Immunity against severe COVID-19 disease begins to wane four months after receipt of the third dose of an mRNA vaccine, such as Pfizer-BionNTech or Moderna, suggests a study from the US Centers for Disease Control and Prevention (CDC) 

According to the study, waning immunity was observed during both the Delta and Omicron variant waves in a similar fashion to how mRNA vaccine effectiveness wanes after a second dose. Although protection decreased with time, a third dose was still highly effective at preventing severe illness with COVID-19. "The mRNA vaccines, including the booster shot, are very effective, but effectiveness declines over time," said co-author Brian Dixon from Indiana University.

"Our findings suggest that additional doses may be necessary to maintain protection against COVID-19, especially for high-risk populations." Overall, the study reported that individuals with second and third doses of an mRNA vaccine had greater protection against hospitalisations (severe disease) than against emergency department/urgent care (ED/UC) visits (symptoms which may not require hospitalisation). Vaccine effectiveness was also lower overall during the Omicron period than during the Delta period.

Vaccine effectiveness against ED/UC visits declined from 97 per cent within the first two months of receipt of a booster to 89 per cent effectiveness at four months or more during the Delta-predominant period (summer/early fall 2021). During the Omicron-predominant period (2021/winter 2021-22), vaccine effectiveness against ED/UC visits was 87 per cent during the first two months after a third dose, decreasing to 66 per cent at four months after a third dose.

After the third dose, protection against Delta variant-associated hospitalisation declined from 96 per cent within two months to 76 per cent after four months or longer. Vaccine effectiveness against Omicron variant-associated hospitalisations was 91 per cent during the first two months declining to 78 per cent at four months.