What did the vaccines show efficacy for? What does efficacy mean for these clinical trials?

Clinical Trials

  • Pfizer-BioNTech:
    • Interim findings, with a median of two months of follow-up, indicate that the Pfizer-BioNTech COVID-19 vaccine was 95.0% effective (95% confidence interval = 90.3%–97.6%) in preventing symptomatic laboratory-confirmed COVID-19 inpersons without evidence of previous SARS-CoV-2 infection.
    • Evidence is based on one large, randomized, double-blind, placebo-controlled Phase II/III clinical trial that enrolled >43,000 participants in the U.S., Germany, Turkey, South Africa, Brazil and Argentina. (median age = 52 years, range = 16–91 years).
    • Consistent high efficacy (≥92%) was observed across age, sex, race, and ethnicity categories and among persons with underlying medical conditions as well as among participants with evidence of previous SARS-CoV-2 infection.
    • Although numbers of observed hospitalizations and deaths were low, the available data were consistent with reduced risk for these severe outcomes among vaccinated persons compared with that among placebo recipients.
  • Moderna (COVE Study)
    • Interim findings, with a median of two months of follow-up, indicate that the Moderna COVID-19 vaccine efficacy after two doses was 94.1% (95% confidenceinterval = 89.3%–96.8%) in preventing symptomatic, laboratory-confirmed COVID-19 among persons without evidence of previous SARS-CoV-2 infection.
    • Evidence is based on one large, randomized, double-blind, placebo-controlled Phase III clinical trial that enrolled approximately 30,000 participants in the United States aged 18–95 years (median = 52 years).
    • High efficacy (≥86%) was observed across age, sex, race, and ethnicity categories and among persons with underlying medical conditions. Ten hospitalizations due to COVID- 19 were documented; nine in the placebo group and one in the vaccine group.
    • Preliminary data suggest that the Moderna COVID-19 vaccine might also provide some protection against asymptomatic SARS-CoV-2 infection.
  • Johnson & Johnson (ENSEMBLE Study)
    • Among all participants from different geographies and including those infected with an emerging viral variant, Johnson & Johnson’s COVID-19 vaccine candidate was 66% effective overall in preventing moderate to severe COVID-19, 28 days after vaccination. The onset of protection was observed as early as day 14. The level of protection against moderate to severe COVID-19 infection was 72% in the United States, 66% in Latin America and 57% in South Africa, 28 days post-vaccination.
    • The vaccine was 85% percent effective in preventing severe disease across all regions studied, 28 days after vaccination in all adults 18 years and older. Efficacy against severe disease increased over time with no cases in vaccinated participants reported after day 49.
    • Evidence is based on a large trial enrolling approximately 44,000 participants and was conducted in eight countries across three continents: 44% (N=19,302) of participants in the United States, 41% (N=17,905) in Central and South America (Argentina, Brazil, Chile, Colombia, Mexico, Peru) and 15% (N=6,576) in South Africa. 

Efficacy In Plain Terms

95% efficacy does not mean that 95% of vaccinated individuals will not get COVID-19 and that the other 5% will get COVID-19.  It means vaccinated individuals will be 95% less likely to get COVID-19.

For example, if 1% of the unvaccinated population develops COVID-19 (similar to the Pfizer control group), getting the vaccine would reduce chances of getting COVID-19 by 95%, resulting in a 0.05% infection rate.

For information about in-vitro efficacy against variants, please see question about variants.

Sources

Updated: March 1, 2021

ICSI

ICSI