Second Booster for COVID-19 Vaccination: Early Results

The U.S. CDC recently authorized a second mRNA COVID-19 vaccine booster (i.e., a fourth dose overall) for middle-aged and older adults (age, ≥50). In part, the CDC's decision was informed by two observational studies from Israel, where a fourth dose of the BNT162b2 vaccine (Pfizer-BioNTech) was approved on January 2, 2022, for older adults and others at high risk.

Taken together, the two studies involved more than 1 million older people (age, ≥60); short-term outcomes for those receiving a fourth dose between January and March 2022 were compared with outcomes among those who had received only 3 doses. The B.1.1.529 (Omicron) variant was dominant during this period. Findings were as follows:

• The fourth dose began to show protection against confirmed SARS-CoV-2 infection during the second week after vaccination; the peak effect occurred at roughly 4 weeks (a roughly 50% reduction in infection, compared with infections in 3-dose recipients), but the effect had disappeared by 8 weeks.

• Protection against a global category of severe COVID-19 also was noted by the second week after the fourth dose; protection was still peaking at 6 weeks (the last time-point for this observation), when the rate of severe infection was lower by roughly two thirds among four-dose recipients, compared with three-dose recipients.

• Between days 7 and 30 after the fourth dose, relative reductions in COVID-19–related hospitalization and death were both roughly 70%; absolute reductions were 180 fewer hospitalizations and 23 fewer deaths per 100,000 people.