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COVID-19 mRNA Vaccines Greatly Reduce Risk of Asymptomatic Infection

Two doses of mRNA COVID-19 vaccine led to 80 percent risk reduction of testing positive for COVID-19

Mar 15, 2021
Mayo Clinic

ROCHESTER, MN—Ten days after receiving a second dose of a messenger RNA, or mRNA, vaccine for COVID-19, patients without COVID-19 symptoms are far less likely to test positive and unknowingly spread COVID-19, compared to patients who have not been vaccinated for COVID-19. The Pfizer-BioNTech and Moderna messenger RNA vaccines for COVID-19 are authorized for emergency use in the US.

With two doses of a messenger RNA COVID-19 vaccine, people with no symptoms showed an 80 percent lower adjusted risk of testing positive for COVID-19 after their last dose. Those are the findings of a Mayo Clinic study of vaccinated patients. These finding appear in the journal Clinical Infectious Diseases.

The authors say these findings underscore the efficacy of messenger RNA vaccines for COVID-19 to significantly limit the spread of COVID-19 by people with no symptoms who may unknowingly spread the infection to others.

The researchers retrospectively looked at a cohort of 39,000 patients who underwent pre-procedural molecular screening tests for COVID-19. More than 48,000 screening tests were performed, including 3,000 screening tests on patients who had received at least one dose of a messenger RNA COVID-19 vaccine. These screening tests were part of routine COVID-19 testing prior to treatments not related to COVID-19, such as surgeries and other procedures. Patients in the vaccinated group had received at least one dose of a messenger RNA COVID-19 vaccine.


Related Article: Allergic Reactions to COVID-19 mRNA Vaccines Are Rare 


"We found that those patients without symptoms receiving at least one dose of the first authorized mRNA COVID-19 vaccine, Pfizer-BioNTech, 10 days or more prior to screening were 72 percent less likely to test positive," says Aaron Tande, MD, a Mayo Clinic infectious diseases specialist and co-first author of the paper. "Those receiving two doses were 73 percent less likely, compared to the unvaccinated group."

After adjusting for a range of factors, researchers found an 80 percent risk reduction of testing positive for COVID-19 among those with two doses of a messenger RNA COVID-19 vaccine.

The study was based on patients receiving screening tests between December 17, 2020, and February 8, 2021, at Mayo Clinic in Minnesota and Arizona, and at Mayo Clinic Health System in Minnesota and Wisconsin.

- This press release was originally published on the Mayo Clinic website