What do the efficacy rates of COVID-19 vaccines mean, and do the efficacy rates impact a population’s herd immunity?

This article was published on
May 31, 2021

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Vaccine efficacy rates tell us how well the new COVID-19 vaccines prevent people from COVID-19 infection in clinical trials. They do not tell us the exact vaccine effectiveness rates that can be expected once the vaccines are used for the public.

Vaccine efficacy rates tell us how well the new COVID-19 vaccines prevent people from COVID-19 infection in clinical trials. They do not tell us the exact vaccine effectiveness rates that can be expected once the vaccines are used for the public.


What our experts say

Vaccine studies share results that tell public health experts, health providers, scientists, and the public how well the vaccines work. Vaccine efficacy and vaccine effectiveness are two common terms that may sound similar. In research, they mean different things. 

Vaccine Efficacy According to the U.S. Centers for Disease Control and Prevention (U.S. CDC), vaccine efficacy is used to describe how well the vaccine protects clinical trial participants from getting sick or getting very sick. Vaccine efficacy refers to results reported from clinical trials in research settings. 

For example, Moderna and Pfizer/BioNTech’s vaccine studies have shown that the shots prevent infection about 95% of the time (two weeks after the second dose).

In statistics, this efficacy value is called the relative risk reduction (RRR). The RRR tells us how much the risk of COVID-19 infection is reduced (or lowered) in the test vaccine group, compared to a control group who did not receive the test vaccine. 

Some experts argue that there is too much focus on RRR and that different measurements, like absolute risk reduction (ARR) (or risk difference) should be reported more publicly. While there is value in considering other measurements, they can be misleading or misunderstood. Even the author of a Lancet paper that suggested that ARR should be reported with vaccine efficacy data has responded to a recent fact-check about his work.

It is important to know a few things about absolute risk reduction (ARR) and vaccine trials: 1. How much a virus is spreading, as well as measures that people take to prevent getting sick (e.g., wearing masks, social distancing), can influence what absolute risk reduction looks like. 2. Clinical trials are relatively short term. Using an ARR for trial data can make a vaccine seem less promising than the effect that a vaccine would have on a person’s risk over a longer period of time.  3. Researchers looked at vaccine efficacy once a specific number of COVID-19 cases had been confirmed in vaccine trials. With this approach, the ARR will always appear low, because it is tied to the number of cases confirmed in the study. Let’s say a study enrolled 20,000 patients into the control group and 20,000 in the vaccine group. In that study, 200 people in the control group got sick and 0 people in the vaccine group got sick. Even though the vaccine efficacy would be a whopping 100%, the ARR would show that vaccines reduce absolute risk by just 1% (200/20,000= 1%). For the ARR to increase to 20% in our example study with a vaccine with 100% efficacy, 4,000 of the 20,000 people in the control group would have to get sick (4,000/20,000= 20%). From a public health perspective, we would not want to wait for more control group infections and a higher ARR before sharing the study findings.

Researchers are still studying how well the COVID-19 vaccines are at limiting spread of COVID-19.

Vaccine Effectiveness Vaccine effectiveness explains how well a vaccine works for the public in real-world conditions. Research about vaccine effectiveness is ongoing. More data will become available as more people around the world are vaccinated.

Herd Immunity “Herd immunity” refers to the percentage of people who need to be immune to a virus so that the community (or herd) is safe. Immunity can be through vaccines or from becoming infected. Herd immunity is important to lower the number of people who need healthcare and protect individuals who may not be able to receive the vaccine for health reasons.

Researchers are still learning about what herd immunity for COVID-19 looks like. Many experts think that we may need at least 60-70% of the population to be vaccinated or recovered from infection in order to achieve herd immunity, but this has not been confirmed. Researchers are also still learning about how vaccine efficacy reported from clinical trials will impact herd immunity. It is currently thought that the percentage of people who agree to get vaccinated will be an important factor for achieving herd immunity.

Vaccinations play an important role for individual health as well as for public health. They can prevent some people from getting sick at all, and they can protect people from getting severely ill or dying. Everyone who is able to get a vaccine is highly encouraged to do so, to help protect themselves as well as others.

Context and background

There is a lot of interest in how well COVID-19 vaccines work, and how the vaccines will help contribute to herd immunity. Unfortunately, there is also vaccine hesitancy among some people. Herd immunity relies upon making sure that people who are able to receive vaccines are willing to take them.


  1. Overview of Vaccine Efficacy and Vaccine Effectiveness (WHO)
  2. Vaccine Efficacy, Effectiveness and Impact (KCE)
  3. “Herd Immunity”: A Rough Guide (CID)
  4. COVID and Herd Immunity (The Atlantic)
  5. Vaccine refusal and herd immunity (Lancet ID)
  6. Outcome Reporting Bias in COVID-19 mRNA Vaccine Clinical Trials (medicina)
  7. Why Number Needed to Treat Can Be Misleading for Vaccines (Medscape)
  8. Fact Check: Comment In Lancet Does NOT 'Confirm' COVID-19 Vaccine Is Not 95% Effective (Lead Stories)
  9. COVID-19 vaccine efficacy and effectiveness—the elephant (not) in the room (Lancet)

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