FAQ On COVID-19 Reinfection: What Does It Mean for Vaccines?

Is COVID-19 reinfection a possibility? Would it have an impact on vaccine? Here’s what you need to know.

A medic collects samples for COVID-19 testing in New Delhi on 15 July. 

(Photo: PTI/Kamal Kishore)

Scientists in Hong Kong recently reported the case of a 33-year-old man who got infected by the novel coronavirus a second time, four and a half months after he first contracted it in late March, according to The New York Times.

Notably, the man did not have any symptoms the second time, suggesting that the immune system was at play after the first bout.

During the first infection, he had symptoms such as cough, fever, sore throat and headache. Akiko Iwasaki, an immunologist at Yale University, upon reviewing the report, told NYT, “The second infection was completely asymptomatic — his immune response prevented the disease from getting worse. It’s kind of a textbook example of how immunity should work.”

The study is to be published in the journal Clinical Infectious Diseases.

Is this the first case of COVID-19 reinfection?

There have been scattered reports about possible reinfection from around the world, as FIT had earlier reported, but the evidence has mostly been anecdotal and attributed to factors such as faulty testing or the lingering viral fragments that may remain in a recovered person.

But researchers at the University of Hong Kong sequenced the virus from both the infections and found they did not match, indicating the two were unrelated. This proved that it was a second infection and not caused by prolonged viral shedding.

“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.

Is reinfection a common probability? Should we be worried?

The answer remains unclear considering the relatively few cases when researchers have sequenced the virus from each of the two infections. But considering the millions of people infected around the world and only the first documented such case, the situation may not be that alarming.

“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 when asked about the Hong Kong report. “And we need to look at something like this at a population level.”

Moreover, since the patient did not show symptoms in the second infection, the immune system could have been providing protection, even if it couldn’t prevent the infection altogether. This immunity is expected to build with each subsequent exposure to the pathogen.  

Malik Peiris, a virologist at the University of Hong Kong who is familiar with the case, told STAT News, “The fact that somebody may get reinfected is not surprising, 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 neutralising antibody response to the first infection or not. Because the vast majority of patients in our experience do mount a good neutralising antibody response. So is this person an outlier or is he likely to be the average person infected?”

In the NYT report, Dr Iwasaki said that the man had no antibodies after the first infection but produced them after the second exposure. Immunity is expected to build with each exposure to a pathogen exactly in this way, she noted.

Dr Kelvin Kai-Wang To, a clinical microbiologist at the University of Hong Kong, cautioned against the possibility of a more severe second infection in some people. “It cannot be generalised yet, because there’s still a possibility that the second infection can be worse,” he told NYT.

How long does one stay protected from COVID-19?

There is no certain answer to this. As cases of reinfection come to the fore, the duration and kind of immunity generated from exposure to the virus remain uncertain.

Until now, it has been found that the immune response among most recovered people involves both antibodies and T cells, indicating protection for some time.

Looking at other coronaviruses suggests that COVID-19 immunity may not be ever-lasting. In an earlier story on FIT, where we examined the logic of 'immunity passports', this is what we wrote:

A study of over 170 people who had been infected by Severe Acute Respiratory Syndrome (caused by another strain of coronavirus, called SARS-CoV) found that SARS-specific antibodies were maintained for an average of two years in their bodies. This meant that SARS patients might be susceptible to reinfection three years after initial exposure. Similar findings were also arrived at for MERS, but since the number of patients affected was limited, the data obtained may be considered inadequate.

As for the seasonal coronaviruses that cause common colds, immunity seems to last for a shorter period of time, even if there are more number of antibodies, which is why getting a common cold is more frequent.

“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 from WHO said, reported STAT News. She added that she was still reviewing the Hong Kong case.

What does this mean for COVID-19 vaccine?

These are important things to consider in reaching definite conclusions about the duration for which vaccines will be effective and how often people may need subsequent or booster doses. Even more importantly, this could mean that people who have once recovered may also need to get vaccinated.

“In order to provide herd immunity, a potent vaccine is needed to induce immunity that prevents both reinfection and disease,” Dr Iwasaki told NYT.

Vaccines continue to remain crucial in our fight against the virus. But following all other precautions such as washing hands and wearing masks remains non-negotiable with looming questions of COVID-19 immunity and how long it lasts.

(This article was first published on FIT and has been republished with permission.)