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Reinfection with the coronavirus is rare, research suggests.
Since the virus emerged at the end of 2019, more than 120 million cases have been confirmed worldwide.
With the vast majority of patients overcoming the infection, experts have tried to ascertain how long immunity against it may last, with most being optimistic survivors are somewhat protected, at least in the intermediate term.
To learn more, scientists from the Danish research centre Statens Serum Institut looked at the country's nationwide coronavirus testing programme, which saw more than two-thirds (69%) of the public being swabbed in 2020.
Results, published in the prestigious journal The Lancet, reveal that just 0.65% of the participants tested positive for the coronavirus in both its first and second wave.
A separate analysis suggests the risk varies according to a person's age, however. People over 65 were found to have 47% protection against a repeat infection, compared with 80% in their younger counterparts.
The scientists stressed that it is unclear how reinfection rates vary according to recently emerged variants of the coronavirus, like the so-called Kent variant. Identified towards the end of 2020, the Kent variant is thought to be up to 70% more transmissible than older versions of the virus.
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"Our study confirms what a number of others appeared to suggest: reinfection with COVID-19 [the disease caused by the coronavirus] is rare in younger, healthy people, but the elderly are at greater risk of catching it again," said author Dr Steen Ethelberg.
"Since older people are also more likely to experience severe disease symptoms, and sadly die, our findings make clear how important it is to implement policies to protect the elderly during the pandemic.
"Given what is at stake, the results emphasise how important it is people adhere to measures implemented to keep themselves and others safe, even if they have already had COVID-19.
"Our insights could also inform policies focused on wider vaccination strategies and the easing of lockdown restrictions."
The Statens scientists analysed 4 million Danes who were swabbed for the coronavirus as part of the country's testing programme, which was open to all residents regardless of whether they had symptoms.
The results reveal that among the participants who tested positive for the coronavirus during Denmark's first wave – March to May 2020 – only 0.65% caught the infection again between September and December, its second wave.
This is around five times fewer than the 3.27% who tested positive during the second wave, but not the first.
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Of the participants under 65 who had the coronavirus in the first wave, 0.6% tested positive again down the line.
This is compared to a second-wave infection rate of 3.6% among individuals of the same age who swabbed negative between March and May.
Among those aged 65 or over, 0.88% who were infected during the first wave tested positive again as little as six months later.
Of the participants in the same age group who had not tested positive for the coronavirus before, 2% were diagnosed in the second wave.
"In our study, we did not identify anything to indicate protection against reinfection declines within six months of having COVID-19," said co-author Dr Daniela Michlmayr.
"The closely related coronaviruses Sars [severe acute respiratory syndrome] and Mers [Middle East respiratory syndrome] have both been shown to confer immune protection against reinfection lasting up to three years, but ongoing analysis of COVID-19 is needed to understand its long-term effects on patients' chances of becoming infected again."
Milder viruses of the same class, like those that trigger common cold-like symptoms, are known to reinfect.
The Statens results are backed up by a separate analysis that looked at almost 2.5 million people who were tested for the coronavirus.
The latter study found only 0.48% of people who had tested positive for coronavirus caught it again three or more months later, compared with an infection rate of 2.2% among those who had initially swabbed negative.
The overall protection against reinfection was therefore estimated to be 78.8%.
This varied according to the participant's age, however, at 80.5% among those under 65 and just 47% for older individuals.
"Set against the more formal reinfection case reports that are based on differential virus sequence data and make reinfection appear an extremely rare event, many will find the data reported by Hansen colleagues about protection through natural infection relatively alarming," Professors Rosemary Boyton and Daniel Altmann, from Imperial College London, wrote in The Lancet.
"Only 80% protection from reinfection in general, decreasing to 47% in people aged 65 years and older are more concerning figures than offered by previous studies.
"Until now, one of the largest datasets has come from Qatar during a period of high disease burden and reported an estimated reinfection risk of 0.2%.
"A key difference between the studies is the Danish study is based on a universally accessible national testing programme for both symptomatic and non-symptomatic individuals, whereas the Qatar data are derived from a programme of testing in the context of symptomatic disease.
"Positive cases within the Danish dataset are thus likely to encompass a far higher proportion of asymptomatic cases presumed to elicit more marginal levels of protective immunity."
The Imperial scientists stressed that the immune response brought about after a natural coronavirus infection is "poor" relative to that induced by vaccines.
"These data are all confirmation, if it were needed, that for [the coronavirus] the hope of protective immunity through natural infections might not be within our reach and a global vaccination programme with high efficacy vaccines is the enduring solution," they wrote.
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The Statens scientists stressed that detailed medical information was only recorded for the patients admitted to hospital with coronavirus complications.
It is therefore unclear whether the severity of an individual's symptoms affects their risk of reinfection.
The coronavirus's genetic material also lingers for more than three months in some patients, after they are thought to have cleared the infection from their airways.
The Statens scientists tried to account for this by assessing patients at two- and four-month gaps between Denmark's first and second coronavirus waves.
Coronavirus tests may also produce a false-positive or false-negative result, but the swabs used in the study are "believed to be highly accurate".
The scientists expect around two false-positives would occur for every 10,000 tests carried out in uninfected people and approximately three false-negatives per 100 swabs in individuals with the infection.