What Does COVID-19 Vaccine Efficacy Mean?

Conceptual image of a gloved hand holding a syringe.

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Key Takeaways

  • The available COVID-19 vaccines are highly effective, and so far, especially effective at preventing hospitalizations and deaths, specifically.
  • A vaccine is still considered effective even if you get sick shortly after because the immune system takes time to develop antibodies.

Since the onset of vaccine distribution, you have likely heard that the Moderna and Pfizer-BioNTech COVID-19 vaccines have 94% and 95% efficacy rates. These percentages represent the vaccines' effectiveness.

Sanjeev Jain, MD, PhD, board-certified allergist and immunologist at Columbia Allergy, tells Verywell that vaccine effectiveness refers to the proportionate reduction in cases among participants that have been vaccinated during a clinical trial.

It's measured by calculating the risk of disease among vaccinated and unvaccinated people, and then determining how the disease reduction risk percentage compares among the two groups.

“The formula for this calculation is risk of contracting illness among unvaccinated (placebo) group minus the risk among vaccinated group, over the risk among unvaccinated groups,” Jain says. 

The greater the percentage reduction of illness in the vaccinated group, the greater the vaccine effectiveness. But what do these percentages mean for your safety from the virus? 

What This Means For You

The currently authorized COVID-19 vaccine efficacy rates are high and comparable to other vaccines, like the chickenpox vaccine. Even COVID-19 vaccines with lower efficacy rates are effective at preventing severe illness and transmission of the virus. When any COVID-19 vaccine becomes available to you, you should get it if you can.

Understanding Efficacy Rates

While the vaccines are not 100% perfect at preventing COVID-19, the Pfizer and Moderna vaccines have high efficacies that are similar to other vaccines, according to Jain. "For comparison, the two recommended doses of the varicella [chickenpox] vaccine are 88 to 98% effective in providing immunity against any form of varicella, and 95 to 100% effective in preventing severe varicella," Jain says.

According to Jain, it is extremely difficult for a vaccine to have 100% effectiveness due to a myriad of factors, including: 

  • Whether the full dosing schedule was received
  • Timing between doses
  • The ability of a person’s immune system to develop antibodies
  • Proper handling of the vaccine 

Some vaccines, such as the AstraZeneca COVID-19 vaccine (not yet authorized for use in the U.S.) only yield 62% efficacy. Despite this 30% difference, Jain says that vaccines—no matter their rate of effectiveness—slow the spread of the virus. "If you receive the vaccine and did not develop an immune response, no real harm is done," Jain says. "However, if you contract the virus or spread it to someone who is unable to fight it off, the harm can be significant. It is best to err on the side of caution and obtain the vaccine."

It's easy to get caught up in the effectiveness percentages. But these vaccines are incredibly effective at keeping people alive. All the percentages refer to disease protection. They are protective against severe disease and completely protective against hospitalizations and deaths. "When the virus enters the person's body, they will have the antibodies that prevent the virus from situating itself in the body and preventing symptoms or an illness," Jain says.

“Moderna reported that no cases of severe COVID-19 resulting in hospitalization or death were reported in the vaccinated group," Jain adds. "The Pfizer-BioNTech vaccine reported five hospitalizations in the placebo group and zero in the vaccine group."

If a person gets the COVID-19 vaccine, but still ends up getting sick, the vaccine still counts as effective, according to Jain. “If you contract COVID-19 after only a single dose of the vaccine or immediately after your second dose, this does not mean your vaccine was ineffective,” he says. 

The immune system takes time to develop antibodies after the vaccine which can take up to two weeks after the second dose. “So if you get sick before your immune system has enough time to respond to the vaccine, this does not mean that the vaccine was ineffective,” Jain says. 

Peter Gulick, MD, associate professor of medicine at Michigan State University, tells Verywell that everyone should get the vaccine in order to decrease overall levels of the virus. “Just get vaccinated because the more people that get vaccinated, the closer we'll get to herd immunity,” Gulick says. 

What Happens After Vaccination? 

Gulick explains that despite getting the vaccine, people may still transmit it to others. “Patients that get the vaccine may still be able to colonize," Gulick says. "They may have the virus up in their nose and it may not cause them disease where they feel symptoms." Because the disease might still be transmitted even after vaccination, Gulick recommends people continue wearing a mask, social distancing, and washing their hands regularly.

“If you have the opportunity to obtain your COVID-19 vaccine, ensure that you are able to receive both doses to allow for an adequate immune response to the vaccine,” Jain says. “In between doses, and for up to two weeks after your second dose, we recommend you continue to take the recommended precautions as if you were still unvaccinated since your immune system has not yet built an adequate immune response to protect against contracting the illness.” 

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. Principles of Epidemiology in Public Health Practice, Third Edition An Introduction to Applied Epidemiology and Biostatistics.

  2. AstraZeneca. AZD1222 vaccine met primary efficacy endpoint in preventing COVID-19.

  3. Food and Drug Administration. FDA briefing document. Pfizer-BioNTech COVID-19 vaccine.

By Kayla Hui, MPH
Kayla Hui, MPH is the health and wellness ecommerce writer at Verywell Health.She earned her master's degree in public health from the Boston University School of Public Health and BA from the University of Wisconsin-Madison.