Why You Shouldn’t Worry About Studies Showing Declining Coronavirus Antibodies

The proportion of people in Britain with detectable antibodies to the coronavirus fell by about 27 percent over a three-month period this summer, the researchers reported Monday, raising fears that immunity to the virus is short-lived. .

But several experts said these concerns were overblown. It is normal for antibody levels to drop after the body clears an infection, but immune cells carry a memory of the virus and can make new antibodies when needed.

“Some of these headlines are silly,” said Scott Hensley, an immunologist at the University of Pennsylvania.

The decline in antibody levels after the acute infection has resolved “is the sign of a normal healthy immune response,” Dr. Hensley said. “It does not mean that these people no longer have antibodies. It doesn’t mean they don’t have protection. “

The research also raised some concerns about the ability of vaccines to help populations achieve herd immunity, the point at which enough people would be immune to the coronavirus to thwart its spread.

It is too early to know how long immunity to the new coronavirus lasts and whether people can be reinfected many months or a year after a first attack of the virus. Still, experts said concerns about vaccines are also unwarranted.

“The vaccine doesn’t have to mimic or mirror natural infection,” said Shane Crotty, a virologist at the La Jolla Institute of Immunology. “It certainly would not be alarmist with this data.”

The new results indicate the prevalence of antibodies against coronavirus in the general population, but not in specific individuals. Several studies looking at antibody levels in individuals have shown that after an initial decline, the levels remain stable for at least four to seven months.

The British report is based on three rounds of antibody blood tests that were carried out on 350,000 randomly selected people from June 20 to September 28. Participants were screened at home for antibodies using fingerstick tests that return a yes or no result. like a pregnancy test.

Over the three-month period, the proportion of people with detectable antibodies in their blood dropped from 6 to 4.8 percent, the researchers reported. The smallest decrease occurred among people aged 18 to 24 and the largest among those over 75.

Looking at the data another way, about 73 percent of people who had antibodies early on still produced a positive result months later, said Dr. Antonio Bertoletti, a virologist at the Duke NUS School of Medicine in Singapore. “That is not such a dramatic decline.”

Antibodies also represent only one arm of the immune response, although the most easily measurable. There are at least three other branches of the immune system that can defend against disease, so antibody levels do not present a complete picture.

“It’s not the entire immune response,” said Dr. Paul Elliott, an epidemiologist at Imperial College London who leads the project.

When the body encounters a pathogen, it quickly produces antibodies that recognize the invader. Once the acute infection resolves, the levels decline, as they should for purely practical reasons.

“Our lymphatic system, where the immune cells are, only has a finite amount of space,” Dr. Hensley said.

Depending on the test used, the small amount of antibodies still circulating in the blood may not be enough for a positive signal. The test used in the study has a sensitivity of 84.4 percent, well below that of laboratory tests that are around 99 percent. That means you can miss anyone with low antibody levels.

For example, people with mild or no symptoms may have produced fewer antibodies than those with severe illness. Most of those who tested positive fell ill in March or April, at the peak of the outbreak in Britain, but about 30 percent did not recall having any symptoms of Covid-19. Even a small decrease in the amount of antibodies can lower their levels below the detection limit.

“We are saying that the antibody response has dropped below the threshold” of detection, Dr. Elliott said. “This is not a surprise to anyone who works in the field.”

Data from monkeys suggest that even low levels of antibodies can prevent serious illness from the virus, if not reinfection. Even if circulating antibody levels are undetectable, the body retains the memory of the pathogen. If it crosses again with the virus, the balloon-shaped cells that live in the bone marrow can produce antibodies en masse in a matter of hours.

A very small number of people may not make any antibodies. But even those people may have immune cells called T cells that can identify and destroy the virus. The vast majority of people infected with the coronavirus develop long-lasting cellular responses, according to several recent studies.

T cells are unlikely to prevent infection, but they can at least prevent serious illness by mitigating the attack, said Dr. Crotty. Given all that, he said, interpreting low antibody levels to mean that immunity wears off or that coronavirus vaccines won’t be effective is “wrong.”

For example, the human papillomavirus “causes a terrible immune response and terrible antibodies,” he said. “But the one-shot vaccine elicits fantastic antibodies that protect 99 percent in people for more than 10 years, a total difference between day and night.”

Vaccines can also be designed to elicit responses much stronger than natural infection, he added.

Although they criticized many of the interpretations of it, the experts said that the results of the new study are an interesting look at the prevalence of antibodies at the population level.

The same research team is also screening hundreds of thousands of people for the presence of the virus. Together, Dr. Elliott said, the studies offer a “really powerful tool” for policymakers to measure the size of a country’s epidemic.