New York, Feb 12 : 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 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 the 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 (late fall 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.
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