Phase 3 studies of vaccines (called efficacy studies) are performed under highly controlled conditions. Study site clinicians and nurses know exactly how to store, handle and administer vaccines, which in the case of the COVID vaccines can be challenging. For this reason, it is important to also perform studies after a vaccine is licensed or approved (called effectiveness studies), to make sure that the vaccine performs the same way in the real world.

On March 29, 2021, the CDC published a study of 3,950 people who had received either one or two doses of the mRNA vaccines made by Pfizer and Moderna between December and March (Thompson MG., Burgess JL, Naleway AL, 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. MMWR Morb Mortal Wkly Rep. 2021 April 2;70(13):495–500.) Participants were followed with weekly PCR testing and monitoring of symptoms. Estimated mRNA vaccine effectiveness for prevention of infection (both asymptomatic and symptomatic) was 80% after one dose and 90% after two doses. The authors concluded, “These findings indicate that authorized mRNA COVID-19 vaccines are effective for preventing SARS-CoV-2 infection, regardless of symptom status, among working-age adults in real-world conditions.”

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

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