The study, conducted during the first two waves of the disease in Denmark, confirmed that only a small percentage (0.65 percent) of individuals had positive PCR tests in both waves of the disease, while the number of people who tested positive after having had a first negative test rose to 3.27 percent.
According to the researchers, these figures translate to an 80 percent protection against reinfection in people who have contracted the disease in the past six months, but protection among the over-65s tested stands at only 47 percent, indicating that they are more susceptible to contracting Covid-19 again.
The study authors found no evidence that protection against reinfection decreased within the six-month follow-up period.
The study's findings, states the research article published in The Lancet, highlight the importance of measures to protect older people during the pandemic, such as social distancing and priority access to vaccines, and suggest that people who have already contracted the SARS-CoV-2 coronavirus, especially older people, should also be vaccinated.
Other recent studies have indicated that reinfections are rare and that immunity to Covid-19 can last for at least six months, but the degree of protection that contracting the disease confers against future reinfections remains poorly understood by researchers, the publication said.
"The results emphasise the importance of people adhering to the measures in place to keep themselves and others safe, even if they have already had Covid-19. Our findings may also support broader vaccination strategies and the relaxation of confinement measures," said one of the study's authors, Steen Ethelberg of the Statens Serum Institut.
The study authors analysed data collected as part of Denmark's national Covid-19 testing strategy, through which more than four million people, or two-thirds of the population (69 percent), were tested in 2020.
However, the authors point out that the period in which the study was conducted did not allow estimation of protection against reinfection with new Covid-19 variants and that further studies are needed to understand how different strains may affect reinfection rates.
Still, another researcher involved in the study, Daniela Michlmayr, stressed that "nothing has been identified to indicate that protection against reinfection decreases within a six-month period".
"The relatively similar coronaviruses SARS and MERS have both been shown to confer immune protection against reinfection for at least three years, but an analysis of Covid-19 continuous over time is needed to understand their long-term effects on the odds of re-acquiring the disease," the researcher stressed.
Among the limitations of their own study, the authors recall it was not possible to assess whether the severity of Covid-19 symptoms has an influence on the degree of protection against reinfection, as detailed clinical data is only collected in hospitalised patients.
In a commentary linked to the publication of this study, professors Rosemary J. Boyton and Daniel M. Altmann of Imperial College London note that "compared to more formal reinfection case reports" that make reinfection seem "an extremely rare event", the data presented are "relatively alarming".
"Only 80 percent protection against reinfection in general, decreased to 47 percent in people aged 65 years and older, are more worrying figures than those offered in previous studies. They confirm that for SARS-CoV-2, hope for group immunity through natural infections may not be within our reach and that a global vaccination programme with highly effective vaccines is the most durable solution," the researchers concluded.