Tests on more than 365,000 people in England have shown that the antibody response to the virus that causes COVID-19 wanes over time.

Led by Imperial College London, analysis of finger-prick tests carried out at home between 20 June and 28 September found that the number of people testing positive dropped by 26.5% across the study period, from almost 6% to 4.4%.

Our study shows that over time there is a reduction in the proportion of people testing positive for antibodies. It remains unclear what level of immunity antibodies provide, or for how long this immunity lasts. Prof Paul Elliott study author, Imperial College London

The downward trend was observed in all areas of the country and age groups, but not in health workers, which could indicate repeated or higher initial exposure to the virus, the authors suggest. The decline was largest in people aged 75 and above compared to younger people, and also in people with suspected rather than confirmed infection, indicating that the antibody response varies by age and with the severity of illness.

People in London had the highest proportion of positive tests across the country, at around twice the national average. Health and care workers, ethnic minority groups, and those living in deprived areas and large households also had the greatest burden of past infection.

These findings suggest that there may be a decline in the level of immunity in the population in the months following the first wave of the epidemic.

These findings from the Real Time Assessment of Community Transmission (REACT) study are available in a pre-print report and will be submitted for peer-review.

Professor Paul Elliott, director of the programme at Imperial, said: “Our study shows that over time there is a reduction in the proportion of people testing positive for antibodies. 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. 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.”

Implications for immunity

The REACT study is using finger-prick testing to detect coronavirus antibodies in the blood. When present these indicate that a person has been previously infected with the virus. The testing kits, called Lateral Flow Tests, detect antibodies above a particular concentration in the blood, and do not measure the amount of antibodies in a particular person.

The tests are sent out to randomly-selected adult volunteers across the country to carry out at home.

This very large study has shown that the proportion of people with detectable antibodies is falling over time. But we don’t yet know whether this will leave these people at risk of reinfection with the virus that causes COVID-19. Prof Helen Ward study author, Imperial College London

The latest report includes findings from three rounds of testing carried out over a three-month period. There were 17,576 positive results across all three rounds, around 30% of whom did not report any COVID-19 symptoms. After accounting for the accuracy of the test, confirmed by laboratory evaluation, and the country’s population characteristics, the study found that antibody prevalence declined from 6.0% to 4.8% and then 4.4% over the three months.

This was found across all ages but the smallest drop was found in the youngest age group, aged 18-24, falling from 7.9% to 6.7% (14.9%), while the largest was found in the oldest group, aged 75 and above, declining from 3.3% to 2.0% (39%).

Professor Helen Ward, one of the lead authors of the report said: “This very large study has shown that the proportion of people with detectable antibodies is falling over time. We don’t yet know whether this will leave these people at risk of reinfection with the virus that causes COVID-19, but it is essential that everyone continues to follow guidance to reduce the risk to themselves and others.”

Prevalence across England

All parts of the country showed a fall in prevalence, except the West Midlands which increased in round two and then declined in round three. London has maintained the highest prevalence of antibodies across the different rounds, with 9% testing positive in round three, compared with the South West which consistently had the smallest proportion.

We rely on this kind of important research to inform our continued response to the disease, so we can continue to take the right action at the right time. Lord Bethell Health Minister

The decline was largest in people who didn’t report a history of COVID-19, dropping by almost two-thirds (64.0%) between rounds one and three, compared to a decrease of 22.3% in people who had an infection confirmed by lab testing.

Health Minister Lord Bethell said: “This study led by Imperial and Ipsos MORI is a critical piece of research, helping us to understand the nature of COVID-19 antibodies over time, and improve our understanding about the virus itself. We rely on this kind of important research to inform our continued response to the disease, so we can continue to take the right action at the right time.

“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.”

Monitoring the coronavirus epidemic

A separate arm of the REACT study is using at-home swab tests to monitor levels of current infection, involving more than 150,000 people each month.

REACT has been commissioned by the Department of Health and Social Care, and is being carried out in partnership with Imperial College London, Imperial College Healthcare NHS Trust and Ipsos MORI.

Kelly Beaver, Managing Director of Public Affairs at Ipsos MORI said: “Over 365,000 members of the public have volunteered to take part in this at home antibody study which involves a small fingerprick test. I’d like to thank all of those that took the time to participate in the study which provides another piece of information to help the Government manage the pandemic for us all.”