For some people, it appears that catching the coronavirus isn't a one-time thing.
Since late August, doctors in Europe, Asia, and the U.S. have confirmed four cases, so far, of COVID-19 patients recovering from the virus and then catching it again.
Researchers have stressed that COVID-19 reinfection is neither wholly unexpected, nor a sign that our immune systems can't protect us against the virus.
Still, it is a new development in the pandemic, so let's recap what we've learned about coronavirus reinfection.
The COVID-19 Reinfection Timeline
On August 24, University of Hong Kong researchers confirmed the first case of COVID-19 reinfection: a 33-year-old Hong Kong man who tested positive for the virus in March, negative in April, and then positive again in August.
"We may see a spectrum of disease with the reinfection."
The next day, European researchers reported two more cases of people catching the coronavirus twice.
One was a Belgian woman in her 50s, who tested positive in March, then negative, and then positive again in June. The other was an older Dutch man with a weakened immune system who had two positive test results three weeks apart (with a negative test in between).
On August 28, U.S. researchers confirmed the fourth case of COVID-19 reinfection: a 25-year-old Nevada man who tested positive in April and then again in June, with two negative tests between the positive ones.
Are We Sure They Recovered in the First Place?
It is possible for a patient to catch COVID-19, falsely test negative for the virus, and then later test positive — and it has happened before — that's not what happened here.
Researchers took samples of the virus during each patient's first and second infections. Using genetic testing, they could then see that the virus responsible for the first infection was a slightly different strain than the one that caused the second.
That means the patients all recovered from the first infection before contracting the virus again.
First Cut Is the Deepest. Usually.
The three overseas patients all had similar experiences with COVID-19 reinfection: their second case was less severe than their first, with either mild symptoms or no symptoms at all.
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This might mean that, while their immune systems might not have been able to prevent a new infection outright, they did provide some protection against the virus.
However, the U.S. patient's second infection was more severe than his first: he ended up developing pneumonia and had to be hospitalized. Researchers aren't quite sure what to make of that yet, and four case studies are too few to draw firm conclusions.
"We see a spectrum of disease with the first infection, and potentially we may see a spectrum of disease with the reinfection, but we just don't know that yet," Krystina Woods, hospital epidemiologist at Mount Sinai West, told TODAY.
More Questions than Answers
The knowledge that COVID-19 reinfection is possible brings with it a host of new questions about the virus: Does immunity fade for everyone after a few months, or is that rare? Can we predict who's most at risk of reinfection? Are reinfected people contagious?
Still, tens of millions of people have contracted COVID-19, so trying to generalize about the disease after just four coronavirus reinfection reports would be way premature.
We'll likely hear about more COVID-19 reinfection cases in the coming weeks and months, and with more data, researchers will have a better shot of finding answers to the questions these first cases have spurred about natural immunity.
In the meantime, the hunt for an effective coronavirus vaccine — which may be capable of producing more robust immunity against the virus than actual infection — continues to move forward.
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