According to scientists, while a previous infection gave those under 65 about 80% protection against re-infection, for those 65 and over it only conferred 47% protection, indicating that they are more likely to catch COVID-19 again.
In the study, the researchers calculated the ratios of positive and negative test results taking into account differences in age, sex, and time since infection, using which they produced estimates of protection against reinfection.
Among people who had COVID-19 in the first wave between March and May 2020, they said only 0.65% tested positive again during the second wave from September to December 2020.
The infection rate was five times higher, at 3.3%, among those who returned a positive test in the second wave after previously testing negative.
Among people under the age of 65 who had COVID-19 in the first wave, 0.60% tested positive again in the second wave, the study noted.
However, the infection rate during the second wave among people in this age group who had previously tested negative was 3.60%.
Researchers noted that older people are at greater risk of re-infection, with 0.88% of people aged 65 or older infected in the first wave testing positive again in the second wave.
During this second wave, 2% tested positive among people 65 or older who had previously not had COVID-19, the study noted.
Due to the length of the study, scientists said it was not possible to estimate protection against reinfection with variants of COVID-19, some of which are known to be more transmissible.
They said more studies were needed to assess how protection against repeated infection might vary with different strains of COVID-19.
Based on the analysis, the researchers suggested that people who have had the virus should still be vaccinated, as natural protection – especially in the elderly – cannot be relied on.
They believe the findings underscore the importance of measures to protect older people during the pandemic, such as better social distancing and prioritization of vaccines, even for those who have recovered from COVID-19.
“Our study confirms what a number of others seem to suggest – re-infection with COVID-19 is rare in younger, healthy people, but older people are at greater risk of catching it again.” , said study co-author Steen Ethelberg, of the Serum Statens Institute.
“Since older people are also more likely to have severe symptoms of the disease and unfortunately die, our results clearly show how important it is to implement policies to protect older people during the pandemic,” Ethelberg said.
Citing the limitations of the study, the researchers said clinical information was only recorded if patients were hospitalized, adding that it was not possible to assess whether the severity of COVID-19 symptoms is affecting the protection of patients against reinfection.
While PCR tests are considered very accurate, the authors expect only about two false positives per 10,000 tests in uninfected people and about three false negatives per 100 tests in infected people.
Scientists said there was still no evidence that protection against repeated COVID-19 infections worsened within six months.
Since COVID-19 was not identified until December 2019, they said the period of protective immunity conferred by the infection remains to be determined.
“In our study, we did not identify anything to indicate that protection against reinfection wanes within six months of COVID-19,” said study co-author Daniela Michlmayr.
“Closely related coronaviruses SARS and MERS have been shown to confer immune protection against reinfection for up to three years, but continued analysis of COVID-19 is needed to understand its long-term effects on the chances of patients being infected again ”. Michlmayr added.