COVID-19 Vaccine Efficacy Estimates

Almost half of the US population is now fully vaccinated against COVID-19. But many people have questions about how effective the available vaccines are given that new variants keep emerging and spreading, some adults are reluctant to receive the vaccine and the fact that vaccines remain unavailable to most children 11 years old and younger.

Researchers can measure how effective a vaccine is at preventing disease by following vaccinated and unvaccinated individuals in clinical trials or observational studies. By comparing the incidence of infections in vaccinated and unvaccinated groups, researchers can estimate what’s known as vaccine efficacy. This term describes the reduction of disease in a vaccinated group of people compared to an unvaccinated group, if all other conditions remain the same. A large reduction of disease in a vaccinated group, when compared to an unvaccinated group, equates to a highly effective vaccine. For example, a vaccine efficacy of 95% means that the vaccinated group experiences 95% less incidence of a given disease or other outcome compared to the unvaccinated group.

Researchers at the University of Florida, the Fred Hutchinson Cancer Center, and the University of Washington are analyzing data on the commercially available COVID-19 vaccines to answer questions about their efficacy. Read the researchers’ most recent full report, or use this brief summary. 

In this work, the researchers report on the effectiveness of vaccines made by:

  • Pfizer
  • Moderna
  • Johnson & Johnson
  • AstraZeneca
  • Sputnik
  • Novavax
  • Sinovac
  • Sinopharm

They produced estimates of vaccine efficacy against:

  • Infections (VES)
  • Infections with disease (VESP)
  • Transmission to others, given infection (VEI)

Their main result is that a full course of all vaccines, with two exceptions, have an average efficacy against any disease with infection of more than 85.3%. The Sinovac vaccine fell below this benchmark, however, and was estimated to have an efficacy of 50.7%, and the Sinopharm efficacy estimate is 78.1%. Other findings were that average vaccine efficacy against severe disease, hospitalization, or death is close to 100%. The study authors derived their estimates by reviewing the results of double-blind, placebo-controlled vaccine trials; these are among the most rigorous trials that can be carried out.

How to read these graphs

The researchers present their findings visually using a graph called a forest plot. These figures are a specific way to report the results of a meta-analysis that averages findings from multiple different studies.

In the forest plots below, the vertical list of vaccine manufacturers is shown on the left; and the data to the right of each maker’s name communicates the average vaccine efficacy (derived from multiple studies) shown as a filled square, while also showing the range of reported results as the “whisker” line that trails in each direction from the filled square. The average vaccine efficacy is then reported numerically on the right, per vaccine maker, with confidence intervals in brackets. Below the list of vaccine makers, an overall pooled vaccine efficacy value is calculated that averages all of the manufacturers’ averages together. This is shown as the oblong line of varying thickness at the bottom.

Most recent meta-analysis graphs

The following graphs are taken from the team's meta-analysis published on July 26, 2021. This page will be updated when the team issues its most recent findings.

Preventing ANY disease




Preventing SEVERE disease




Preventing hospitalization


Preventing death



Preventing transmission to others


Vaccine efficacy for variants of concern

Alpha (B.1.1.7)




Gamma (P.1)



Beta (B.1.351)




Delta (B.1.617.2)



This meta-analysis was posted on July 28, 2021 based on the research team's most recent meta-analysis dated July 26, 2021. These graphs and findings will be updated as the findings from new meta-analyses are issued. The original research paper was first posted to MedRxiv.