Recently, CDC researchers published data on how long protection against severe disease lasted following two doses of either the Pfizer or Moderna mRNA vaccines in adults (Tenforde MW, Self WH, Zhu Y, et al. Protection of mRNA vaccines against hospitalized COVID-19 in adults over the first year following authorization in the United States. Clin Infect Dis. 2022 May17;ciac381. doi: 10.1093/cid/ciac381).

The incidence of hospitalization was examined in 10,078 patients who were or were not vaccinated with two doses of mRNA vaccine (Pfizer or Moderna) between March and December 2021, a period when the predominant circulating strain was the delta variant. About 80% of those studied had at least one comorbidity that increased their risk of severe disease. The authors found that among immunocompetent adults, protection against hospitalization was 90% three months post-immunization and 82% nine months post-immunization. For Pfizer’s vaccine, efficacy against hospitalization declined from 88% to 79% during that six-month period; for Moderna’s vaccine, efficacy dropped from 93% to 87% during that period. For those 18 to 64 years of age, efficacy from three to nine months dropped from 91% to 87%, and for those 65 years of age and older, it dropped from 87% to 78%.

The authors concluded that during a time when the delta variant was the predominant circulating strain, “effectiveness of two mRNA doses against COVID-19-associated hospitalization was largely sustained through nine months.” The push for third and fourth doses of mRNA vaccines in the United States has largely been the result of the emergence of immune-evasive strains, specifically omicron and omicron subvariants.

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