Can you please help us understand vaccine efficacy vs. effectiveness?


A: Why yes we can! Vaccine efficacy and effectiveness both measure the % reduction in disease in the vaccinated group compared to the unvaccinated group. Vaccine efficacy measures this under *optimal conditions*, such as a vaccine trial, and vaccine effectiveness measures this in *real world settings*.

How was vaccine efficacy evaluated in vaccine trials?

In vaccine trials, the primary outcome of interest was *symptomatic COVID-19 disease*. Scientists monitored individuals who were vaccinated to see how many developed symptoms and tested positive for COVID-19 and did the same in the group that received the placebo shot. They then estimated the % reduction in symptomatic COVID-19 disease in those who were vaccinated compared to those who were not. In the Pfizer trial, for example, there was a 95% reduction in symptomatic COVID-19 disease in the vaccinated group compared to the placebo group (i.e., for every 100 individuals who developed symptomatic COVID-19 disease in the placebo group, only 5 did in the vaccine group).

While not the primary outcome of interest, some data has also been released from trials on vaccine efficacy for *overall infection* (infection among those with and without symptoms) as well as *asymptomatic infection*, specifically. See our prior post discussing these results here.

What factors affect vaccine effectiveness in the real world?

In the real world, differences in population characteristics (i.e., age distribution, rates of underlying comorbidities and/or baseline immune function), the way the vaccine is administered (i.e., adherence to dosing schedule and storage and handling guidelines) and circulation of different variants of the SARS-CoV-2 virus in the population, compared to in clinical trials, can all impact how effective the vaccine is in practice.

How are scientists assessing COVID-19 vaccine effectiveness in the real world?

Researchers have begun to utilize observational data to try to assess the effectiveness of vaccines for reducing symptomatic COVID-19 disease, as well as *any* infection, outside of clinical trial settings. For example, in a not yet peer-reviewed study, researchers in Israel utilized data on individuals enrolled in a health maintenance organization (HMO) that covers more than half of all Israelis to compare the number who tested positive for COVID-19 among 600,000 individuals vaccinated with the Pfizer vaccine to the number who tested positive in a group of approximately the same size who had similar medical histories, but did not receive the vaccine. They estimated a 95% reduction in symptomatic COVID-19 disease among the vaccinated group seven or more days after receiving the second dose.

In another study from Israel that was recently published in The Lancet, researchers found among 7,214 hospital staff that there was an 85% reduction in symptomatic COVID-19 disease and an overall reduction of infection (both symptomatic asymptomatic) of 75%, 15-28 days after their first dose of the Pfizer vaccine.

Data from studies aimed at assessing vaccine effectiveness in real world settings is rapidly emerging (these are just two examples) and will continue to shed light on the effectiveness of currently available vaccines across a range of populations, different modes of administration, and among those who have been infected with different SARS-CoV-2 variants. The Nerdy Girls will continue to update you as new information becomes available!

For more on the HMO data reported in Israel see here.

To read the Lancet study visit here.

For more examples of on ongoing COVID-19 vaccine effectiveness research being carried out by CDC in the U.S. visit here.

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