Message: #1 2022-07-16 
Vaccine efficacy, effectiveness and protection
14 July 2021
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This article is part of a series of explainers on vaccine development
and distribution. Learn more about vaccines – from how they work and
how they’re made to ensuring safety and equitable access – in
WHO’s Vaccines Explained series.
COVID-19 vaccines have proven to be safe, effective and life-saving.
Like all vaccines, they do not fully protect everyone who is
vaccinated, and we do not yet know how well they can prevent people
from transmitting the virus to others. So as well as getting
vaccinated, we must also continue with other measures to fight the
pandemic.
Vaccine efficacy and effectiveness
All COVID-19 vaccines approved by WHO for emergency use listing have
been through randomized clinical trials to test their quality, safety
and efficacy. To be approved, vaccines are required to have a high
efficacy rate of 50% or above. After approval, they continue to be
monitored for ongoing safety and effectiveness. But what is the
difference between efficacy and effectiveness?
A vaccine’s efficacy is measured in a controlled clinical trial and
is based on how many people who got vaccinated developed the
‘outcome of interest’ (usually disease) compared with how many
people who got the placebo (dummy vaccine) developed the same outcome.
Once the study is complete, the numbers of sick people in each group
are compared, in order to calculate the relative risk of getting sick
depending on whether or not the subjects received the vaccine. From
this we get the efficacy – a measure of how much the vaccine lowered
the risk of getting sick. If a vaccine has high efficacy, a lot fewer
people in the group who received the vaccine got sick than the people
in the group who received the placebo. more... |