Danish patients gained between 60% and 94% protection against reinfection, depending on the COVID variant wave, the researchers found.
The findings were published Nov. 22 in the open-access journal PLOS Medicine.
"In our study, we find a significant vaccine effectiveness against SARS-CoV-2 reinfection, and this shows the importance of vaccination also for those who might be protected by natural immunity," study author Katrine Finderup Nielsen said in a journal news release. She is a researcher at Statens Serum Institut in Denmark.
The team of investigators analyzed infection and vaccination data from Danish registers that included all people living in Denmark who tested positive for the virus or were vaccinated between January 2020 and January 2022.
This included more than 200,000 people who had tested positive during the alpha, delta and omicron waves.
Researchers found that vaccination offered up to 71% protection against reinfection during the alpha period. It offered 94% protection during the delta period. By the omicron period, that protection was down to about 60%. The added protection lasted up to nine months.
This insight into vaccine effectiveness may help with planning future vaccination strategies, the authors said.
The study was too short to determine whether the vaccine protected against severe outcomes, such as death and hospitalization. Future studies with longer follow-up times could answer this question, researchers said.
The U.S. Centers for Disease Control and Prevention has more on COVID-19 vaccines.
SOURCE: PLOS Medicine, news release, Nov. 22, 2022