Vaccine efficacy and duration of protection

To what extent can vaccines prevent people from getting infected with COVID-19?


COVID-19 vaccines that have been granted Emergency Use Listing by WHO are highly effective at preventing severe disease, hospitalization and death. But post-introduction studies also indicate that these vaccines have demonstrated strong (but less) prevention of both symptomatic and asymptomatic infection. Several studies also show a reduction – about 50% – in risk of transmission to members of the same household, particularly for Pfizer-BioNTech and AstraZeneca vaccines. Accordingly, the vaccines likely reduce transmission, lowering the risk of disease in unvaccinated people, in addition to helping to stop the spread of variants.

This is supported by vaccine effectiveness against symptomatic disease and infections demonstrated by multiple studies with various vaccines, as well as by clinical trials and post-introduction observational studies, demonstrating vaccine effectiveness against asymptomatic infections for some vaccines, such as AstraZeneca, Janssen, Moderna, and Pfizer-BioNTech.

Additionally, some studies have demonstrated reductions in viral load if infected, and shortened duration of viral shedding for some vaccines, such as AstraZeneca, Moderna, Pfizer-BioNTech. For these three vaccines, there is evidence for direct reduction in household transmission, but more studies are needed to support conclusively their ability to reduce transmission.


  • COVID-19 vaccines are highly effective in preventing severe illness and deaths.
  • They also reduce, but do not eliminate, the risk of infection and transmission.