A 25-year-old male seems to have the United States’ first case of reinfection with SARS-CoV-2, the virus that causes COVID-19.

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Researchers wonder under which circumstances reinfection is possible and what it might imply.

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The resident of Reno, Nevada tested positive for infection with SARS-CoV-2 in mid-April 2020 after the onset of mild symptoms, such as a sore throat, a cough, headaches, nausea, and diarrhea.

After recovering, he twice tested negative for the virus in May. But at the end of that month, just 48 days after the first positive test, he tested positive again, this time with more severe symptoms, including a fever.

He was later admitted to a hospital after developing difficulties breathing and received supplemental oxygen.

When researchers, led by the Nevada State Public Health Laboratory (NSPHL), sequenced the virus particles in each of the positive test samples, they found substantial differences.

For the virus that caused the first infection to mutate from one sequence to the other within 48 days would require a very fast mutation rate — one considerably faster than any previously observed in this virus.

The scientists therefore believe that the second case was a new infection, not a resurgence of the original infection after persisting without symptoms in the intervening period.

They also note that the male did not appear to have a suppressed immune system, tested negative for HIV, and was not taking immunosuppressant drugs.

To rule out the possibility that the two samples came from different individuals, as a result of mishandling or mislabeling, the researchers called on the services of the Washoe County Sheriff’s Office. Forensic testing by its scientists confirmed that the specimens were indeed from the same person.

The findings have not yet been peer-reviewed or published in a journal, but the researchers have posted them as a preprint on the server SSRN.

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“If reinfection is possible on such a short timeline, there may be implications for the efficacy of vaccines developed to fight the disease. It may also have implications for herd immunity,” says Mark Pandori, Ph.D., director of the NSPHL and senior author of the research.

If reinfection were widespread, it would suggest that a vaccine may not produce sufficiently long-lasting immunity against the virus. By the same token, long-term herd immunity — the point at which so many people in a population have immunity that it halts the spread of the infection — would be unattainable.

However, there is currently no evidence that reinfection is anything more than a rare occurrence. In their paper, the authors write:

“An implication of this finding is that initial exposure to the SARS-CoV-2 virus may not result in a level of immunity that is 100% protective for all individuals. […] It is crucial to note that the frequency of such a phenomenon is not defined by a singular case study. This may represent a rare event.”

In late August 2020, scientists in Hong Kong reported the world’s first confirmed reinfection with SARS-CoV-2.

In this case, reported in Clinical Infectious Diseases, the person experienced no symptoms of the second infection, which occurred 142 days after the first.

This provides evidence for a more optimistic scenario, in which individuals who have had the infection once are able to mount faster, more robust immune responses to a second infection. As a result, they may experience few, if any, symptoms.

In this scenario, either vaccinations or herd immunity could still potentially protect populations from severe infections.

Overall, there remains considerable uncertainty about the degree of protection provided by an initial infection.

Medical News Today recently reported on research indicating that even people who have had mild or asymptomatic COVID-19 — and who have no antibodies capable of neutralizing the virus — may be protected against future infection.

The researchers behind this study found that immune cells called memory T cells appeared to provide long-lasting defenses against the virus.

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