The preliminary report provides what researchers say is the world's first documented case of reinfection.

By Claire Gillespie
Updated August 25, 2020
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We all have a million and one questions about the new coronavirus. When it comes to whether you can get COVID-19 twice, or if you become immune to it, there's no definitive answer. But there is new evidence. 

The strongest indication yet that it may be possible comes from Hong Kong. Preliminary research slated for publication in the journal Clinical Infectious Diseases documents the case of a 33-year-old man who was infected a second time, more than four months after his initial diagnosis, according to the New York Times. Per local protocol, the man was hospitalized in March, even though he had only mild symptoms, and he was released after testing negative twice.

Then, on August 15, he tested positive again, although he had no symptoms. This time he was at the Hong Kong airport after a trip to Spain via the UK, the Times and others reported on Monday. Researchers say it's the world's first documented case of reinfection.

So what does it mean? It's believed the man contracted a strain of the virus that was circulating in Europe in July and August, different from his initial infection. Researchers say it proves that he had the virus a second time.

Paul Offit, MD, director of the Vaccine Education Center and professor of pediatrics at Children's Hospital of Philadephia, who was not involved in the study, told CNN that it was "great news" that the man had no symptoms after his second bout.  It suggests that his body mounted an antibody response, and that bodes well for the prospect of a COVID-19 vaccine. "It's encouraging for the vaccine that his first infection induced an immune response that protected against disease," Dr. Offit told the cable news organization.

Others experts aren't so sure. As Columbia University virologist Angela Rasmussen told Science: “I disagree that this has huge implications across the board for vaccines and immunity,” because the patient in the study may be a rare example of someone not mounting a good immune response to the first infection.

Questions about whether people can get the coronavirus, recover, and become infected a second time have grown since it was reported in March that patients in China who had two negative tests and were discharged from a hospital subsequently had to be readmitted because they later tested positive. “In the case of the Chinese patients, the patients had no symptoms and were showing signs of improvement," Mahmoud Loghman-Adham, MD, a principal at California-based life sciences consulting firm Innopiphany, told Health after the report. Another patient, from Japan, reportedly also developed symptoms and tested positive for COVID-19 a second time, after being given the all-clear. But these reported reinfections lacked clear-cut documentation.

It’s understandable that people are worried about getting a serious disease twice and requiring repeat hospitalizations and/or quarantine. While experts are still learning about how COVID-19 behaves, they do have deeper knowledge of similar viral diseases. “In general, patients make antibodies to viruses and become immune to a repeat infection from the same virus,” explains Dr. Loghman-Adham. 

Most current testing for the coronavirus is based on polymerase chain reaction (PCR) amplification, which measures the number of copies of RNA (ribonucleic acid, a nucleic acid present in all living cells) the virus produces. 

“The test can just confirm that viral RNA is present in large amounts, but this RNA can be a small piece of the viral RNA and thus unable to cause infection,” says Dr. Loghman-Adham. “The test would thus be ‘false positive,’ especially if the virus copy numbers are less than a certain threshold.”

Collecting samples from multiple sites on the body, such as the nose, throat, stools, and blood, will increase the chance of detecting the virus RNA. But this is only performed for research purposes. 

“To know whether the samples contain infectious viruses, they are placed into a Petri dish containing special cells and the cells are allowed to grow and multiply. If infectious viruses are present in a sample, the cells will become infected and die,” explains Dr. Loghman-Adham. “The test is cumbersome and time-consuming, and it can also expose the technicians to a dangerous virus.”  

Another test looks for proteins, or "antigens," to SARS-CoV-2, the virus that causes COVID-19. So far, the US Food and Drug Administration has granted emergency use of three such tests. The latest, developed by London-based LumiraDx, provides results in under 12 minutes, according to a statement issued by the company last week. FDA previously granted emergency use authorizations to San Diego-based Quidel Corporation and Franklin Lakes, New Jersey-based BD (Becton Dickinson) for their antigen tests.

While the Hong Kong case suggests that repeat infection is possible, more research is needed before scientists will know for sure. "There's been more than 24 million cases reported to date, and we need to look at something like this at a population level," Maria Van Kerkhove, the World Health Organization's technical lead on COVID-19, said at a press conference on Monday.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

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