29 septiembre, 2021

To Boost or Not to Boost …

Five analyses provide insights into the relative benefits of COVID-19 vaccine booster doses.

Perhaps the most pressing issue under active consideration by the FDA and CDC concerns the possible need for COVID-19 vaccine booster doses. Five new or continuing studies provide key information that is influencing the decisions of both agencies.

In two CDC-based studies with test-negative designs, researchers assessed the effectiveness of the three COVID-19 vaccines currently authorized in the U.S.

  • Thompson and colleagues evaluated outcomes in adults aged ≥50 arising from >41,500 hospital admissions to 187 hospitals and >21,500 emergency department and urgent care visits to 221 facilities between January 1 and June 22, 2021. The effectiveness of full immunization with mRNA vaccine (Pfizer-BioNTech or Moderna) was 89% (RT-PCR–confirmed SARS-CoV-2 infection necessitating hospitalization), 90% (ICU admission), and 91% (emergency department or urgent care visits). The Johnson & Johnson AD26.COV2.S vaccine had 68% and 73% effectiveness, respectively, against infection requiring hospitalization and emergency department or urgent care visits. The effectiveness of either mRNA vaccine was similar in various geographic regions, in patients aged ≥85, in Black and Hispanic populations, in those with chronic underlying medical conditions, and for the interval from full vaccination through day 112.
  • Pilishvili and colleagues studied mRNA vaccine effectiveness in 109,865 healthcare workers tested for SARS-CoV-2 between December 28, 2020, and May 19, 2021, of whom 8365 tested positive. The adjusted effectiveness of complete immunization with either vaccine was 90.4% overall, but only 39.1% among participants with immunocompromising conditions. Effectiveness was slightly lower for postvaccination weeks 9 through 14 compared with earlier (although the 95% confidence intervals overlapped).

Two follow-up analyses (one a preprint) of the placebo-controlled Moderna vaccine trial (NEJM JW Infect Dis Feb 2021 and N Engl J Med 2021; 384:403) have been published.

  • The initially reported efficacy of 94.1% was based on a median follow up of 64 days; now, El Sahly and colleagues have evaluated safety and efficacy through a median of 183 days (with a cut-off of March 26, 2021). No new safety concerns were identified. Efficacy was 93.2% against COVID-19, 98.2% against severe disease and 63.0% against asymptomatic SARS-CoV-2 infection, and was comparable across ethnic, racial, and age groups.
  • In the second analysis, Baden et al. assessed COVID-19 incidence (between July 1 and August 27, 2021) in trial participants who had been initially randomized to receive vaccine compared with incidence (between December 2020 and April 2021) in those who had been randomized to receive placebo and were subsequently vaccinated. Among the initially vaccinated participants, disease incidence was 77.1 (any COVID-19) and 6.2 per 1000 person-years (severe COVID-19); for the more recently vaccinated participants, incidence was 49.0 (any) and 3.3 (severe), for a reduction of 36% and 46%, respectively.

Bar-On et al. examined the effectiveness of a third dose of Pfizer-BioNTech mRNA vaccine in recipients aged ≥60 using the Israeli Ministry of Health database (information on 1,137,804 individuals between July 30 and August 31, 2021). They found that ≥12 days after the third dose, the rate of confirmed SARS-CoV-2 infection in boosted individuals was 11.3-fold lower than in those who had received two doses, and the rate of severe illness was 19.5-fold lower.


While the three studies with analyses completed during the early spring of 2021 did not assess vaccine effectiveness against different SARS-CoV-2 variants (especially Delta), all show that the available vaccines — both mRNA vaccines in particular — maintained impressive effectiveness against mild and severe COVID-19. Counterbalancing these data, the analysis of the Moderna vaccine’s effectiveness in early versus later recipients during the summer of 2021 (when Delta rose to predominance in the U.S.), while limited by a small sample size and short observation period, indicates that immunity appears to wane beyond 6 months. The study in Israel shows that a booster dose of Pfizer vaccine can provide significantly greater protection against severe (or any) SARS-CoV-2 infection, with the caveat that the reported follow-up period for that trial only went to 25 days post vaccination.

Determining the appropriate immunization schedule for any vaccine will be difficult, particularly as we don’t yet have scientific or societal consensus on whether the goal of vaccination is to prevent all, or only the most severe, COVID cases. I personally concur with the FDA advisory committee’s recommendation and CDC’s decision to allow a booster dose of Pfizer vaccine for those aged ≥65 or at high risk for complications or occupational or institutional exposure — and I anticipate that, for all three vaccines in use in the U.S., boosters eventually will be recommended for all individuals aged ≥12.

Dr. Hana El Sahly is an author of two of the summarized papers and a member of the NEJM Journal Watch Infectious Diseases board but had no role in selecting this feature.

At the time we reviewed the paper by Baden et al. it had not undergone formal peer review, and subsequent changes may be made.

Créditos: Comité científico Covid

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