ACIP — The Advisory Committee on Immunization Practices (ACIP) recommended to the CDC that booster doses of Comirnaty be administered to persons who are ≥65 years old, adult residents of long-term care facilities, and persons aged 50-64 years who have an underlying medical condition. The ACIP also recommended that booster doses be considered for younger adults with underlying medical conditions, but advised against their use on the basis of institutional or occupational exposure to SARS-CoV-2.

CDC — The CDC, which regulates US public vaccination programs, accepted most of the ACIP's recommendations, but stated that booster doses may indeed be considered for adults 18-64 years old who are at increased risk for severe COVID-19 because of frequent institutional or occupational exposure to SARS-CoV-2 (see Table 2).2

DOSAGE AND ADMINISTRATION — The booster dose of Comirnaty is the same as the dose for primary immunization (30 mcg IM). A single booster dose may be given ≥6 months after completion of a 2-dose primary series.3

CLINICAL STUDIES — Expansion of the EUA to include booster doses was based on cohort data from the US and Israel and on the results of an immunogenicity study.

Waning Immunity – In a retrospective cohort study of ~3.4 million persons ≥12 years old in the US, those who received two doses of Comirnaty were significantly less likely to be infected with SARS-CoV-2 than those who were not vaccinated, but the relative risk reduction associated with vaccination declined from 88% at ≤1 month to 47% at ≥5 months after the second dose. Vaccination was also associated with a lower risk of hospitalization due to COVID-19; the relative risk reduction did not change significantly over time (87% at ≤1 month; 88% at ≥5 months).4

In a study that examined positive PCR test results for SARS-CoV-2 infection in Israel over 3 weeks in July 2021, adults ≥60 years old who completed a 2-dose primary series of the Pfizer/BioNTech vaccine in the second half of January 2021 had a significantly higher rate of infection than those who completed their series in the second half of March 2021 (3.2 vs 1.6 cases/1000 persons). Similarly, adults ≥60 years old who completed their series in January had a significantly higher rate of severe COVID-19 than those who completed it in March (0.29 vs 0.15 cases/1000 persons).5

Booster Immunogenicity – In an unpublished longitudinal immunogenicity study (summarized in the FDA Fact Sheet), 210 adults 18-55 years old who had completed a 2-dose primary series of Comirnaty about 6 months previously and had no evidence of prior SARS-CoV-2 infection were given a booster dose. Geometric mean titers of anti-SARS-CoV-2 neutralizing antibodies 1 month after the booster dose were 3.29-fold higher than they were 1 month after the second primary-series dose.3

Booster Efficacy – In a one-month cohort study in ~1.1 million Israeli residents who had completed a 2-dose primary series of the Pfizer/BioNTech vaccine ≥5 months previously, persons who received a booster dose had significantly lower rates of SARS-CoV-2 infection (by 11.3-fold) and severe COVID-19 (by 19.5-fold) beginning 12 days after administration compared to those who did not.6

ADVERSE EFFECTS — Adverse effects with a third dose of an mRNA-based COVID-19 vaccine appear to be similar to those with the second primary-series dose.7 In the immunogenicity study in adults 18-55 years old, lymphadenopathy occurred more commonly with the booster dose of the Pfizer/BioNTech vaccine than with primary-series doses (5.2% vs 0.4%), but no cases of hypersensitivity reactions, Bell’s palsy, or myocarditis/pericarditis were reported in booster dose recipients.3,8

OTHER VACCINES — Booster doses of the mRNA-based COVID-19 vaccine manufactured by Moderna (Spikevax) and the adenovirus-based COVID-19 vaccine manufactured by Johnson & Johnson (Janssen) have not been authorized to date. The FDA and CDC are expected to consider the appropriateness of booster doses for recipients of these vaccines in the near future.

CONCLUSION — The FDA has authorized use of a single booster dose of the Pfizer/BioNTech mRNA-based COVID-19 vaccine (Comirnaty) administered ≥6 months after completion of the 2-dose primary series in certain adults. The efficacy of primary immunization with Comirnaty in preventing SARS-CoV-2 infection appears to decrease over time, especially in older persons, and administration of a booster dose has been associated with decreased rates of infection and severe COVID-19. Whether booster doses of Comirnaty will eventually be recommended for the general population remains to be determined. Booster doses of the Moderna and Johnson & Johnson COVID-19 vaccines are not currently recommended.

REFERENCES