Repeat COVID-19 Infections Are Rare, But People Over 65 Are At Greater Risk of Catching COVID Again – Lancet Study
Nurses preparing a diagnostic test for COVID-19 at a drive-through testing center at the University of Washington Northwest Hospital & Medical Center in March 2020.

Previous infection with COVID-19 protects most people against reinfection for over six months, but those over the age of 65 are more likely to get infected again, found a peer reviewed study published in The Lancet

In the world’s first large-scale study of COVID-19 reinfections, researchers from Denmark, Sweden and the United Kingdom discovered that protection provided by prior SARS-CoV2 infection dropped from 80% for those under 65 years of age to 47% in those over 65. 

“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 Steen Ethelberg, co-author of the study and senior researcher in the epidemiology of zoonotic infections at the Statens Serum Institut in Denmark, in a press release. 

The study used national PCR test data from 2020 in Denmark to compare the infection rates between individuals who tested positive and negative during the first and second surge of the pandemic. 

Between February and December, Denmark’s testing capacity increased rapidly and managed to test approximately 10% of the population each week by the end of 2020. The population-level observational study was conducted in Denmark due to the country’s investment in high intensity testing and its free-of-charge testing strategy. 

Some four million individuals took a PCR test in 2020. During the first surge, from March to May, of the 533,381 people tested, 2.11% tested positive. Among the PCR-positive people from the first wave, 0.65% tested positive again during the second surge, from September to December, compared to 3.27% of those who tested negative during the first surge. 

There were 5.35 positive tests per 100,000 people among those who had previously tested positive, compared to 27.06 positive tests per 100,000 people among those who tested negative. 

Approximately 0.88% of those over 65 years of age who tested positive during the first wave were infected again in the second surge. Among those in this age group who tested negative, 2% tested positive during the second wave. 

The researchers estimated that protection against repeat SARS-CoV2 infection was 80.5%, but the protection among those aged 65 years and older was reduced to 47.1%.

The increased likelihood of testing positive again among older individuals could be due to age-related changes in the immune system, which affect the coordination of immune responses and adaptive immune system, said the authors. This makes individuals over 65 more susceptible to emerging infectious diseases. 

These findings could inform decisions on which groups should be vaccinated and the implementation of public health measures, including physical distancing and mask wearing. 

People wearing face masks to prevent the spread of SARS-CoV2 as they commute inside a metro station amid the COVID-19 pandemic in Colombia.

“Given what is at stake, 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,” Ethelberg said. 

“Our insights could also inform policies focused on wider vaccination strategies and the easing of lockdown restrictions,” he added.

These findings, however, could be limited. The number of infected older people included in the study was small. In addition, the researchers only examined diagnostic test results, so it is possible that those who became reinfected were asymptomatic or had mild symptoms and were not tested.

Duration of Protective Immunity

The longevity of the protection against repeat infection is still unknown, but the researchers found no evidence that the protection waned in six months after infection. 

A separate longitudinal study found that 95% of participants retained immunity for up to eight months after infection, but concentrations of antibodies decreased moderately. 

“The closely related coronaviruses SARS and MERS 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,” said Daniela Michlmayr, co-author and researcher on bacteria and parasites at the Staten Serum Institut. 

The authors of the study concluded that natural protection cannot be relied upon to achieve long term herd immunity.

“These data are all confirmation, if it were needed, that for SARS-CoV2 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,” said Rosemary J. Boyton and Daniel M. Altmann, professors at Imperial College London, in a comment linked to the study. 

Healthcare professional administering a COVID-19 vaccine in the United States in February.
Protection Against Reinfection with COVID Variants

The study analyzed infection with the original COVID-19 strain, as variants were not yet present in Denmark during the study period. 

More longitudinal studies and molecular surveillance are needed to assess if the protection against repeated infection differs with the COVID-19 variants that have emerged over the past several months, according to the authors. 

Some of the variants of concern are known to be more transmissible and could escape from natural and vaccine-induced immunity. This could complicate protective immunity, potentially pushing immunity below a protective margin.

Image Credits: Flickr – Trinity Care Foundation, University of Washington Northwest Hospital & Medical Center, Flickr: IMF Photo/Joaquin Sarmiento, ABC7 News.

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