Previous infection with SARS-CoV-2 gives good protection against reinfection for more than 7 months in under 65s, but this protection decreases significantly in older individuals, show results from a large Danish study.
The degree of protection that an earlier infection offers to people who have had COVID-19 has been much discussed, but was previously unclear. This has led to some anxieties about reinfection particularly with the recurring waves of the virus occurring around the world over the last year.
This study, led by Steen Ethelberg, Ph.D., a researcher at the Statens Serum Institut and professor at the University of Copenhagen in Denmark, sought to address this lack of knowledge by following up people who were infected with SARS-CoV-2 during the first wave in Europe (March–May 2020) during the subsequent second wave (Sep–Dec 2020) to assess their risk for reinfection.
The researchers used the Danish Microbiology Database as a resource to track tests for COVID-19. From 525,339 people tested in the first wave, 11,068 were positive for SARS-CoV-2 infection and eligible for follow up in the second wave.
Out of this group of more than 11,000 individuals, only 72 tested positive for the virus again during the second wave. For comparison, 16,819 of the group who tested negative during the first wave tested positive in the second wave.
As described in an article in The Lancet, earlier infection gave people around 80% protection against reinfection overall. Notably, this did not vary significantly between those who had been infected 3-6 months previously and those whose infection was at least 7 months earlier, suggesting that this protection could last long term. Gender also did not appear to influence risk for reinfection.
“In our study, we did not identify anything to indicate that protection against reinfection declines within six months of having COVID-19,” said Daniela Michlmayr, Ph.D., a co-author on the study also from the Staten Serum Institut.
“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.”
Unfortunately, individuals over the age of 65 years did appear to be at increased risk of reinfection as previously infected people in this group only had 47% protection against reinfection.
“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. 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,” emphasized Ethelberg.
“Given what is at stake, the results emphasize how important it is that people adhere to measures implemented to keep themselves and others safe, even if they have already had COVID-19. Our insights could also inform policies focused on wider vaccination strategies and the easing of lockdown restrictions.”
There were some restrictions with this study. For example, it was not possible to assess the impact of disease severity on reinfection risk and sampling errors may have occurred (although PCR testing was used throughout). The study was also carried out before recent variants such as the South African or UK variants became more wide spread so it’s unclear what impact the mutations may have on reinfection.
But, nevertheless, the large number of people included in this study gives a good population level view on what the risks for infection might be in any future waves and reinforces the need for vaccination to protect the elderly population.