A third of the population in England is still susceptible to being infected with the Delta variant of Covid-19, according to a scientific adviser.
Professor Matt Keeling, from the University of Warwick and a member of Scientific Pandemic Influenza Group on Modelling (Spi-M), which informs ministers, said his team estimate that, by July 19, there will have been 15.3 million symptomatic and asymptomatic infections in the country.
This means that 27.4% of the English population will have been infected and therefore have natural immunity, leaving the rest either vaccinated or unvaccinated.
When taking account of vaccines, which do not work perfectly, the modellers at Warwick calculated that 33% of the population remains susceptible to the Delta variant, which was first identified in India.
Prof Keeling told a briefing: “There’s still a large number of susceptibles out there and we expect infection, cases and hospital admissions to keep increasing between now and July 19th.”
He said keeping hospital admissions low “and below what we saw in January” really does rely on individual behaviour, as he backed calls for people to take it slowly when restrictions are released.
He said experts do not know about drops in immunity in any great detail and “any waning immunity” could alter the figures, pushing up cases.
He added: “You almost need to think of this like a spring-like system, and, if you suddenly release it, you get a much, much bigger wave than if you gradually let things change.”
Professor Graham Medley, from the London School of Hygiene and Tropical Medicine and a member of the Scientific Advisory Group for Emergencies (Sage), told the briefing he thinks the country will reach the herd immunity threshold “one way or another”.
At that point, the reproduction number (the R) will be around 1 and “that is herd immunity”, he added.
Prof Medley said he agrees that UK will always experience a wave of infection when it releases restrictions “because the vaccines are not perfect and we’re not vaccinating everybody, so there is room for another wave of infection”.
“That’s going to be the case for all countries, not just this one,” he added.