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New research finds that mRNA vaccines reduce asymptomatic COVID-19 cases. FREDERIC J. BROWN/Getty Images
  • People with SARS-CoV-2, which is the virus that causes COVID-19, who show no symptoms may account for more than half of all transmission cases of the disease.
  • A new study helps alleviate concerns that people who have been vaccinated may still be vulnerable to symptom-free, or “asymptomatic,” COVID-19 and will, therefore, spread the virus to others.
  • The study suggests that people who have had two doses of a messenger RNA (mRNA) vaccine are 80% less likely to develop asymptomatic COVID-19 than people who have not been vaccinated.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date.

Clinical trials of approved COVID-19 vaccines, including mRNA vaccines, suggest that they prevent 70–95% of symptomatic cases.

However, there remains some uncertainty about whether or not the vaccines protect people against “asymptomatic,” or symptom-free, COVID-19.

This is a crucial question because researchers estimate that more than half of all transmission cases of SARS-CoV-2 are from people who are not showing any symptoms.

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Therefore, governments and officials who are planning the safest way to ease restrictions need to know whether or not people who have been vaccinated can still experience asymptomatic infections.

A new study by researchers at the Mayo Clinic in Rochester, MN, now offers reassurance that the Pfizer-BioNTech and Moderna mRNA vaccines provide a high degree of protection against asymptomatic infection.

Their analysis suggests that the risk of such an infection is 79% lower among those who received a first dose of either vaccine more than 10 days previously, compared with people who have not been vaccinated.

The researchers estimate that protection from asymptomatic infection is slightly better after a second dose of either vaccine, at 80%, compared with people who have not been vaccinated.

The research now appears in the journal Clinical Infectious Diseases.

The scientists analyzed data from adults that had been recorded shortly before a medical or surgical procedure in the Mayo Clinic Health System.

Before their procedures, all the adults underwent routine screening for symptoms of COVID-19 via a standardized phone or electronic questionnaire. On arrival at the hospital or clinic, they underwent screening once again and received polymerase chain reaction tests for SARS-CoV-2.

Between December 17, 2020, and February 8, 2021, 39,156 people who were apparently free of symptoms underwent a total of 48,333 tests for the virus.

The researchers chose December 17, 2020, as their start date because this was the first day of the COVID-19 vaccination rollout in this health system.

Out of 3,006 tests in people who had been vaccinated against COVID-19, 42 (1.4%) were positive. By contrast, out of 45,327 tests in people who had not been vaccinated, 1,436 (3.2%) were positive.

The researchers adjusted their findings to account for other factors that affect a person’s risk of COVID-19. They say that these factors included “age, sex, and race/ethnicity.”

After making these adjustments, they found that people who had received a single dose of either vaccine more than 10 days previously were 79% less likely to test positive than people who had not been vaccinated.

People who had received a second dose of either vaccine were 80% less likely to test positive than people who had not been vaccinated.

Most of the people who had been vaccinated (94%) received the Pfizer-BioNTech mRNA vaccine. The remainder received the Moderna mRNA vaccine.

In their paper, the researchers conclude:

“We observed a significant decrease in asymptomatic infection, consistent in timing and magnitude with what has been observed in clinical trials evaluating the prevention of symptomatic infection after vaccination with mRNA vaccine.”

The authors note some limitations to their study. For example, most of the participants were white, non-Hispanic, and under the age of 65 years, so the results may not be representative of other groups.

In addition, the researchers could not rule out the possibility that some of the participants had mild, unrecorded symptoms of COVID-19 or that they went on to develop symptoms after screening.

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