In a new study from CDC, researchers found that immunity against severe COVID-19 disease begins to wane four months after receipt of the third dose of an mRNA vaccine (Pfizer or Moderna).
Waning immunity was found 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 findings suggest that additional doses may be necessary to maintain protection against COVID-19, especially for high-risk populations.
In the study, the team found that individuals with second and third doses of an mRNA vaccine had greater protection against hospitalizations (severe disease) than against ED/UC visits (symptoms that may not require hospitalization).
Vaccine effectiveness was also lower overall during the Omicron period than during the Delta period.
Vaccine effectiveness against ED/UC visits declined from 97% within the first two months of receipt of a booster to 89% 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% during the first two months after a third dose, decreasing to 66% at four months after a third dose.
After the third dose, protection against Delta variant-associated hospitalization declined from 96% within two months to 76% after four months or longer.
Vaccine effectiveness against Omicron variant-associated hospitalizations was 91% during the first two months declining to 78% at four months.
These findings confirm the importance of receiving a third dose of mRNA COVID-19 vaccine to prevent moderate-to-severe COVID-19 illness, especially among those with comorbidities
That protection conferred by mRNA vaccines waned in the months following a third vaccine dose supports further consideration of booster doses to sustain protection against moderate-to-severe COVID-19 illness.
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The study was conducted by Brian Dixon et al.
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