Most COVID-19 survivors can weather risk of reinfection, study says

COVID-19 survivors tend to have a roughly tenfold increase in protection against the virus, according to a government-funded study published Wednesday.

Why it matters: There have been some documented cases of reinfection leading to concern survivors don't gain any immunity. While there remain questions on how much or how long immunity lasts and what the impact of variants will be, this large set of observational data bolsters evidence there's some protection.

The latest: The study, published in JAMA Internal Medicine, examined commercial SARS-CoV-2 antibody test data from 3.2 million U.S. patients from Jan. 1 and Aug. 23, 2020.

• Out of those who had tested antibody-negative initially and were later tested for active infection, they found 3% were positive for SARS-CoV-2 90 or more days later.
• Out of those who were antibody-positive initially and were later tested for active infection, they found only 0.3% were positive for SARS-CoV-2 90 or more days later.
• "There's a tenfold decrease, which is essentially a 90% reduction in risk for people who are antibody positive," says Doug Lowy, co-author and deputy director of the National Cancer Institute, which conducted the study.
• "It's something that has been hypothesized for a long time, but our study is by far the largest study to look at this, especially in the United States," Lowy says.

Caveat: Because the study examines real-time data and was not done in a clinical trial setting, there are could be "confounders," or distorting factors, that affect results, Lowy points out. This means the tenfold protection is a rough average — in actuality, "maybe it's a threefold difference, and maybe it's a twentyfold difference."

• However, the results do closely match another recent NEJM study from the U.K. that also found a roughly tenfold difference, he says.

What they're saying: Jennifer Juno, a senior research fellow at the University of Melbourne's Doherty Institute who was not part of the study, says "several studies now suggest that prior infection does indeed provide protection against re-infection, as we would expect."

• "The key questions we need to address now include understanding the duration of this protection, and the specific immune responses that are most strongly associated with protection," she says.

Juno co-authored a different paper published last week in Nature Communications looking at the level of antibodies in people over a four-month period after infection. They found:

• People tend to have strong neutralizing antibodies initially that rapidly decline by about 50% within 55 days, but that decline slows and plateaus.
• And then other immune system actors pick up. The level of B cells that produce antibodies to the coronavirus spike protein increased over time in their study participants, rather than declined, Juno says.
• "This is encouraging news, as it suggests that the immune system is generating a robust memory response to infection, which is likely to play a role in providing some protection from reinfection," she adds.

The big picture: Vaccination is still highly recommended for those who've been infected before, both Lowy and Juno say.

• "Early studies suggest that individuals who were previously infected show substantial boosting of their antibody levels after receiving one dose of a COVID vaccine, which points to a great benefit of receiving the vaccine even if you have been previously infected," Juno says.

Go deeper: The hurdles we face before reaching herd immunity