Experts have warned that it may be unwise for elderly people given the Covid-19 vaccine to hug their loved ones at Christmas.
As the NHS vaccination programme continue across the UK, scientists suggested only half of people may have immunity after a single dose of the vaccine and should wait until they are fully protected with a second dose.
It comes after some of the people receiving the new Pfizer/BioNTech vaccine said they were looking forward to hugging their relatives.
Dr Simon Clarke, associate professor in cellular microbiology at the University of Reading, said: “The gift of the Pfizer vaccine rollout has not quite come in time for Christmas.
“It takes four weeks from the first jab of the Pfizer vaccine for someone to develop immunity to Covid-19.
“If someone is in receipt of the vaccine today, they will have to wait three weeks to get the second jab and then another week for immunity to develop.”
Dr Julian Tang, a clinical virologist from the University of Leicester, said: “Typically, vaccine responses may peak after two to four weeks – and these may take longer in the elderly due to ‘immunosenescence’, where the immune system in older people responds more slowly and less aggressively to foreign antigens.
“As people only started receiving the Pfizer-BioNTech vaccine on December 8, and the second dose is due at 21 days after the first dose, no one will have had the second dose by Christmas.
“Recently released data from Pfizer suggests that the efficacy might only be 50-60% after the first dose – which is quite low.
“The trial data shows that the maximum protection of (over) 90% is only achieved after the second dose.
“So people still have to be careful despite having had one dose of the vaccine, as maybe only half of those might be protected – until the second dose has been given.”
It comes as Professor David Salisbury, a former Department of Health director of immunisation, said severe adverse reactions to vaccines are extremely rare.
He was commenting after the Medicines and Healthcare products Regulatory Agency (MHRA) said on Wednesday that people with a history of significant allergic reactions should not have the Pfizer/BioNTech jab.
Two NHS workers with a history of serious allergies – and who carried adrenaline pens – suffered a reaction after receiving their Covid-19 vaccine.
Prof Salisbury told BBC Radio 4’s Today programme that adverse reactions happen “of the order of about one in a million doses”.
He added: “Severe adverse reactions, allergic ones to vaccines, do happen but they are very rare.
“They happen of the order of about one in a million doses. And everybody who administers vaccines in this country is trained to deal with them.”
Asked if there could be a range of reactions as the vaccine is rolled out across the world, he said: “They will happen. What will also happen is there will be events after vaccination that actually have nothing to do with it.
“We need to be very careful to separate out coincidence from causality.”