Study: COVID-19 Reinfections Are Rare, Although More Common in Older Adults

Researchers found no evidence that protection against COVID-19 reinfection declined within a 6-month follow-up period.

Although prior infection with COVID-19 protects most people against reinfection, new research has found that people over 65 years of age have a greater risk, with only 47% protection against repeat infection compared to 80% protection for younger people.

A large-scale assessment of reinfection rates in Denmark confirmed that only 0.65% of people returned a positive PCR test twice. However, their findings also noted that adults 65 years of age or older were at a significantly higher risk.

The investigators analyzed data collected as part of Denmark’s national COVID-19 testing strategy, through which more than two-thirds of the population were tested in 2020. Free, national PCR testing is a central part of Denmark’s strategy to control COVID-19, which differentiates it from other countries, according to the study authors.

With data spanning the country’s first and second waves, the investigators said they were able to estimate protection against reinfection with the original strain of COVID-19. Importantly, the investigators were not able to estimate protection against reinfection with COVID-19 variants, some of which are known to be more transmissible. Further research is needed to assess how protection against repeat infection might vary with different COVID-19 strains.

The researchers said their findings highlight the importance of measures to protect elderly people during the pandemic, including enhanced social distancing and prioritization for vaccines, even for patients who have recovered from COVID-19. The analysis also suggests that those who have had the virus should still be vaccinated, as natural protection may not be enough, especially in older adults.

Among people who had COVID-19 during the first wave in Denmark, only 0.65% tested positive during the second wave. Notably, with a rate of 3.3%, the rate of infection was 5 times higher among people who returned a positive test during the second wave having previously tested negative.

Of patients under 65 years of age who had COVID-19 during the first wave, 0.6% tested positive again during the second wave. The rate of infection during the second wave among people in this age group who had previously tested negative was 3.60%. Notably, 0.88% of those 65 years of age and over who were infected during the first wave tested positive again in the second wave. Among people 65 years of age and older who had previously not had COVID-19, 2% tested positive during the second wave.

Furthermore, the authors found no evidence that protection against reinfection declined within a 6-month follow-up period, according to the study. Because the virus was only identified in December 2019, the investigators said the period of protective immunity conferred by infection has not yet been determined.

“In our study, we did not identify anything to indicate that protection against reinfection declines within 6 months of having COVID-19,” said Daniela Michlmayr, PhD, from the Staten Serum Institut, in the press release. “The closely related coronaviruses [severe acute respiratory syndrome] and [Middle Eastern respiratory syndrome] have both been shown to confer immune protection against reinfection lasting up to 3 years, but ongoing analysis of COVID-19 is needed to understand its long-term effects of patients’ changes of becoming infected again.”

REFERENCE

The Lancet: Study finds COVID-19 reinfections are rare, more common for those above age 65 [news release]. EurekAlert; March 18, 2021. https://www.eurekalert.org/pub_releases/2021-03/l-tls031821.php. Accessed March 23, 2021.