Understanding efficacy levels of the covid-19 vaccines

Understanding efficacy levels of the covid-19 vaccines

The new COVID-19 vaccines have been produced at record speed due to unparalleled investment and global scientific collaboration.
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The new COVID-19 vaccines have been produced at record speed due to unparalleled investment and global scientific collaboration.3

  • The vaccines can prevent serious illness, hospitalization, or death after being infected by COVID-19.1
  • We know this because several clinical trials of approved COVID-19 vaccines found up to 100% efficacy in preventing severe symptoms, which could cause hospitalization and death from COVID-19 infection.1,2
  • People are considered vaccinated two weeks after the second vaccine dose - or the first dose for single-shot vaccines. The current advice is to continue social distancing and wearing a mask when with unvaccinated people.6


Clinical trials of the approved vaccines have found different efficacy rates – but what does that mean?1

Efficacy vs. effectiveness

Although they may sound similar, in medical research the two terms actually have specific meanings4:

  • Efficacy refers to what the COVID-19 vaccine can achieve in controlled conditions like a clinical trial where many factors that could affect results are
  • Effectiveness is what the intervention can achieve in real life where there are more factors that can have an impact on

Comparing the vaccines

Comparing the efficacy of one vaccine against another is not really possible, as the clinical trials were all set up slightly differently. The trials looked at severe symptoms that could lead to hospitalization and even death, but some also looked at mild symptoms or tracked COVID-19 through testing when there were no symptoms.1 Each vaccine was also studied in various populations against the COVID-19 variants circulating at that time.5

Determining a good level of efficacy

Vaccine efficacy is set against the disease they prevent. Due to COVID-19 being considered a serious disease, the threshold for efficacy was set at 50%. This means that on the balance of risk, any  COVID-19 vaccine with an efficacy of 50% or over is classed as worth taking.1

Choosing to wait for a preferred vaccine

COVID-19 vaccines are currently in limited supply, and are being rolled out according to prioritization.7 Everyone offered a vaccine has the opportunity not only to protect themselves but also those closest to them from serious illness, even death, and to help stop transmission in the wider community.

Waiting for a preferred vaccine would ultimately mean you and others being at risk for longer.

Another concern is that the more the virus passes through the population the more chance it has to create a vaccine-resistant mutation.7 Therefore getting as many people vaccinated as soon as possible is vital. Emerging research from vaccination rollouts in Israel and UK show vaccines are highly effective at preventing deaths and serious illness from COVID-19.9,10


Looking for more information?

Visit the World Health Organization or the Centers for Disease Control and Prevention to learn more.
If you have any questions about COVID-19 vaccinations, please contact your Cigna representative.


  1. What is vaccine efficacy? GAVI – The Vaccine https://www.gavi.org/vaccineswork/what-covid-19-vaccine-efficacy. Updated March 4, 2021. Ac- cessed March 9, 2021.
  2. Baden LR, El Sahly HM, Essink B, et Efficacy and Safety of the mRNA-1273 SARS-CoV-2 Vaccine. N Engl J Med 2021; 384:403-416. DOI:10.1056/NEJ- Moa2035389.
  3. Lurie N, Saville M, Hatchett R, Halton Developing Covid-19 Vaccines at Pandemic Speed. N Engl J Med 2020; 382:1969-1973. DOI:10.1056/NE-JMp2005630.
  4. Efficacy Effectiveness. New Health Advisor. https://www.newhealthadvisor.org/Efficacy-vs-Effectiveness.html. Accessed March 9, 2021.
  5. Why COVID vaccines are so difficult to Nature 591,16-17;2021. DOI:10.1038/d41586-021-00409-0.
  6. When You’ve Been Fully Vaccinated. https://www.cdc.gov/coronavirus/2019-ncov/vaccines/fully-vaccinated.html. Updated March 8, 2021. Accessed March 9, 2021
  7. Roadmap for prioritizing population groups for vaccines against COVID-19. World Health Organization. https://www.who.int/immunization/sage/meet- ings/2020/october/Session03_Roadmap_Prioritization_Covid-19_vaccine.pdf. Published September 27, 2020 - Accessed March 9, 2021.
  8. Can Covid-19 Vaccines Keep up with an Evolving Virus? eu. https://www.labiotech.eu/trends-news/emergex-covid-19-vaccine. Updated Febru- ary 11, 2021. Accessed March 9, 2021.
  9. COVID-19: analysing first vaccine effectiveness in the UK. UK Government. https://publichealthmatters.blog.gov.uk/2021/02/23/covid-19-analysing-first-vaccine-effectiveness-in-the-uk. Updated February 23, Accessed March 9, 2021.
  10. Dagan N, Barda N, Kepten E, et BNT162b2 mRNA Covid-19 Vaccine in a Nationwide Mass Vaccination Setting. N Engl J Med. 2021 Feb 24. DOI:10.1056/NEJMoa2101765.

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