COVID-19 vaccines teach our immune system to recognize SARS-CoV-2, the virus that causes COVID-19, so that if we are exposed to the virus, our immune system can leap into action and keep us healthy.

Vaccines can protect us from COVID-19 by preventing us from getting infected at all, and by preventing us from getting sick, ending up in the hospital, or dying – even if we are infected. Vaccines can also prevent us from transmitting the virus to others. COVID-19 vaccines can prevent some or all of these health outcomes and the evidence from clinical trials helps us understand how vaccines keep us healthy.

In clinical trials, these are some of the health outcomes that researchers can assess to determine a vaccine’s efficacy – how well a given vaccine candidate works and protects us.

How is vaccine efficacy determined?

Most phase 3 clinical trials report the vaccine efficacy (VE) for symptomatic disease as the primary outcome. That tells us how well the vaccine prevents us from getting sick and showing symptoms. Or, in other words, the vaccine efficacy tells us how much less likely a vaccinated person is to get sick compared to an unvaccinated person. Since vaccines can prevent many health outcomes, it’s important to understand which outcomes were being assessed in clinical trials.

For example, if a COVID-19 vaccine’s efficacy is reported to be 80% against symptomatic disease, participants who received the vaccine were 80% less likely to get sick with COVID-19 and experience symptoms, compared to participants in the control group (the group that did not receive the vaccine). A vaccine clinical trial can report a different outcome, or multiple outcomes, so it’s important to ask what outcome the vaccine efficacy estimate is referring to.

In addition to the primary outcome, vaccines can have multiple benefits. For example, in addition to preventing symptomatic disease (the most common primary outcome), many COVID-19 vaccine clinical trials showed that the vaccine was 100% effective in preventing severe disease (in particular, hospitalizations and deaths). In other words, none of the vaccinated participants, who got infected, were hospitalized or died of COVID-19, a remarkable finding!

Finally, what’s the difference between vaccine efficacy (“efficacy”) and vaccine effectiveness (“effectiveness”)?

Vaccine efficacy (“efficacy”) is a measure of how well the vaccine works in a clinical trial, whereas vaccine effectiveness (“effectiveness”) determines how well the vaccine works in the real world. While these numbers are likely to be very similar, they may not be identical.

Getting vaccinated is the single most effective decision you can make to prevent COVID-19. Yet, no vaccine has been proven to be 100% effective against all outcomes for all age groups and for all variants, so it’s important to continue to follow local health authority guidelines to reduce the risk of SARS-CoV-2 infection even after vaccination.

Learn more about which health outcomes were evaluated in clinical trials by reviewing a vaccine’s clinical trial records!