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 the 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.
    • In a phase 3 trial of adolescents (age 12-15) without evidence of prior COVID-19 infection, data shows the vaccine had 100% efficacy and led to robust antibody responses. The trial enrolled 2,260 adolescents in the United States. 18 cases of COVID-19 were observed in the placebo group versus none in the vaccinated group.  Vaccination elicited a strong antibody response in a subset of adolescents one month after the second dose.

Special Note: New data released April 1, 2021 from continued follow up of the Pfizer-BioNTech phase 3 trial shows: 

    • Vaccine is still highly effective with 91.3% efficacy observed against symptomatic COVID-19, measured seven days through up to six months after the second dose.
    • Vaccine was 100% effective in preventing severe disease as defined by the CDC and 95.3% effective in preventing severe disease as defined by the FDA.
    • Efficacy was generally consistent across age, gender, race and ethnicity demographics, and across participants with underlying conditions.
    • Vaccine efficacy was 92.6% in the United States; the vaccine 100% effective in preventing COVID-19 cases in South Africa where the B.1.351 lineage is prevalent.
    • No serious safety concerns were observed in trial participants up to six months after the second dose.
  • 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.
    • New data (released in NEJM on April 6, 2021) showed follow-up from phase 1 study:
      • This study analyzed 33 healthy adult participants in Moderna’s Phase 1 study at 6 months following the second dose (day 209). Antibody activity remained high in all age groups at day 209, with those age 18 to 55 having the highest titers.
      • New data from the Phase 3 COVE Study: Updated cases show continued strong efficacy, including greater than 90% against cases of COVID-19 and greater than 95% against severe cases of COVID-19, with approximately 6 months median follow-up post-dose 2.
  • Johnson & Johnson (ENSEMBLE Study)
    • Interim findings from this clinical trial indicate that the Janssen COVID-19 vaccine efficacy against symptomatic, laboratory-confirmed COVID-19 was 66.3% ≥14 days after vaccination and 65.5% ≥28 days after vaccination.
    • Efficacy varied geographically and was highest in the United States (74.4%), followed by Latin America (64.7%) and South Africa (52.0%). Regional differences in SARS-CoV-2 variants were noted; in South Africa, 94.5% of virus sequences from trial participants were from the B.1.351 lineage, whereas in Brazil, the P.2 lineage accounted for 69.4% of virus sequences.
    • The vaccine was 85% effective in preventing severe disease across all regions studied
    • Vaccine efficacy for the prevention of COVID-19–associated hospitalization was high: 93% at 14 days or more post-vaccination and 100% at 28 days or more post-vaccination.
    • No COVID-19 associated deaths occurred in the vaccine group, seven COVID-19–associated deaths occurred in placebo recipients. Vaccine efficacy against all-cause death was 75%
    • Preliminary data suggest benefit in protecting against asymptomatic infection (among a subset of participants with SARS-CoV-2 serology results 71 days after vaccination, 0.7% of vaccine recipients had no symptoms of COVID-19 but had documented seroconversion to a non–spike protein, a marker of natural infection, compared with 2.8% of placebo recipients)
    • 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: April 7, 2021 

 

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