SARS-CoV-2 Reinfection is Possible

But don’t freak out. Here’s why.

Repeat symbol courtesy of ClipArtKey

By Amy Rogers, MD, PhD

Today (August 24) scientists in Hong Kong released solid evidence that a person can get COVID-19, recover, and later get infected again.

Before you freak out, let’s consider first that this is NOT a surprise, at all. We know that you can repeatedly catch the coronaviruses that cause “common colds” in humans, so I’ve been expecting this news for a while. Anecdotal reports have been popping up for months about patients who were diagnosed with COVID-19, recovered, and then tested positive again weeks or months later. (For example, this and this.) The weakness of those reports is this: you couldn’t tell whether the second positive test was a new infection, or whether the original infection had lingered in their bodies for a long time.

Today’s report from Hong Kong changes that (described by STAT News here and elsewhere; original paper preprint accepted to Clinical Infectious Diseases here). Researchers found “a 33-year-old man who was first infected by SARS-CoV-2 in late March and then, four and a half months later, seemingly contracted the virus again while traveling in Europe.” His reinfection was discovered by screening upon his return from Spain. How do they know this was not a positive re-test of the same infection he’d had before? By chance, the virus from his first infection had been sequenced–the exact nucleotide sequence of its genome was on record. The coronavirus he brought home had a different genetic sequence–one that matches the dominant strain circulating in Europe this summer.

So what does this mean?

  • As expected, natural immunity to SARS-CoV-2 will not last forever. It might last less than one year. Implications: A well-designed vaccine can provoke a stronger, longer-lasting immune response than a mild natural infection. When such a vaccine is available, everyone should get it, whether they have had COVID or not.
  • If natural immunity wanes over time, then repeat vaccination (boosters) may be necessary even with a good vaccine
  • Immunity to SARS-CoV-2 may be dose-dependent. The individual in this report had an infection with moderate symptoms this winter. It’s possible this wasn’t enough to provoke a powerful, long-lived immune response. People with more severe COVID might get better protection.

Also, “immunity” isn’t one thing, an all-or-nothing response. It has many components.

  • Immunity can protect you from getting sick when you are re-infected by a virus. In this case, the traveler had no symptoms from his re-infection, possibly because his previous exposure protected him. Implications: As we know is the case with influenza (flu), vaccination or natural exposure can reduce the severity and duration of illness. This is probably true for COVID as well.
  • Immunity can protect you from re-infection. The best kind of immunity is called sterilizing immunity. It’s an immune response so fast and powerful that if you are re-exposed to a virus, the virus is not able to set up shop in your body at all. You don’t get any symptoms because you don’t even get infected. In this case, the traveler did not have sterilizing immunity. Implications: To naturally break the chain of transmission in the community, people must have sterilizing immunity, or their immune systems must have the virus locked down enough so they cannot spread it to others. This is what we mean by herd immunity, when the number of new infections naturally declines without special effort on our part. In this case, we don’t know if the traveler was infectious, but he might have been.

Does this mean we can’t have herd immunity against SARS-CoV-2?

I don’t think it does. First, as I just said, the person in this report was clearly reinfected but we don’t know if he was contagious. Second, we don’t have quantitative data on this. Yes, now we know that genuine re-infection is possible. But how frequent is it? Twenty-three million people have been diagnosed with COVID. One guy has documented reinfection. Are there dozens of people like him? Hundreds? Thousands? Millions? The number determines the impact on herd immunity.

Also, we are not relying on natural infection to get us to herd immunity. We’re expecting a vaccine or vaccines to help. A good vaccine will generate better immune protection than this guy had.

I’ll be posting more about the latest thinking on herd immunity, and the coronavirus endgame as I see it now.

Amy Rogers, MD, PhD, is a Harvard-educated scientist, novelist, journalist, and educator.

Originally published at on August 24, 2020.

Amy Rogers, MD, PhD, is a Harvard-educated scientist, novelist, journalist, and educator. She blogs about coronavirus at

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