All of the authorized and approved vaccines are effective at preventing severe COVID-19.
Against earlier forms of the coronavirus, the vaccines were highly effective at preventing symptomatic illness. For example, the Pfizer/BioNTech vaccine, which was the first COVID-19 vaccine to receive full approval from the Food and Drug Administration, showed a final efficacy of 91% against symptomatic illness in its phase 3 trial, meaning that under the conditions of the trial the vaccine reduced the risk of getting sick by 91%.
Johnson & Johnson, which partly tested its vaccine in South Africa when the beta variant emerged, reported an efficacy of 66% in preventing moderate to severe COVID-19 and an efficacy of 85% in preventing severe or critical COVID-19 in its clinical trial.
Subsequent studies have demonstrated that the vaccines are effective under real-world conditions, including against the highly contagious delta variant. Against omicron, however, the vaccines haven’t fared as well and are much worse at preventing infection or mild disease. The shots are still good at preventing hospitalization or death, though, particularly if someone has received a booster dose.
Booster shots are recommended for most people, as these increase and prolong protection against severe disease and also provide some temporary protection against infection and milder illness. For instance, a Centers for Disease Control and Prevention study found that during the omicron wave, adults who had received two doses of the Pfizer/BioNTech or Moderna vaccines had a 79% lower risk of dying or needing a ventilator if hospitalized with COVID-19, compared with unvaccinated adults. Those who had received a booster vaccine dose as well had a 94% lower risk.
People over the age of 50 and people who are immunocompromised are also eligible for a second mRNA booster.