Here’s Why Experts Aren’t Surprised by Coronavirus Reinfection

While you might be worried about documented incidents of coronavirus reinfection making headlines, infectious disease experts are less concerned. 

On Monday, a study published in The Lancet Infectious Disease confirmed a 25-year-old Nevada man tested positive for SARS-CoV-2, the viruses that causes COVID-19, on two separate occasions: April 18 and June 5. He tested negative for the virus in between this time period, indicating he recovered and became reinfected. According to the researchers, his symptoms were more severe during the second infection.

This is not the first confirmed case of reinfection. In late August, researchers from the University of Hong Kong said they’d identified the first case of reinfection with SARS-CoV-2. An overview of the findings was published in Clinical Infectious Diseases on August 25.

According to the researchers, a 33-year-old Hong Kong man who was initially infected with SARS-CoV-2 in March tested positive again in August after returning home from Spain. During the first infection, his symptoms were mild. Unlike the Nevada man, during the second infection, he was asymptomatic. The only reason his infection was identified at all was because of a routine airport screening for COVID-19, which is standard in China.

Is this a good thing or a bad thing? The good news is that it proves the immune system is responding to SARS-CoV-2, according to infectious disease experts not involved with the study.

“This patient initially had a mild case of COVID-19, so this suggests that even a mild infection by the coronavirus can result in an immune response,” William Li, MD, a physician scientist and president and medical director of Angiogenesis Foundation, tells Verywell via email. 

Previous research suggests that a more severe SARS-CoV-2 infection means a more robust immune response. From that standpoint, if this patient had experienced more than just mild symptoms the first time around, it's possible his immune system would have become primed enough to prevent him from contracting the virus again. But Li explains a more severe case of COVID-19 could actually have the opposite effect. 

“We do know from hospitalized cases of severely ill patients with COVID-19 that a massive and overwhelming immune response can lead to a potentially lethal cytokine storm in some patients,” he says. “It is still uncertain what constitutes an adequate immune response to avoid COVID-19.”

Immunologist Robert Quigley, MD, D.Phil, senior vice president and regional medical director of risk mitigation company International SOS, says this recent case study suggests a SARS-CoV-2 infection may be protective against later infections, but not completely preventative. 

“Any immunity that resulted from the first infection was not enough to block reinfection, but enough to protect the patient from disease,” Quigley tells Verywell. “This result, be it in just one case report, may lend credence to the fact that herd immunity from natural infection is not likely to eliminate SARS-CoV-2 and that the pursuit of a vaccine is definitely the path towards herd immunity.”

Scientists Expect Multiple Versions of SARS-COV-2

Genome sequencing of the patient’s respiratory samples shows he was infected with two separate SARS-CoV-2 viruses. In other words, he contracted SARS-CoV-2, recovered, and then contracted it again. It wasn’t one long, mild infection. 

Quigley says that at this point, he can’t say that one of these versions of SARS-CoV-2 is worse than the other. But he can say that multiple viral strains are normal.

“Viruses are constantly changing and defining strains, variants and lineages,” he says. “This is part of the normal evolution of an RNA virus. Mutations, although common, do not typically create a more virulent strain of the virus—although they can.”

In the case of this reinfected patient, the two viruses he was infected with were different by only 23 nucleotides, which Li describes as “the building blocks of genetic material like RNA and DNA.” For scale, SARS-CoV-2 is made of 29,811 nucleotides. It’s not a numerically significant difference, but the amount isn't what matters.

“It is not so much the difference of only 23 nucleotides, but rather whether those changes altered the virulence—or any characteristic—of the virus,” he says. “This could happen with just two nucleotides changing.”

Why Haven’t We Identified Reinfection Before?

Because viral mutations and variants are a normal part of an RNA virus's life cycle, experts aren't surprised by the fact someone became reinfected. But if it's so normal, shouldn't the medical community be seeing more of it?

"I’m going to speculate reinfection is probably much more ubiquitous than we think," Quigley says. "There might be lots of reinfections we don’t know about because we’re not re-testing people. There may be people getting reinfected with variants or mutant strains of SARS-CoV-2 and we have no idea. This just happens to be the first case that made it to press."

According to Li, we should expect to start seeing more incidents soon.

"The fact that reinfection occurs is not surprising; we can get a cold several times a year," he says. "The important thing is knowing it's possible to be reinfected and knowing your immune system has the ability to fight [the virus] so you don’t get sick. This needs to be studied in larger numbers. The single case will expand into more shortly, without a doubt."

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.