New research adds to evidence that people who have been infected with COVID-19 and developed antibodies still benefit from vaccination.
Dr Katrine Finderup Nielsen, of the Statens Serum Institut, Denmark, and colleagues investigated the protection added by COVID vaccines to natural post-infection immunity.
They looked at nationwide Danish registers of infection and vaccination from January 2020 to January 2022, covering 209,814 people infected around the Alpha variant period, 292,978 around the Delta period, and 245,530 around the Omicron period.
"Waning immunity following vaccination was observed and was most pronounced during the Omicron period," the authors report in yesterday’s *PLoS Medicine*.
They add: "This study shows that in previously infected individuals, completing a primary vaccination series was associated with a significant protection against SARS-CoV-2 reinfection compared with no vaccination."
Their estimates show that these individuals gain a 60% to 94% additional protection against reinfection, depending on the variant, for the subsequent nine months.
"Even though vaccination seems to protect to a lesser degree against reinfection with the Omicron variant, these findings are of public health relevance as they show that previously infected individuals still benefit from COVID-19 vaccination in all three variant periods," the authors conclude.
They call for further studies on the impact of the vaccine on severe outcomes, such as death and hospitalisation, for those with previous infection.
Dr Finderup Nielsen commented: “This shows the importance of vaccination also for those who might be protected by natural immunity.”
Nielsen, K. F. et al. Vaccine effectiveness against SARS-CoV-2 reinfection during periods of Alpha, Delta, or Omicron dominance: A Danish nationwide study. *PLoS Medicine* 22 November 2022 doi: 10.1371/journal.pmed.1004037
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