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A Project of The Annenberg Public Policy Center
SciCheck’s COVID-19/Vaccination Project

How effective are the vaccines?


This article is available in both English and Español

All of the authorized vaccines are effective at preventing symptomatic disease.

According to the results of the phase 3 trials, the Pfizer/BioNTech and Moderna vaccines had an efficacy of 94% or higher, which means your approximate risk of getting sick is cut by 94% or more if you are vaccinated. It’s important to keep in mind that the effectiveness of the vaccines outside the controlled setting of a clinical trial is typically somewhat lower. A study by the Centers for Disease Control and Prevention found these two vaccines were 90% effective in real-world conditions, two weeks after the second dose, and 80% effective two weeks after the first dose. The study monitored 3,950 health care personnel, first responders and other essential workers for 13 weeks.   

Johnson & Johnson’s phase 3 trial began in late September, nearly two months after the Pfizer and Moderna trials and shortly before a new variant of the coronavirus emerged in South Africa. Globally, J&J reported an efficacy of 66.1% in preventing moderate to severe COVID-19 and an efficacy of 85.4% in preventing severe or critical COVID-19. But efficacy was higher in the U.S. population (72% efficacy in preventing moderate to severe disease and 85.9% efficacy in preventing severe or critical disease) than in South Africa (64% and 81.7% efficacy in preventing moderate to severe or severe/critical disease, respectively).

Data from the Moderna trial also demonstrate that the vaccine protects against severe COVID-19. The Pfizer/BioNTech vaccine probably does as well. But because so few participants in that trial developed severe COVID-19, this hasn’t yet been conclusively shown. The J&J trial showed 100% efficacy in preventing COVID-19 that would require medical intervention — meaning hospitalization, ICU admission, mechanical ventilation or a life support machine.

Although the trials show the vaccines protect against symptomatic disease, what is still unknown is whether these vaccines protect against infection with the coronavirus, and if not, whether they can prevent someone from spreading the virus to others. (See our video on the distinction between virus and disease.) However, the CDC has said that “a growing body of evidence” suggests those who are fully vaccinated are “potentially less likely to transmit SARS-CoV-2 to others.”