Contrary to what health experts previously believed, Covid-19 reinfections are relatively common and can occur in a shorter time than expected, Sumathi Reddy reports for the Wall Street Journal.

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How common are Covid-19 reinfections?

CDC data suggests that more than half of the U.S. population has been infected by the coronavirus as of February—and many of these cases may have been due to reinfections.

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According to a study from the United Kingdom, reinfections in the country were 10 times higher during the recent omicron surge than the delta surge in 2021. People who were unvaccinated, younger, and living in "more deprived" areas were at the highest risk of reinfection between July 2020 and March 2022. In addition, people who had a mild first infection with a low viral load had a higher risk of reinfection.

Currently, around 12% of Covid-19 infections in the United Kingdom are reinfections, and this number will likely increase over time, Reddy writes. So far, it is not clear how many reinfections have occurred in the United States since the country does not track reinfection data broadly.

How serious are reinfections?

Although many reinfections are typically mild in healthy individuals, health experts warn that some reinfections may be more serious.

"On average at a population level, the people who get reinfected have milder symptoms," said Francois Balloux, an infectious disease epidemiologist and director of the UCL Genetics Institute. "That doesn't mean that some people might not have a worse infection the second or even third time."

According to a preprint study that examined more than 300,000 patients part of the VA health system, roughly 9,200 patients tested positive for Covid-19 at least 90 days after first testing positive. Of these patients, 17% were hospitalized. An additional 189 patients had a third positive test at least 90 days after their second test, and around 26% were hospitalized.

"This data suggests to me that it is reasonable to be cautious about reinfection," said Theodore Iwashyna, a physician in the ICU at a VA hospital in Michigan and the study's lead author.

How quickly can reinfections occur?

Although physicians and health agencies say that most people are protected from a Covid-19 reinfection for at least 90 days, there is growing evidence that people can be reinfected much more quickly.

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For example, a CDC report from April identified 10 people, including eight children and two adults, who had tested positive for both the delta and omicron variant within 90 days. The interval between the two infections ranged from 23 to 87 days, with a median of 54.5 days.

In addition, researchers in Spain reported a case study in which a 31-year-old female health care worker was infected by the coronavirus twice within three weeks. The woman was fully vaccinated and had received a booster shot just 12 days before her first infection by the delta variant. Then, just 20 days later, she tested positive again, this time for the omicron variant.

Reinfection with the same variant can also occur, Reddy writes. According to a preprint study from Denmark, 47 people who were first infected by the original omicron variant BA.1 were later reinfected by the omicron subvariant BA.2 between 20 to 60 days later.

What do reinfections mean for long Covid?

According to some health experts, the risk of long Covid may be reduced after a reinfection since the immune system will activate more quickly than it did during the initial infection.

"The first infection is the one most likely to lead to complications based on what we know with many other endemic respiratory infectious diseases," said Amesh Adalja, an infectious disease physician and senior scholar at the Johns Hopkins University Center for Health Security.

However, other health experts argue it is too soon to say how reinfections affect long Covid and that there are many unanswered questions about the condition.

"I don't think we should make any bets on someone who has a reinfection having a less likelihood of having long Covid," said Amy Duckro, an infectious disease specialist with Kaiser Permanente.

Currently, a research group affiliated with the patient support group Body Politic is using a $3 million grant to conduct a study on the effects of reinfection on long Covid patients. According to Gina Assaf, a co-founder and co-leader of the research group, some patients have reported developing long Covid after a reinfection.

"We don't know to what degree it's happening, but it is happening," she said. (Reddy, Wall Street Journal, 4/27)