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Researchers in Hong Kong on Monday reported what appears to be the first confirmed case of Covid-19 reinfection, 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.

The case raises questions about the durability of immune protection from the coronavirus. But it was also met with caution by other scientists, who questioned the extent to which the case pointed to broader concerns about reinfection.

There have been scattered reports of cases of Covid-19 reinfection. Those reports, though, have been based on anecdotal evidence and largely attributed to flaws in testing.


But in this case, researchers at the University of Hong Kong sequenced the virus from the patient’s two infections and found that they did not match, indicating the second infection was not tied to the first. There was a difference of 24 nucleotides — the “letters” that make up the virus’ RNA — between the two infections.

“This is the world’s first documentation of a patient who recovered from Covid-19 but got another episode of Covid-19 afterwards,” the researchers said in a statement.


Experts cautioned that this patient’s case could be an outlier among the tens of millions of cases around the world and that immune protection may generally last longer than just a few months. They said that ongoing studies tracking patients who had recovered from Covid-19 would help reach more definitive conclusions. They also noted that the man’s second case was milder than his first, indicating that his immune system was providing some level of protection, even if it could not prevent the infection entirely.

“There’s been more than 24 million cases reported to date,” Maria Van Kerkhove, a coronavirus expert at the World Health Organization, said at a briefing Monday, when asked about the Hong Kong report. “And we need to look at something like this at a population level.”

The question of how long someone is protected from Covid-19 after being infected and recovering looms large.

Studies are increasingly finding that most people who recover from the illness mount a robust immune response involving both antibodies (molecules that can block the virus from infecting cells again) and T cells (which can help clear the virus). This has suggested that people would be protected from another case for some amount of time.

But based on what happens with other coronaviruses, experts knew that immunity to SARS-CoV-2 would not last forever. People generally become susceptible again to the coronaviruses that cause the common cold after a year or even less, while protection against SARS-1 and MERS appears to last for a few years.

“What we are learning about infection is that people do develop an immune response, and what is not completely clear yet is how strong that immune response is and for how long that immune response lasts,” Van Kerkhove said. She added she was still reviewing the Hong Kong case.

The strength and durability of the immune response is also a crucial factor in how long vaccines will be effective for, and for how often people might need a booster dose.

In the Hong Kong case, the man had traveled to Spain and returned to Hong Kong via the United Kingdom. A saliva sample was taken upon arrival in Hong Kong as part of a screening protocol and tested positive for SARS-CoV-2 on Aug. 15.

During his second infection, the man did not have any symptoms. Some patients go through their course of Covid-19 without showing symptoms, but researchers have also hypothesized that secondary cases of the coronavirus will generally be milder than the first. Even if immune systems can’t stop the virus from infecting cells, they might still rally some level of response that keeps us from getting sicker. During his first case, the patient had classic Covid-19 symptoms of cough, fever, sore throat, and headache.

Experts said it was also important to consider the immune response the patient generated after his first infection. While most people seem to mount a solid response, there has been indication that some people do not produce neutralizing antibodies — those that can block the virus from infecting cells — at very high levels, for unclear reasons.

“The fact that somebody may get reinfected is not surprising,” said Malik Peiris, a virologist at the University of Hong Kong, who is not an author of the paper describing the reinfection but is familiar with the case. “But the reinfection didn’t cause disease, so that’s the first point. And the second thing is that it is important to know whether the patient mounted a neutralizing antibody response to the first infection or not. Because the vast majority of patients in our experience do mount a good neutralizing antibody response. So is this person an outlier or is he likely to be the average person infected?”

Even if the Hong Kong case is an outlier, it points to a few implications: For one, people who have recovered from Covid-19 should also be vaccinated, the researchers said. And they should continue following precautions like wearing a mask and physical distancing.

Helen Branswell contributed reporting.

  • Reinfection? How about colonization? Did the researcher determine the virus was alive or did they swab viral debris in the nasopharyngeal area? RT-PCR can not tell the difference between live or dead RNA. And how do we make that quantum leap from this incomplete, possibly incorrectly interpreted information to everyone needs a vaccine?

    • Well, if we think that we’re all identical to this Hong Kong case, then every time we get it after the first, it’ll be mild and no big deal.
      But we’re not identical. I’ve never visited Spain in my life.
      I think Covid 19 will become like colds and flu; they recur, usually not very seriously, but sometimes people die from it. But we’ll see.

  • From the extensive immunity studies in canines and other species we KNOW that all individuals do not automatically mount identical immune responses. There are individuals with strong and efficient immune systems, those with weaker systems, and non-responders who do not mount any response at all to vaccinations. In part due to individual genetic dispositions and in part due to the overall health and efficiency of the immune system. Humans in general live a very UNhealthy lifestyle bombarded with toxins, not the least of which are pesticides and do little in diet or lifestyle to support their immune systems which is a controllable factor in the inconsistency in immunologic response and duration.

  • I’m curious to learn if he was/is contagious during his second infection. Are we to assume that if the virus is present enough for a positive test result, we’re contagious? Or is the test picking up viral fragments?

  • One person in 24 million has had a confirmed reinfection, which caused no actual illness.

    The final paragraph of this article is not a logical conclusion, it’s propoganda. But that’s okay, because everyone who is paying attention is learning just how little we can trust the media and the pharmaceutical industry.

  • “… And they should continue following precautions like wearing a mask and physical distancing…”.
    I am not sure if the author means in the above quote that people should wear masks indefinitely. Is so, that is not practical.

    • H cerri, you said, “I am not sure if the author means in the above quote that people should wear masks indefinitely. Is so, that is not practical.”
      I want to know why you say that.

    • May not be want he meant but sure looking like it’s headed that way.Wear it so you don’t get, still wear if you did get it so you don’t get it again, and then keep wearing because the vaccine is not going to protect you from the 6 different strains the UK found that this crap has. So yeah, indefinitely.

  • Could be similar to the flu where infection does not confer permanent immunity, nor immunity from different strains of the virus. Coronavirus immunization may be an annual event much like flu shots.

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