Survivors of Covid-19 are significantly less likely than the rest of the population to catch the novel coronavirus — but their risk of reinfection is not zero, according to a study published Wednesday in JAMA Internal Medicine.

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Most Covid-19 survivors have significant protection against coronavirus

For the JAMA study, researchers looked at deidentified data on more than 3.2 million patients who had a recorded antibody test for the novel coronavirus between Jan. 1 and Aug. 26 of last year. Of those patients, 378,606 tested positive for coronavirus antibodies, indicating they had been infected with the novel coronavirus at some point previously.

The researchers then examined whether that subset of patients later tested positive on a diagnostic test for the virus, which would indicate they were currently infected. The researchers reviewed the results of diagnostic nucleic acid amplification tests conducted on the patients within 30 days, 60 days, 90 days, and longer intervals after antibody testing.

According to the researchers, 11.3% of the patients who tested positive for antibodies also had a positive diagnostic test for the coronavirus within 30 days, 2.7% had a positive diagnostic test result within days 31 to 60, 1.1% had a positive result within days 61 to 90, and 0.3% had a positive result at more than 90 days.

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In comparison, among the 2.9 million patients who had tested negative for coronavirus antibodies at the outset, 3.9% had a positive diagnostic test for the coronavirus within 30 days. According to the researchers, "That proportion remained relatively consistent at approximately 3.0% over all subsequent periods of observation, including after 90 days."

According to the researchers, those findings show that although reduction of reinfection risk "was not seen in the first 30 days after an initial antibody test, it became pronounced after 30 days and progressively strengthened through the 90-day observation period and beyond."

Doug Lowy of the National Cancer Institute, who co-authored of the study, said, "That's a tenfold decrease, which is essentially a 90% reduction in risk for people who are antibody positive."

"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," he added.

Lowy cautioned that the study used real-time data and wasn't conducted in a clinical setting, meaning "confounders"—or factors that distort the data—could be present. That means the tenfold increase in protection should be taken as an estimate, Lowy said, because in reality, "maybe it's a threefold difference, and maybe it's a twentyfold difference."

However, Lowy noted the study's results match research from the United Kingdom that's been published in the New England Journal of Medicine. That study similarly found a tenfold increase in protection among people with coronavirus antibodies.

But experts still have some questions regarding coronavirus antibodies and protection against reinfection.

Jennifer Juno, a senior research fellow at the University of Melbourne's Doherty Institute who was not involved in the study, said the new study's findings offer "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." But she added, "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."

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CDC study suggests some patients could face more severe Covid-19 with reinfection

It's important to note, however, that while the study indicated coronavirus antibodies protect against reinfection to a significant extent, it also suggested that reinfection can occur.

A CDC Morbidity and Mortality Weekly Report published Friday had similar findings, and further suggested that some patients could develop more severe cases of Covid-19 upon reinfection than they experienced the first time were infected with the novel coronavirus.

For the report, researchers looked at two separate outbreaks that occurred three months apart at a skilled nursing facility in Kentucky. The first outbreak at the facility occurred in July 2020, with a total of 20 of the facilities' 115 residents and five of the facilities' 143 health care providers testing positive for the coronavirus. In total, eight of the residents were hospitalized for Covid-19 and five died. None of the health care providers were hospitalized or died as a result of the outbreak.

According to the researchers, the facility continued to conduct regularly testing of its providers and residents over the next three months. From Sept. 1 to Oct. 29 of last year, the facility conducted 928 diagnostic tests for its providers and residents, and all of those tests came back negative.

But on Oct. 30, 2020, the facility reported to the Kentucky Department for Public Health that two of its residents were experiencing symptoms of Covid-19 and had tested positive for the coronavirus. Ultimately, between Oct. 30 and Dec. 7, 2020, a total of 85 residents and 43 health care providers at the facility tested positive.

According to the researchers, among 12 of the residents who tested positive during the first outbreak and were still living at the facility in October, five—ranging in age from 67 to 99—tested positive again during the second outbreak.

Three of the five residents had asymptomatic infections during the first outbreak, but all residents experienced symptoms of Covid-19 during the second infection. One required hospitalization and later died, the researchers noted.

Further, according to the researchers, the two residents who had symptomatic cases of Covid-19 during the first outbreak experienced more severe symptoms during their second infection.

The researchers said their findings were "noteworthy" because they suggest that people with asymptomatic cases of Covid-19 during their first infections "do not produce a sufficiently robust immune response to prevent infection" again.

Further, the researchers added that their findings "suggest the possibility that disease can be more severe during a second infection" (O'Reilly, Axios, 2/25; Higgins-Dunn, CNBC, 2/25; Walker, MedPage Today, 2/25; Harvey et. al., JAMA Internal Medicine, 2/24; Cavanaugh et. al., Morbidity and Mortality Weekly Report, 2/26).