A study of hundreds of thousands of people across England suggests immunity to the coronavirus is gradually wearing off - at least according to one measure.
Researchers who sent out home finger-prick tests to more than 365,000 randomly selected people in England found a more than 26% decline in Covid-19 antibodies over just three months.
“We observe a significant decline in the proportion of the population with detectable antibodies over three rounds of national surveillance, using a self-administered lateral flow test, 12, 18 and 24 weeks after the first peak of infections in England,” the team wrote in a pre-print version of their report, released before peer review.
“This is consistent with evidence that immunity to seasonal coronaviruses declines over 6 to 12 months after infection and emerging data on SARS-CoV-2 that also detected a decrease over time in antibody levels in individuals followed in longitudinal studies,”
The study was published Monday by Imperial College London and Ipsos MORI, a market research company. At the beginning of the study, in June, 6% of those who took the tests had IgG antibody responses to the coronavirus, they reported. By September, just 4.4% of them did. For health care workers, the rates stayed about the same.
Antibodies are the proteins your body naturally generates to fight infection. IgG are one type - the tests were not designed to detect other types of antibodies. Other research teams have found that other types of antibodies may persist longer than IgG does.
The results also confirm earlier studies that showed that people who did not have symptoms of Covid-19 are likely to lose detectable antibodies sooner rather than those who had more severe infections.
Younger people who had recovered from Covid-19 had a slower loss of antibodies, compared to people older than 75 who had survived an infection, the researchers found.
Still, not enough is known to determine if antibodies provide any effective level of immunity to Covid-19, or how long people may be immune to reinfection with the coronavirus.
Some infections, such as measles, cause what’s known as sterilizing immunity. People infected once have antibodies that can be detected for many years after infection.
With coronaviruses, scientists know less. It’s also unclear what contribution T cell immunity and the body’s memory responses to threats like Covid-19 will play in providing protection if someone is exposed again to the novel coronavirus. More research is needed to better understand the ongoing risks of reinfection.
The study has limits. The samples were not taken from the same people over and over again, but from different people over time. It’s possible people who had been exposed to the coronavirus were less likely to take part over time and that may have skewed the numbers, researchers said.
“This very large study has shown that the proportion of people with detectable antibodies is falling over time,” Helen Ward, who is on the faculty of medicine at the school of public health at Imperial College London, said in a statement.
“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,” added Ward, who worked on the study.
Earlier national prevalence surveys that determined how many people have had Covid19 in Iceland recorded a durable antibody response over four months from the time of infection. Other studies have shown that different factors may impact how quickly antibodies decline. Age, co-morbidities, and the initial severity of illness all seem to play a role.
The UK’s Health Minister, Lord James Bethell, called the study a “critical piece of research” that could help inform the British government how to take the right action to control the spread of Covid-19.
“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,” Bethell said in a news release. “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.”
Dr. Claudia Hoyen, who specializes in pediatric infectious diseases at University Hospitals of Cleveland, thought the study was interesting and encouraging, since it suggests that at least where antibodies are concerned, this coronavirus acts like other coronaviruses. As with a cold, antibodies wane and people can get a cold more than once. Also as with a cold, people with robust immune systems, typically younger people, typically don’t see as quick a drop in antibodies as with people with older immune systems.
“This study is really like the first piece of the puzzle that actually gives us the indication that, yes, these antibodies don’t seem to stick around for everybody,” Hoyen said. “At least in this case, this virus is sort of acting like we can predict, which is a good thing because everything about this virus has been so off the wall.” Hoyen said the study also “cements the fact that we’re going to be in masks for a while.”
“I know that we’re all hoping that this is going to end soon, but I just don’t think it will,” she said.
Get CNN Health's weekly newsletter
Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.
“I think the sooner we resolve ourselves to the fact that this is what we have to do to get through this, we can accept it and move on. This data clearly shows that your antibodies go away. So just because you have had it once, doesn’t make you’ immune and it also means you can be contagious again.”