outside the ExCeL in east London which is the site of one of a number of Nightingale hospitals prepared last year at the start of the coronavirus pandemic and which the NHS says is being reactivated and made ready to admit patients as hospitals in the capital struggle as Covid patients numbers rise. (Photo by Yui Mok/PA Images via Getty Images)
Embargoed to 1630 Saturday January 2, 2021 Assistant Technical Officer Lukasz Najdrowski unpacks doses of the Oxford University/AstraZeneca Covid-19 vaccine as they arrive at the Princess Royal Hospital in Haywards Heath, West Sussex.
Police patrol Barry Island, Wales, which usually draws crowds for the New Year day swim, but is relatively empty as the country is in Level four of its coronavirus restrictions, the highest level available, due to the surge in coronavirus cases.
EDINBURGH, SCOTLAND - JANUARY 01: Members of the public brave cold waters in River Forth to take a New Year dip in front of the Forth Rail Bridge on January 1, 2021 in South Queensferry. Restrictions have been placed on many new year festivities around the world including the annual Loony Dook as countries attempt to curb spikes in new coronavirus cases. (Photo by Jeff J Mitchell/Getty Images)
LONDON / UNITED KINGDOM - JANUARY 1:Two women celebrate the new year in front of the Big Ben in London, England on January 1 2021. London Mayor Sadiq Khan cancelled the show and the celebrations this new years eve. (Photo by Tayfun Salci/Anadolu Agency via Getty Images)
Fireworks and drones illuminate the night sky over London as they form a light display as London's normal New Year's Eve fireworks display was cancelled due to the coronavirus pandemic. (Photo by Victoria Jones/PA Images via Getty Images)
LONDON / UNITED KINGDOM - JANUARY 1: Big Ben shows the arrival of the new year in London, England on January 1 2021. London Mayor Sadiq Khan cancelled the show and the celebrations this new years eve. (Photo by Tayfun Salci/Anadolu Agency via Getty Images)
A man wearing a face covering walks past graffiti on the Lower Newtownards Road in Belfast with a message reading 'Wear a mask, it hasn't gone away you know'. (Photo by Liam McBurney/PA Images via Getty Images)
Mounted Police officers attempt to disperse crowds gathered near Westminster Bridge in London, Britain, 31 December 2020. The government is discouraging people from celebrating New Years Eve and urging them to stay home as coronavirus cases in the UK continue to surge. (Photo by Maciek Musialek/NurPhoto via Getty Images)
CARDIFF, WALES - DECEMBER 31: Two people wearing protective face masks walk down an empty St. Mary's Street on December 31 in Cardiff, Wales. Wales went into a Level 4 lockdown from midnight on December 19. All non-essential shops have closed and people have been advised to stay home in a bid to curb the infection rates in one of the worst hit areas of the UK. (Photo by Matthew Horwood/Getty Images)
LONDON, UNITED KINGDOM - DECEMBER 31: A couple pose for a friend's photograph as they hold a sparkler in front of the London Eye, in what would normally be a ticket-only area filled to capacity waiting for the annual fireworks display, on December 31, 2020 in London, United Kingdom. New Year's Eve Celebrations have been curtailed in the UK this year dues to Coronavirus pandemic restrictions. With most of the UK in tiers three and four, socialising is off limits. (Photo by Leon Neal/Getty Images)
LONDON, UNITED KINGDOM - DECEMBER 31: Police officers speak to two revellers near to the London Eye on December 31, 2020 in London, United Kingdom. New Year's Eve Celebrations have been curtailed in the UK this year dues to Coronavirus pandemic restrictions. With most of the UK in tiers three and four, socialising is off limits. (Photo by Leon Neal/Getty Images)
Christmas lights adorn trees in a deserted street in the centre of Edinburgh on New Year's Eve, December 31, 2020, as authorities hope people stay at home and do not come to celebrate Hogmanay this year in the city. - The streets of Scotland's capital are normally packed at Hogmanay, as people from around the world flock to see in the New Year in raucous style. As with so much else, the coronavirus outbreak has put paid the celebrations, putting Scots in a gloomy mood made worse by Britain's parting of the ways with Europe. (Photo by Andy Buchanan / AFP) (Photo by ANDY BUCHANAN/AFP via Getty Images)
Revellers enjoy a night out in central Liverpool, northwest England, on December 30, 2020 before hightened Tier 3 restrictions closing hospitality venues, and bars indoor and outdoor socialising between households are imposed in the city amid surging cases of the novel coronavirus. - Tighter coronavirus restrictions will be extended across England from on December 30 the British government announced, with a new Covid-19 variant blamed for a rapid surge in cases. Liverpool enters Tier 3, which closes hospitality venues, and bars indoor and outdoor socialising between households with the highest Tier 4 rules, which require all non-essential shops, hairdressers as well as leisure and entertainment venues to close, will now apply to more than 44 million people. (Photo by Oli SCARFF / AFP) (Photo by OLI SCARFF/AFP via Getty Images)
Note to eds: picture has been pixelated by PA Picture Desk to protect identity of patient. A patient arrives by ambulance at Southend University hospital in Essex. Hospitals in the county have declared a major incident and local authorities, concerned about the number of Covid-19 cases, have asked for military help to increase hospital capacity, with fears over critical care, bed capacity and staff sickness. (Photo by Joe Giddens/PA Images via Getty Images)
LONDON, UNITED KINGDOM - 2020/12/30: A lady wearing a face mask a preventive measure against the spread of covid 19.walks on the street. As daily Covid19 infection rate hits record high in London, the government is pinning its hopes on the Oxford/AstraZeneca jab. (Photo by Pietro Recchia/SOPA Images/LightRocket via Getty Images)
LONDON, UNITED KINGDOM - 2020/12/30: General view of an empty and deserted Camden High Street, London. Most businesses and shops remain closed as much of England moves into Tier 4 restrictions and the UK sees a record number of coronavirus cases. (Photo by Vuk Valcic/SOPA Images/LightRocket via Getty Images)
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The impact of obesity

Early in the pandemic, experts warned obesity raised the risk of coronavirus complications.

This was undoubtedly a worrying discovery, given just under a third (29%) of UK adults were classified as obese in 2019.

"Obesity can affect our inner metabolism," said Professor Sir Stephen O'Rahilly from the University of Cambridge.

"It expands your fat and puts fat in the wrong places, which disturbs metabolism and you get very high levels of insulin in the blood.

"It increases inflammatory cytokines [and] reduces a protective molecule called adiponectin that protects the lungs.

"We may [also] get fat in the lung itself."

Other experts have suggested that as the coronavirus infects more cells "the demand for oxygen in the tissues is much higher for an obese individual than it is for a lean individual".

"Eventually the obese body becomes overwhelmed by the lack of oxygen getting to the major organs," Dr Dyan Sellayah from the University of Reading previously said.

The blame may once again fall on cytokines.

"Obese people tend to have dysfunctional immune systems; their fat tissue for example becomes a reservoir for immune cells known as macrophages," said Dr Sellayah.

"While these cells reside in our fat under normal circumstances, in obesity they are at higher frequency and become more troublesome.

"They start to secrete inflammatory cytokines. It is likely the immune response to the virus is therefore ineffective in obese individuals".

BAME groups more at risk

Statistics have repeatedly shown people of black, Asian or minority ethnic (BAME) backgrounds are more likely to become seriously ill with the coronavirus than their white counterparts.

A Public Health England report released in June found that among confirmed cases, non-white people were between 10% and 50% more likely to die.

An explanation as to why this occurs has been somewhat muddled.

It has been suggested a BAME person may be more likely to be obese.

According to Professor NaveedSattar from the University of Glasgow, different ethnicities vary in where they deposit fat.

BAME communities also have higher rates of undiagnosed diabetes and kidney disease than their white counterparts, he added.

"[These conditions] make your small blood vessels more damaged already [and] then less able to cope with the immune response [triggered by the coronavirus]," said Professor Sattar.

Recent data from the Office for National Statistics suggested, however, the higher risk of death among BAME patients is down to where they live and their occupation, not their overall health.

BAME individuals may be more likely to have front-line jobs – like care home staff, bus drivers or supermarket employees – raising the risk they will be exposed to the virus.

They may also live in crowded, multi-generational households, where social distancing is almost impossible.

The impact on the elderly

Older people are more at risk of infections in general, hence why people aged 65 or over are offered a free flu vaccine on the NHS.

When it comes to the coronavirus, Professor TracyHussell from the University of Manchester said: "If you're over 65, you're 23 times more at risk of death and complications."

"In an ageing immune system, the viral immune signals are much slower," she said.

"The thymus [an organ of the immune system] is around 40% its original size at 65, which decreases the number of [immune-boosting] T-cells [that are produced].

"T-cells are critical for helping many other aspects of the immune system, so all immune components are affected."

As to why this occurs, Professor Hussell suggested "the immune system may become exhausted by a lifetime of repeated exposure to infections".

The elderly may also be less able to physically clear an infection from their throat.

"Physical changes to the lungs of older people [also] mean they have poorer cough strength and reduced functioning of cilia, the fine hairs that line the airways," Professor Arne Akbar from the British Society for Immunology previously said.

"[Reduced cilia function] results in a decreased ability to clear mucus, which traps infections in the lungs, where they can build up."

The older we get, the more likely we also are to develop conditions that raise the risk of coronavirus complications, like high blood pressure, heart disease and type 2 diabetes.

This article originally appeared on Yahoo

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