Antibodies are proteins made by the immune system to fight infections like viruses and might help to ward off future occurrences by those same infections.
Antibodies can take days or weeks to develop after exposure to COVID-19, the U.S. Food and Drug Administration wrote, and it is unknown how long they stay in the blood.
However, tests led by Imperial College London on more than 365,000 people in England show the antibody response to the virus that causes COVID-19 wanes over time. Their analysis of finger-prick tests carried out at home from June 20 to September 28 found the number of people testing positive for antibodies 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,” said Paul Elliott, the study’s lead author and a professor at Imperial College London. “It remains unclear what level of immunity antibodies provide, or for how long this immunity lasts.”
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 said in an article for the college. The decline was largest in people 75 and older compared to younger people, and in people with suspected rather than confirmed infection. This indicates the antibody response varies by age and with the severity of illness.
According to the college article, London residents 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, from the Real Time Assessment of Community Transmission, suggest a decline in the level of immunity in the population in the months following the first wave of the epidemic.
The REACT study used finger-prick testing to detect coronavirus antibodies in the blood, which, when present, indicates a person has been infected.
The study found that antibody prevalence declined from 6.0% to 4.8% and then to 4.4% over the three months of testing. The researchers observed this decline across all ages, but the smallest drop was in the youngest age group —18-24 — falling from 7.9% to 6.7% (14.9%); the largest was found in the oldest group — 75 and older — 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.”
Health Minister Lord Bethell said of the study: "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.”
