The proportion of people in England with antibodies after Covid-19 infection dropped by more than a quarter over three months, a new study has shown, suggesting people’s immune response to SARS-CoV-2 wanes over time.

The findings from the REACT-2 study led by Imperial College London showed that the number of people who tested positive for antibodies at the end of September had dropped by 26.5 percent compared to tests carried out mid-June, going from just under 6 percent of those tested showing antibodies to 4.4 percent. This indicates people’s immune response to Covid-19 diminishes over time, and the level of immunity in a population declines after the first wave of an epidemic.

The REACT study, an antibody surveillance study that tracks past Covid infections across England using home finger-prick tests, released its first set of data from 100,000 participants back in August, showing that just under 6 percent of those screened between June 20 and July 13 had antibodies.

Now, new data has been released ahead of peer-review that shows after a second round of testing antibody levels dropped to 4.8 percent, and a third round showed that for those tested between September 15 and 21, the number dropped again to 4.4 percent.

Each round had over 100,000 adult participants, with results from 365,000 people in total from all parts of England, selected at random. The decline in antibodies – blood proteins produced by the body in response to an attacking antigen – was seen across all age groups, but the largest drop was seen in the 75+ age group (a decline of 39 percent) and the smallest in the 18-24 age group (14.9 percent).

The results also showed that asymptomatic infections appeared to lose detectable antibodies faster than those who exhibited Covid-19 symptoms.

Antibodies would be expected to decline over time, as they do with other coronaviruses, like the common cold, but how fast and how far they fall is unknown for Covid-19. It is also not yet known what level of antibody is needed for someone to be protected from infection or reinfection of Covid-19.

“Our study shows that over time there is a reduction in the proportion of people testing positive for antibodies,” said Imperial’s Professor Paul Elliot. However, “Testing positive for antibodies does not mean you are immune to Covid-19. It remains unclear what level of immunity antibodies provide, or for how long this immunity lasts.”

This does not necessarily mean that immunity from vaccination would be short-lived, but it does undermine the idea that naturally occurring herd immunity among a population is a viable response to Covid-19, something already indicated in results from Sweden, one of the few countries that did not impose a lockdown in the hopes of achieving herd immunity. Swedish epidemiologists had expected around 40 percent of the country to have developed antibodies by May after the virus first reached it in March, and were surprised to find just 6.1 percent of the population had. It's currently estimated that without a vaccine, 60 percent of a population would need to be protected against Covid-19 for it to stop the spread. 

The React-2 data is yet to be peer-reviewed, so more data will be needed to verify its findings, but antibodies are still a good indicator of protection against reinfections. Though reinfection cases have been reported, they are rare, which may be because it's still too soon for antibodies to have declined to the point of people becoming susceptible to the infection again – something we will discover with time. 

“We don’t yet know whether this will leave these people at risk of reinfection with the virus that causes Covid-19," Professor Helen Ward, one of the study’s lead authors, said, "but it is essential that everyone continues to follow guidance to reduce the risk to themselves and others.”