- People with SARS-CoV-2, which is the virus that causes COVID-19, who have no symptoms can be responsible for more than half of all cases of transmission of the disease.
- A new study helps to alleviate the concern 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 received two doses of a messenger RNA (mRNA) vaccine are 80% less likely to develop asymptomatic COVID-19 than people who have not been vaccinated.
Clinical trials of approved COVID-19 vaccines, including mRNA vaccines, suggest that they prevent 70–95% of symptomatic cases.
However, some uncertainty remains about whether or not vaccines protect people from “asymptomatic” or symptom-free COVID-19.
This is a crucial question because researchers estimate that more than half of all cases of SARS-CoV-2 transmission are from people who have no symptoms.
Therefore, governments and officials who are planning the safest way to ease restrictions need to know whether people who have been vaccinated can still have asymptomatic infections.
A new study by researchers at the Mayo Clinic in Rochester, MN, now offers assurances that Pfizer-BioNTech and Moderna mRNA vaccines provide a high degree of protection against asymptomatic infections.
The analysis suggests that the risk of such an infection is 79% lower among those who received the first dose of any of the vaccines more than 10 days ago, compared to people who have not been vaccinated.
The researchers estimate that protection against asymptomatic infections is slightly better after a second dose of either vaccine, at 80%, compared to people who have not been vaccinated.
The research now appears in the newspaper Clinical Infectious Diseases.
The scientists analyzed data from adults that were recorded shortly before a medical or surgical procedure in the Mayo Clinic’s Health System.
Before the procedures, all adults underwent routine screening for symptoms of COVID-19 using a standard telephone or electronic questionnaire. Upon arrival at the hospital or clinic, they were screened once again and received polymerase chain reaction tests for SARS-CoV-2.
Between December 17, 2020 and February 8, 2021, 39,156 people apparently without symptoms were subjected to a total of 48,333 tests for the virus.
The researchers chose December 17, 2020 as the starting date as this is the first day of implementation of the COVID-19 vaccination in this health system.
Of the 3,006 tests on people vaccinated against COVID-19, 42 (1.4%) were positive. In contrast, of 45,327 tests on people who have not been vaccinated, 1,436 (3.2%) were positive.
The researchers adjusted their findings to take into account other factors that affect a person’s COVID-19 risk. They say these factors include “age, sex and race / ethnicity”.
After making these adjustments, they found that people who had received a single dose of any of the vaccines more than 10 days ago were 79% less likely to test positive than people who had not been vaccinated.
People who received a second dose of either vaccine were 80% less likely to test positive than people who were not vaccinated.
The majority of vaccinated people (94%) received the mRNA vaccine from Pfizer-BioNTech. The remainder received the Modern mRNA vaccine.
In their article, the researchers conclude:
“We observed a significant decrease in asymptomatic infection, consistent in time and magnitude with what was observed in clinical trials that evaluated the prevention of symptomatic infection after vaccination with the mRNA vaccine.”
The authors note some limitations in their study. For example, the majority of participants were white, non-Hispanic and under the age of 65, 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 might have mild, unrecorded symptoms of COVID-19 or that they would develop symptoms after screening.
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