The share of people in Britain with detectable antibodies for the new coronavirus sharply declined over a period of three months, according to new data published Tuesday—but experts say that shouldn't necessarily spark concerns about people's immunity to Covid-19. Here's why.

What does it mean if you test positive for coronavirus antibodies? It's complicated.

Study details

Researchers from the Imperial College London on Tuesday released new data from the ongoing REACT-2 study. The new data is based on three rounds of antibody tests conducted in a random sample of more than 365,000 U.K. adults ages 18 and older from June 20 to Sept. 28.

The antibody tests used for the study were self-administered at home using finger-prick assays, which provide users a yes or no result. According to the Times, these antibody tests have a sensitivity of 84.4%, which is lower than the nearly 99% sensitivity of lab-based tests. As a result, they may not have detected antibodies among participants with low antibody levels—such as people who were infected but experienced mild or no Covid-19 symptoms—since the smallest decline in overall antibodies may put their levels below the tests' limit of detection.

Key findings

According to the researchers, the new data showed the share of participants with detectable antibodies in their blood declined by nearly 27%, from 6% in June to 4.8% in September. The researchers found the smallest decline in the share of participants with detectable antibodies occurred among adults ages 18 to 24, while the largest decline occurred among adults over the age of 75. 

The researchers also found younger adults had a higher prevalence of antibodies since the beginning of the study, while older adults had a lower prevalence of antibodies.

Helen Ward, a public health professor at Imperial College London, said antibodies declined more rapidly among adults with asymptomatic cases of Covid-19, the disease caused by the new coronavirus.

The researchers found most of the participants who tested positive for antibodies were sick with Covid-19 in March or April, but nearly 30% of the participants who tested positive did not remember experiencing Covid-19 symptoms.

What the new data means for our immunity to Covid-19

The new data has sparked concerns about people's immunity to Covid-19 being short-lived, the Times reports. However, experts say these fears may be unwarranted.

Antonio Bertoletti, a virologist at Duke NUS Medical School in Singapore, said if you look at the data from a different perspective, it shows that 73% of participants who tested positive for antibodies in June continued to have detectable antibodies three months later. "That's not such a dramatic decline," he said.

Moreover, according to several experts, the new data shouldn't come as a surprise since people's antibody levels normally decline once their bodies clear an infection, but their immune cells maintain a memory of the virus responsible for the infection and can crank out a fresh set of antibodies as needed.

Scott Hensley, an immunologist at the University of Pennsylvania, said people's antibody levels drop because "[o]ur lymphatic system, where immune cells are, only has a finite amount of space." He added that a drop in antibody levels after an acute infection has resolved "is the sign of a normal healthy immune response. It doesn't mean that those people no longer have antibodies. It doesn't mean that they don't have protection."

In addition, according to the Times, the new data indicates only the prevalence of antibodies to the novel coronavirus in the broader population—not the specific levels in individuals. And while several studies suggest that individuals' antibody levels remain stable for at least four to seven months, it's too early to definitively say how long a person's immunity to a virus lasts and how quickly people can become reinfected with the pathogen, the Times reports.

Wendy Barclay, the head of Imperial College London's infectious disease department, said, "We don't yet know what level of antibody is needed in a person's blood to prevent reinfection."

Further, since antibodies only represent a small part of the immune system, Elliott said the study's new data does not provide a full picture of the body's immune response to the new coronavirus. At least three other parts of the human immune system can fight infections, the Times reports. For instance, people who don't develop antibodies may still generate T cells that identify and attack the pathogen, a defense that while not necessarily preventing infection can potentially help avert serious cases of Covid-19.

Moreover, Shane Crotty, a virologist at the La Jolla Institute for Immunology, said it's "wrong" for people to assume that low antibody levels mean a future coronavirus vaccine may not help populations achieve herd immunity. "The vaccine doesn't have to mimic or mirror the natural infection. Certainly, I wouldn't be alarmist about these data," Crotty said.

As an example, Crotty pointed to the human papillomavirus, which he said "elicits a terrible immune response and lousy antibodies. But the vaccine with a single immunization elicits fantastic antibodies that are 99% protective in people for 10-plus years, just a complete night-and-day difference" (Collis, Politico, 10/27; Jenkins, The Hill, 10/27; Mandavilli, New York Times, 10/27).