Antibody levels have dipped in older people as they await second vaccine doses, with the country still some way from herd immunity, according to an official survey.
Some 54.9 per cent of people in England had antibodies in the week ending March 28 —unchanged from two weeks earlier.
Vaccination has now outstripped the proportion of people showing antibodies, suggesting some waning of immune response. However, experts have stressed that this does not mean that people without detectable antibodies are not protected by other elements of the immune system.
People aged 65 to 69 had the highest level of detectable antibodies at 84.5 per cent, suggesting that the effect is most visible in people a few weeks after vaccination.
The lowest levels of antibodies are found in people aged 16 to 24, at 38.7 per cent, where previous infection with Covid is likely to play a significant role.
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In people over 70 there has been a dip in antibody levels in the weeks after near-100 per cent coverage was achieved, falling to 77.6 per cent in the over-80s, down from 85 per cent at the start of March.
The findings reflect the impact of the vaccination programme which began in December, using the AstraZeneca and Pfizer jabs. The tests cannot distinguish which vaccines are responsible for the production of antibodies, and also include the immune response to infection.
The ONS said that “by mid-March we had begun to see a decrease in detectable antibodies among age-groups prioritised for vaccination” but there were signs that it had “reversed slightly, likely reflecting many of these same age groups now receiving their second dose of vaccine”.
It added: “There is a clear pattern between vaccination and testing positive for antibodies. However, the detection of antibodies alone is not a precise measure of protection granted by vaccines. It is possible that antibody levels in some people are now too low to be detected by our tests but still high enough to grant a level of protection.”
Professor Oliver Johnson of the University of Bristol said that it was a “big disappointment” that overall levels of population immunity had not increased. “This may put herd immunity arguments to bed for a few more weeks,” he said.
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Some academics had claimed that Britain was approaching the point where so many are immune that the virus peters out naturally. However, the faster-spreading Kent variant probably requires about three quarters of the country to be immune before this happens. Some have suggested that this will not be possible through vaccines alone, given that they are only 80 to 90 per cent effective and 20 per cent of the population is under 18, so will not be vaccinated.
Government scientific advisers believe that herd immunity is likely only to be reached after another wave of infections in late summer or autumn, after restrictions are lifted.
Johnson added that it was a “good job we’re moving on to second doses”, given the fall in antibodies in older age groups.
It takes two to three weeks after vaccination for the body to produce sufficient numbers of antibodies to fight infection, and how long immunity lasts is still not known for sure. Even if antibodies are no longer detectable in the blood they may be there at low levels, while other levels of the immune system are also thought to play a role in keeping people protected against Covid.