The number of people in England with COVID-19 antibodies dropped by more than one-quarter in three months, a new study from Imperial College London found.
The exact relationship between antibody presence and coronavirus immunity remains unclear, but one study author said falling antibody levels could indicate that population immunity may also be waning.
"On the balance of evidence I would say, with what we know for other coronaviruses, it would look as if immunity declines away at the same rate as antibodies decline away," Professor Wendy Barclay said.
The presence of coronavirus antibodies dropped by more than a quarter in England in the space of three months this summer, a new large-scale study found. The authors suggest their results may indicate that immunity to the virus within the general population also rapidly faded after the first wave of the pandemic.
Scientists at Imperial College London found that the share of people who tested positive for antibodies to COVID-19— part of the body's defense against re-infection — fell by 26.5% from June to September: from 6.0% of the population to 4.4%.
The exact relationship between antibody presence and immunity to the coronavirus remains unclear. Although the presence of antibodies suggests a person is likely immune to the virus, that's not the only indicator, and a lack of antibodies in someone who was previously infected may not necessarily mean they've lost their protection.
According to an August study, other layers of protection against reinfection, most notably T cells, are likely also critical in the body's long-term immunity.
Nonetheless, the authors of the Imperial report suggested that falling levels of antibody prevalence could indicate falling population immunity, given evidence from other human coronaviruses.
"We can see the antibodies and we can see them declining and we know that antibodies on their own are quite protective," said Professor Wendy Barclay, an author of the report and the head of Infectious Disease at Imperial College London, in comments cited by Reuters.
"On the balance of evidence I would say, with what we know for other coronaviruses, it would look as if immunity declines away at the same rate as antibodies decline away, and that this is an indication of waning immunity at the population level."
Professor Helen Ward, another co-author of the report, told the BBC that the study suggests "immunity is waning quite rapidly, we're only three months after our first [round of tests] and we're already showing a 26% decline in antibodies."
Paul Elliott, professor of epidemiology and public health at Imperial College and another of the study's authors, said in a press release emailed to Business Insider:
"Our study shows that over time there is reduction in the proportion of people testing positive for antibodies against the virus that causes COVID-19. It remains unclear what level of immunity antibodies provide, or for how long this immunity lasts. If someone tests positive for antibodies, they still need to follow national guidelines including social distancing measures, getting a swab test if they have symptoms and wearing face coverings where required."
The study, which has not yet been peer-reviewed, involved a random selection of 365,000 people over three non-overlapping rounds, who tested themselves for coronavirus antibodies at home between June 20 and September 28 using a finger-prick method.
The tests are not full-proof on a person-to-person level. Imperial College says the type of tests used were found to "correctly identify individuals with coronavirus antibodies over 80% of the time while correctly ruling out those who don't in more than 98% of tested individuals."
The smallest decline in antibody levels was observed among 18-24 year olds, where the proportion with antibodies fell by 14.9%. The largest was in those over 75, among whom the proportion fell by 39%.
For healthcare workers, the proportion of antibodies in the study group did not change over the observation period, which could either reflect they had greater initial exposure to the virus, or could be due to repeated infection, the authors suggested.
Commenting on the report in a press release emailed to Business Insider, UK Health Minister Lord Bethell said: "It is also important that everyone knows what this means for them – this study will help in our fight against the virus, but testing positive for antibodies does not mean you are immune to COVID-19."
"Regardless of the result of an antibody test, everyone must continue to comply with government guidelines including social distancing, self-isolating and getting a test if you have symptoms and always remember Hands, Face, Space."
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