A study has found that people who have contracted SARS-CoV-2 remain immune to the virus for at least 5–7 months and probably much longer.

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Recent alleged cases of reinfection with SARS-CoV-2, the coronavirus that causes COVID-19, have raised concerns that the human immune system may only provide short-term protection against the virus.

In addition, scarce research has suggested that the number of antibodies in a person’s bloodstream that is capable of disabling the virus declines sharply after an initial infection.

However, scientists at the University of Arizona (UArizona) College of Medicine in Tucson have now found evidence of long lasting immunity in people who have had COVID-19.

They tested for the presence of antibodies to the virus in nearly 6,000 individuals and then followed them up for several months.

“We clearly see high quality antibodies still being produced 5–7 months after SARS-CoV-2 infection,” says Dr. Deepta Bhattacharya, an associate professor of immunobiology at the university, who co-led the research.

“Many concerns have been expressed about immunity against COVID-19 not lasting. We used this study to investigate that question and found immunity is stable for at least 5 months.”

Bhattacharya points out that people who contracted the SARS-CoV virus responsible for the 2002–2004 outbreak of SARS were still immune 12–17 years after infection. This virus is very similar to SARS-CoV-2.

“If SARS-CoV-2 is anything like the first one, we expect antibodies to last at least 2 years, and it would be unlikely for anything much shorter,” he says.

In their paper, published in the journal Immunity, the scientists also note that out of nearly 30 million cases of COVID-19 since December 2019, there have been only about 10 confirmed cases of reinfection.

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The researchers began recruiting volunteers for testing in Pima County, AZ, on April 20, 2020.

The main objective of the research was to develop an accurate antibody blood test that could identify individuals who have contracted the virus.

Existing tests that screen for antibodies against a single target on the virus particle tend to produce a lot of “false positives.” This means that some people who have never contracted the virus will test positive.

To get around this problem, the scientists developed a test that first looks for antibodies to one target on the virus. If the test detects them, the sample is then screened for antibodies to another target.

A person will only test positive if they have antibodies to both targets.

One target of the new test is the receptor-binding domain, or RBD, which is the part of the virus’s spike protein that it uses to invade host cells. The other target is another section of the spike called S2.

To confirm how well the test worked, the researchers incubated plasma samples with the live virus in a high security lab. They then exposed cell cultures to the mixture to see whether antibodies in the samples had neutralized the virus.

These checks revealed that only one test out of 5,882 had given a false-positive result. In other words, if a population were screened, fewer than 0.02% of tests would falsely indicate that someone had contracted the virus.

In August 2020, the Food and Drug Administration (FDA) gave the new test “emergency use authorization.” As a result, Bhattacharya and his team have now tested nearly 30,000 people in Arizona.

The scientists have also tracked how long immunity to the virus lasted among those who initially tested positive.

This showed that levels of immunity were strongest among those who experienced the most severe infections. However, the strength of immunity did not appear to vary according to age or sex.

In addition, their research revealed that after an initial decline, levels of immunity stabilized and persisted for at least 5–7 months of the follow-up.

“Whether antibodies provide lasting protection against SARS-CoV-2 has been one of the most difficult questions to answer. This research not only has given us the ability to accurately test for antibodies against COVID-19 but also has armed us with the knowledge that lasting immunity is a reality.”

– Dr. Michael D. Dake, co-author and Senior Vice President, UArizona Health Sciences

Other studies have suggested that immunity is short lived, in particular, among those with mild infections.

The authors of the new study believe that these studies made the mistake of extrapolating long-term immunity from the early falls they saw in neutralizing antibodies.

They write that in a typical viral infection, there is an initial wave of short-lived antibody-producing immune cells, which then quickly decline in number. However, what follows is the creation of a smaller number of longer lived antibody-producing cells, which can last for years.

“Thus, the decay in antibody production after infection or vaccination is not linear and cannot be extrapolated from early timepoints, demonstrating the need for longer-term follow-up studies,” they write.

The authors note two limitations of their study. Firstly, they say there is a possibility that some individuals who had contracted the virus lost their immunity before they took the antibody test and, therefore, tested negative.

Secondly, the maximum follow-up was 226 days after the onset of the disease. There remains a possibility that antibody levels may decline sharply thereafter.

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