But so far, the available research suggests just a tiny subset of the more than 30 million people in the United States — and 132 million globally — who’ve had a confirmed Covid-19 case have become infected again. Studies indicate that the vast majority of people mount a robust and long-lasting immune response after being infected for the first time with SARS-CoV-2, the virus that causes Covid-19. A study that followed health care workers in U.K. for six months, for example, found that those who had an initial Covid-19 infection carried protective antibodies for the length of the study period; the few who tested positive again generally had no symptoms (it’s unclear, though, if they could still transmit the virus). A study out of Denmark also found reinfections were rare, though they were more common in people 65 and up.
UPDATE April 10: An April 9 Lancet study of UK health workers found good immunity after having had Covid:
A previous history of SARS-CoV-2 infection was associated with an 84% lower risk of infection, with median protective effect observed 7 months following primary infection. This time period is the minimum probable effect because seroconversions were not included. This study shows that previous infection with SARS-CoV-2 induces effective immunity to future infections in most individuals.
Even CDC now says, "Confirmed and suspected cases of reinfection have been reported, but remain rare."
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