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Understanding Vaccine Efficacy

Updated: Jul 28, 2021

By Julia Garaffa

There is a lot of information and statistics circulating about the COVID-19 vaccines. We hear many questions from our network about which vaccine to choose and how to understand the flurry of efficacy rates and numbers in the media and online. In this article, we break down how vaccine efficacy is measured, the efficacy of each COVID-19 vaccine, and why getting vaccinated is so important.

Vaccine Efficacy Simplified

Vaccine efficacy is a measurement of how well a vaccine reduces the risk of getting sick. It is calculated by comparing the number of fully vaccinated people that become sick to the number of unvaccinated people (called control group) who become sick.

Another common term that is used is vaccine effectiveness. Vaccine effectiveness is measured the same way as vaccine efficacy. The difference is that efficacy is measured in a controlled setting, like a clinical trial, while effectiveness is measured in a real-world setting [1].

How well do the COVID-19 vaccines work?

There are currently three COVID-19 vaccines available in the US: Moderna, Pfizer-BioNtech, and Johnson & Johnson / Janssen. All three vaccines are safe and effective, but differ in their technology and administration. The Moderna and Pfizer-BioNtech vaccines are mRNA vaccines, which use a blueprint of a protein (called mRNA) to teach the body to defend itself against COVID-19. The Johnson & Johnson / Janssen vaccine is a viral vector vaccine, which uses a harmless virus different from the one that causes COVID-19 to teach the body to fight the COVID-19 virus. The Johnson & Johnson vaccine is a single dose while Moderna and Pfizer are 2 doses, given 28 days (Moderna) or 21 days (Pfizer) apart [2].

Here is the breakdown of efficacy rates among the three vaccines:

  • The Moderna vaccine’s clinical trials showed it to have 94.1% efficacy at reducing the risk of getting COVID-19 and 100% efficacy at preventing severe COVID-19 [3].

  • Pfizer’s clinical trials show it to have 95% efficacy against COVID-19 and 100% efficacy against severe COVID-19 [4].

  • Studies have found that in real-world healthcare settings, the effectiveness of Moderna and Pfizer is 91% [5].

  • Johnson & Johnson has 66% efficacy in preventing COVID-19 infection, and 85% efficacy at preventing severe COVID-19 [6].

  • All three vaccines were shown to be 100% effective in clinical trials at preventing hospitalization and death [3,4,6].

Does higher efficacy mean a better vaccine?

When considering how well a COVID-19 vaccine works, looking at efficacy alone can be misleading. Vaccines that have a lower total efficacy, such as Johnson & Johnson, are still highly effective at preventing severe illness, hospitalization, and death from COVID-19, which is the most important aspect to consider [7]. Getting vaccinated is similar to wearing a helmet while riding a bike. Those with helmets may still risk getting bruised if they crash, but serious head injuries are much less likely, just like how vaccinated people may still get mild COVID-19, but are protected from serious cases. Just like a helmet, any of the three vaccines is much more effective at keeping you safe than no vaccine.

Additionally, there are other factors to take into consideration when deciding which vaccine is best. Individuals who have known allergies to ingredients in one of the vaccines may opt for another [2]. Those who cannot travel to a vaccine clinic twice for a two-dose series may prefer the Johnson & Johnson single dose vaccine [7]. Pfizer is the only option currently approved for those 12 to 17 years old [2].


Overall, what is most important is getting vaccinated with whichever vaccine is available, especially for individuals with a substance use disorder who are at a higher risk of COVID-19 complications [8]. The more people that are vaccinated, regardless of which vaccine they receive, the better we can slow the spread of COVID-19 and reduce the number of life-threatening infections, which is a vital step in a return to normalcy.

If you would like help finding where you can get vaccinated or have any other questions, the Addiction Policy Forum’s cost-free Vaccine Navigator Initiative can help. Go to and fill out the form to get started.


  1. Center for Disease Control and Prevention. (2011). Lesson 3 - Section 6: Measures of Public Health Impact. Principles of Epidemiology in Public Health Practice, Third Edition: An Introduction to Applied Epidemiology and Biostatistics. Retrieved from

  2. Center for Disease Control and Prevention. (2021). Different COVID-19 Vaccines. Retrieved from

  3. Baden L.R, El Sahly H.M., Essink B., et al. (2020). “Efficacy and Safety of the mRNA-1273 SARS-CoV-2 Vaccine” New England Journal of Medicine, 384(5) 403-416.

  4. Polack, F.P., Thomas, S.J., and Kitchin, N., et al. (2020). “Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine” New England Journal of Medicine 383(27) 2603-2615.

  5. Center for Disease Control and Prevention. (2021). CDC COVID-19 Study Shows mRNA Vaccines Reduce Risk of Infection by 91 Percent for Fully Vaccinated People. Retrieved from

  6. Johnson & Johnson. (2021). Johnson & Johnson COVID-19 Vaccine Authorized by U.S. FDA For Emergency Use - First Single-Shot Vaccine in Fight Against Global Pandemic. Retrieved from

  7. University of Rochester Medical Center. (2021). Why the Johnson & Johnson vaccine is more effective than you think. Retrieved from

  8. Center for Disease Control and Prevention. (2021). COVID-19 and People Who Use Drugs or Have Substance Use Disorder. Retrieved from


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