Individuals previously infected with SARS-CoV-2, the virus that causes COVID-19, still benefit from vaccination and receive 60% to 94% protection against reinfection, depending on the variant. A new study led by Katrine Finderup Nielsen of the Statens Serum Institut, Denmark, reports these findings Nov. 22.nd in the open access journal PLOS medicine.
During the recent pandemic, vaccination was one of the best resources available to contain the spread of COVID-19. People infected with the virus are known to develop long-lasting natural immunity, but Finderup Nielsen and her team wanted to know if these individuals would still benefit from receiving the vaccine. The team analyzed infection and vaccination data from national Danish registries, including all people living in Denmark who tested positive for the virus or were vaccinated between January 2020 and January 2022. The dataset included more than 200,000 people who tested positive for SARS-CoV – 2 during each of the Alpha, Delta and Omicron waves. Their analysis found that for people with previous infections, vaccination provided up to 71% protection against reinfection during the Alpha period, 94% during the Delta period, and 60% during the Omicron period, with protection up to nine months.
These findings show that vaccination protected people against SARS-CoV-2, in addition to the protection afforded by natural immunity during all three variant waves. The authors point out that the current study was too short to determine whether the vaccine protects against serious outcomes, such as death and hospitalization, and that future studies with longer follow-up times will be needed to answer this question. From a public health perspective, this understanding of vaccine effectiveness can help decision-makers plan the timing and implementation of vaccination strategies to make them most effective.
In our study, we find significant vaccine effectiveness against SARS-CoV-2 reinfection, and this demonstrates the importance of vaccination, including for those who may be protected by natural immunity.”
Katrine Finderup Nielsen of the Statens Serum Institut, Denmark
Nielsen, K.F., et al. (2022) Vaccine effectiveness against SARS-CoV-2 reinfection during periods of Alpha, Delta or Omicron dominance: a Danish nationwide study. PLOS medicine. doi.org/10.1371/journal.pmed.1004037.