Real World Effectiveness of Pfizer and Moderna Vaccines: Microbial Minutes

April 15, 2021

Do Pfizer and Moderna vaccines prevent SARS-CoV-2 infection? Can you transmit the virus after you’ve been vaccinated? How effective are mRNA vaccines in real world scenarios?

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Randomized, placebo-controlled Phase III clinical trials have demonstrated that mRNA vaccines are >90% effective at preventing symptomatic COVID-19, and are especially effective at preventing severe disease. But whether vaccination prevents SARS-CoV-2 infection in real world conditions has yet to be determined. 

In other words, we have known for awhile that COVID-19 vaccines are highly effective at preventing people from getting sick and requiring hospitalization or dying from COVID-19; but we have not known whether vaccination prevents the virus from infecting people altogether, or if it’s possible for people to still acquire asymptomatic SARS-CoV-2 infection and unknowingly spread the virus after being vaccinated. 
On March 29, 2021, the CDC’s Morbidity and Mortality Weekly Report published data on the subject. We dive into that research here.

How do mRNA COVID-19 vaccines perform in real world scenarios?

Thompson M.G. et al. Interim Estimates of Vaccine Effectiveness of BNT162b2 and mRNA-1273 COVID-19 Vaccines in Preventing SARS-CoV-2 Infection Among Health Care Personnel, First Responders, and Other Essential and Frontline Workers — Eight U.S. Locations, December 2020–March 2021. Morbidity and Mortality Weekly Report. March 29, 2021.

Take-Away Points

  • mRNA COVID-19 vaccines have demonstrated greater than 90% efficacy at preventing symptomatic COVID-19 in Phase III clinical trials. 
  • Data now indicate that mRNA vaccines are 80% effective at preventing SARS-CoV-2 infection 14 days or more after the first dose and 90% effective at preventing SARS-CoV-2 infection 14 days or more after the second dose of vaccine outside of clinical trials.
  • This data represents some of the first large-scale evidence that mRNA vaccines are effective at preventing SARS-CoV-2 infection in real situations where the risk of exposure may be higher than normal.
  • Reduced infection suggests that vaccination also limits viral transmission, which is important to controlling the spread of SARS-CoV-2.

Author: Ashley Hagen, M.S.

Ashley Hagen, M.S.
Ashley Hagen, M.S. is the Science Communications Specialist at the American Society for Microbiology.