Researchers at the University of Hong Kong have confirmed the first instance of a patient being reinfected with coronavirus a second time, suggesting that immunity to the virus may be short-lived. The patient, a 33-year-old man, tested positive for the virus in March and then again in August after returning to Hong Kong from a trip to Spain. But it’s not all bad news; this man’s case demonstrates that the immune response, while not able to completely ward off a second infection, may be robust enough to prevent less serious illness the second time around.   

Almost since the beginning of the pandemic, stories have been popping up in the United States, China, and South Korea from doctors claiming their patients had again tested positive for the virus after recovering. These accounts were anecdotal and the cases never confirmed as two separate infections; they could have resulted from a resurgence of the original infection, or the result of a faulty test result.  

But this case out of Hong Kong is different. It is the first proven case of reinfection because genetic sequencing showed that the patient had contracted two separate strains of the virus, in March and in August. Hong Kong researchers said that the strain the patient contracted the second time had been circulating in Europe in July and August.       

“The difference here is that the doctors were able to sequence the virus twice to say that this is actually two separate infections. And we haven’t had that information,” Yahoo News Medical Contributor Dr. Dara Kass explains. “We have known since the beginning that there were multiple varieties of the coronavirus in even America. Ones that came from China, ones that came from Europe — and that viruses generally mutate and will have slightly different presentation depending on where you get the infection.”

But experts have cautioned against jumping to conclusions about the study.

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“We need to put this into context,” Dr. Maria Van Kerkhove, technical lead of the World Health Organization’s COVID-19 response, said of the Hong Kong study during a live Q&A on Wednesday.  

“I want to reiterate. This is one example out of 23.5 million cases so far.”

World Health Organization Technical Lead Maria Van Kerkhove speaks during a press briefing on COVID-19 at the WHO headquarters in Geneva on March 9, 2020. (Photo by FABRICE COFFRINI/AFP via Getty Images)

WHO and others also emphasize that these findings shouldn’t be all that shocking. Since the early days of the pandemic, WHO has reiterated that precautions like wearing a mask and social distancing should still be taken by those who have recovered from the virus, and that “there is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection.”     

Subsequent studies have also demonstrated that the form of immunity conveyed by antibodies (proteins that help fight off infections, carried in blood plasma) may be short lived. A U.K. study published on July 11, which had not yet been peer-reviewed, found that antibodies may start to decline 20 to 30 days after the onset of COVID-19 symptoms. And a Chinese study published in June found that antibody levels in patients who had recovered from COVID-19 fell sharply within two to three months after infection.

(There is a second level of immunity defense, in the form of white blood cells, which may persist longer. Research is ongoing on this.)

“What we need people to understand is that this is not surprising to really anybody in the medical community and should not change at all how you act,” Kass says of the results of the Hong Kong study. “Masks, social distancing, and washing your hands are still the best defense to any coronavirus, no matter the strain.”

Kass notes that in the family of coronaviruses, immunity is generally temporary and lifelong immunity was unlikely to be acquired through one infection alone.    

“Think about a common cold,” she says. “We know people have recovered from viruses and get them again — usually not as severe, maybe they have some protection from the previous exposure.”

This was true of the patient observed in the Hong Kong study. The 33-year-old had mild symptoms including a cough, fever and sore throat for several days during his first infection, but was asymptomatic the second time he contracted the virus. 

A lab technician sorts blood samples for COVID-19 vaccination study at the Research Centers of America in Hollywood, Florida on August 13, 2020. (Photo by CHANDAN KHANNA / AFP)

“His immune response prevented the disease from getting worse,” Akiko Iwasaki, an immunologist at Yale University who was not involved in the study told the New York Times. “It’s kind of a textbook example of how immunity should work.”  

“That’s important,” Kass added of the fact that the patient in Hong Kong had no symptoms the second time he was infected by the virus. “We need to remember that it may mean that immunity is incomplete, but still better than having never been infected in the first place.”

But this scenario may not be the case with all reinfections. A study published on Thursday by researchers at the University of Nevada, Reno School of Medicine and the Nevada State Public Health Laboratory, which has not yet been peer-reviewed, identified the first case of reinfection in the U.S. And in this instance, the patient’s symptoms from the second infection were more severe than the first.    

The results from these studies will likely inform how a coronavirus vaccine will be developed and administered. Since immunity may not last, multiple doses of a vaccine may be necessary to build up or maintain an immune response.

The possibility of waning immunity and potential for reinfection demonstrated by the study also punches more holes in the notion of natural herd immunity as a possible solution to the pandemic.   

“This reinfection shows us that that is probably not a viable strategy,” Kass says of herd immunity. “In fact, it was probably never a viable strategy, because if natural infection is not longstanding, and more importantly if there are multiple strains of the virus that can infect you, then herd immunity, natural immunity, does not really have a leg to stand on.”


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