March 17, 2021
You can be infected with Covid-19 twice, as multiple case studies from around the world have shown. But how common is reinfection?
Catching Covid Twice Is Rare, But More Likely In Older People
The first large-scale study of Covid-19 reinfections, published in The Lancet journal, found repeat infections with the virus are rare, but more common for those aged 65 and over. Most people who have had Covid-19 are protected from catching it again for at least six months, the research also found.
This chimes with evidence from Public Health England’s SIREN study which revealed people infected with Covid-19 are likely to be protected against reinfection for at least five months. However, experts warned those with immunity may still be able carry the virus in their nose and throat and therefore have a risk of transmitting it to others.
This new study, which assessed reinfection rates in Denmark in 2020, found only a small number of people (0.65%) returned a positive PCR test twice.
While prior infection gave those under the age of 65 years around 80% protection against reinfection, for people aged 65 and older it provided just 47% protection, indicating that this age group is more likely to catch the virus again.Researchers said the findings highlight the importance of measures to protect elderly people during the pandemic, such as enhanced social distancing and prioritisation for vaccines. 
The analysis also suggests those who have had the virus should still be vaccinated, as natural protection alone is not reliable enough. In the UK, those aged 60 and over have been prioritised – 1.6 million have had two doses, which gives maximum protection, while almost 25 million have had the first jab.
The authors analysed data collected as part of Denmark’s national Covid-19 testing strategy, through which more than two-thirds of the population (4 million people) were tested in 2020.
Free, national PCR testing – open to anyone, regardless of symptoms – was one of the central pillars of Denmark’s strategy to control the virus. In the UK, people are offered PCR tests if they have one or more of three key symptoms of the virus: a continuous cough, fever, or loss of smell/taste.How likely is reinfecton? The study found that of those who had Covid-19 during the first wave between March and May 2020, only 0.65% – that’s 72 out of 11,068 people – tested positive again during the second wave from September to December 2020.
Among those under the age of 65 who had Covid during the first wave, 0.60% (55 out of 9,137 people) tested positive again during the second wave, but of those aged 65 or older who were infected during the first wave, 0.88% (17 out of 1,931 people) tested positive again in the second wave. 
Because the data was collected in 2020, it could not estimate protection against reinfection with new Covid-19 variants, some of which are known to be more transmissible. Further studies are needed to assess how protection against repeat infection might vary with different strains, researchers said.
“Our study confirms what a number of others appeared to suggest: reinfection with Covid-19 is rare in younger, healthy people, but the elderly are at greater risk of catching it again,” said Dr Steen Ethelberg, from the Statens Serum Institut in Denmark.
“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.”
The results emphasise how important it is that people adhere to measures implemented to keep themselves and others safe, even if they have already had Covid-19, added Dr Ethelberg.Similar results emerged from analysis of a separate cohort. Test data from almost 2.5 million people was assessed to determine reinfection rates throughout the epidemic, not just during the second wave. Only 0.48% of people who had previously tested positive for Covid-19 caught it again at least three months later and estimated protection against reinfection was 78.8%.
Protection against repeat infections varied little among people under the age of 65, with authors estimating 80.5% protection for this group. However, protection against reinfection was much lower among people over the age of 65 years, with estimated protection of just 47%.
The authors highlighted certain limitations to their study, including that errors in testing may have occurred. While PCR tests are believed to be highly accurate, they can throw up the occasional false positive result. 
Responding to the study, Professor Rosemary Boyton and Professor Daniel Altmann, from Imperial College London, said many will find the data about protection through natural infection “relatively alarming”.
“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,” they wrote, adding that the data suggests “a global vaccination programme with high efficacy vaccines is the enduring solution”.Related...Does The Vaccine Give Better Protection Than Having Had The Virus?Does The Vaccine Give Better Protection Than Having Had The Virus?What Are The Covid Variants In The UK? Here’s How They’re SpreadingHere's What Experts Think Will Happen To Covid Come WinterPeople Who Recover From Covid 'Can Still Carry And Transmit The Virus'Coronavirus Antibodies Last For 'At Least 6 Months' For Most PeopleWhat The New Covid Variants Mean For Vaccines And Infections
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Coronavirus disease (COVID-19) is an infectious disease caused by a newly discovered coronavirus.

Most people infected with the COVID-19 virus will experience mild to moderate respiratory illness and recover without requiring special treatment. Older people, and those with underlying medical problems like cardiovascular disease, diabetes, chronic respiratory disease, and cancer are more likely to develop serious illness.

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