Does getting Covid-19 prevent you from getting it again? How well and for how long people are protected by an immune response to the SARS-CoV-2 virus will affect transmission of the virus and shape the course of the pandemic. Although some research shows reinfection is rare, especially among younger adults, fast-spreading variants first reported in Brazil and South Africa appear to increase the risk.

1. How many people have been reinfected?

A tracker maintained by the Dutch news agency BNO News recorded 72 confirmed cases globally as of mid-April 2021. That compares with more than 143 million confirmed Covid-19 cases in total. On April 22 Singapore reported 10 suspected cases at a dormitory. Reports of reinfections are biased toward detecting people who develop the symptoms of Covid-19, meaning more asymptomatic infections may be occurring but aren’t being detected. Researchers in Denmark used the country’s vast national health network to look more systematically for reinfections among about 4 million people (or 69% of the population) who underwent 10.6 million tests in 2020. According to a study published March 17 in the Lancet medical journal, 0.65% of people who tested positive for SARS-CoV-2 during Denmark’s first Covid-19 surge returned a positive test again during the second wave.

2. Does that suggest most Covid-19 survivors are protected from reinfection?

Yes, though seniors maybe less so. The Danish study estimated protective immunity to be about 80–83% in people younger than 65 years, but that fell to about 47% in those 65 years and older.

3. Why is age a factor?

Researchers in Denmark found natural, age-related changes in the immune system, sometimes referred to as immune senescence, may help explain it. These changes affect numerous components of the immune system as well as its coordination, and result in older people being more susceptible to a number of infectious diseases.

4. How long does immune protection from an infection last?

The protective effect was observed to last for an average of seven months following an initial SARS-CoV-2 infection in a study of more than 25,000 U.K. health-care workers who were tested every two-to-four weeks. The study, published April 9 in the Lancet, found a previous infection reduced the incidence of infection by at least 84% when all possible and probable reinfections were included.

5. Is reinfection with a virus unusual?

Not at all. Only some viruses, such as the one that causes measles, provide their victims with something close to lifelong protection against a second bout. Common viruses which, like SARS-CoV-2, cause respiratory disease -- such as those responsible for colds and flu -- are prevalent largely because of their ability to reinfect. For many respiratory viruses that don’t provide lifetime immunity after recovery, reinfections are less severe -- and less infectious -- than the initial bout. Factors that can allow for repeat infection include an insufficient response by the immune system initially, waning immunity and genetic mutations in the virus that enables it to evade detection by the immune system.

6. Are mutations increasing the risk of reinfections?

Yes. Two fast-spreading SARS-CoV-2 variants in particular have been associated with an increased risk of reinfection due to the strains being more likely to escape infection-induced immunity. These are the so-called P.1 variant spotted in Brazil in December and 501Y.V2 first reported in South Africa in early August 2020. Of the two, P.1 appears more likely to cause reinfection. The strain, reported in at least 45 countries, has been blamed for driving a deadly surge in Covid-19 cases in Brazil, where researchers have estimated it to be 2.5 times more transmissible than the previously circulating variant, with an average reinfection probability of 6.4%. Some scientists say that shows herd immunity by infection isn’t an option. The study of reinfections in U.K. health-workers found natural infection conferred protection against the B.1.1.7 variant dominant there. Pharmaceutical companies have started developing new versions of their Covid-19 vaccines to counter these variants.

7. How is a case of reinfection distinguished from a single case of prolonged illness?

To demonstrate reinfection, scientists have to isolate the microbial culprit each time, check its genetic fingerprint, and show that each infection was caused by a different strain of SARS-CoV-2.

8. What happens in the second infection?

It seems to vary. The BNO News tracker has described a similar frequency of cases of reinfection that have been both milder and more severe than the initial infection. This might reflect a range of factors that can mitigate or worsen illness. These include:

• the quality of the immune response to the initial infection

• the interval between infections

• the significance of mutations in the virus responsible for the subsequent infection

• the amount of virus, or infectious dose, an individual is exposed to

• the health status of the individual infected.

9. What do the reinfections say about prospects for an effective vaccine?

Not necessarily anything. One theory is that the immune system requires a tough challenge to be adequately prepared to take on the virus again. Vaccines seem better at providing that training, and not necessarily just by triggering the production of antibodies. Covid-19 vaccines prompt the production of a less well-known immune player, T cells. While preventing infection with SARS-CoV-2 is the ultimate goal, researchers say that a vaccine that protects against illness or even against severe disease is very useful.

(Updates section one to add Singapore.)

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