- People with SARS-CoV-2, which is the virus that causes COVID-19, which shows no symptoms, can account for more than half of all transmission cases of the disease.
- A new study helps alleviate concerns that vaccinated people may still be vulnerable to symptom-free or “asymptomatic” COVID-19, and thus spread the virus to others.
- The study suggests that people with two doses of a messenger RNA vaccine (mRNA) are 80% less likely to develop COVID-19 asymptomatically than people who have not been vaccinated.
Clinical trials of approved COVID-19 vaccines, including mRNA vaccines, indicate that they occur in 70-95% of symptomatic cases.
However, there is some uncertainty as to whether the vaccines protect humans against ‘asymptomatic’ or asymptomatic, COVID-19.
This is an important question because researchers estimate that more than half of all SARS-CoV-2 transmission cases come from people who show no symptoms.
Therefore, governments and officials who plan the safest way to alleviate restrictions should know if vaccinated people can still experience asymptomatic infections.
A new study by researchers at the Mayo Clinic in Rochester, MN, now provides the assurance that the vaccines Pfizer-BioNTech and Modern mRNA offer a high degree of protection against asymptomatic infection.
Their analysis indicates that the risk of such an infection is 79% lower among those who received a first dose of one of the vaccines more than ten days earlier, compared to people who have not been vaccinated.
The researchers estimate that protection against asymptomatic infection is slightly better after a second dose of any vaccine, 80%, compared to people who have not yet been vaccinated.
The research now appears in the journal Clinical infectious diseases.
The scientists analyzed data from adults that were recorded in the Mayo Clinic Health System shortly before a medical or surgical procedure.
Prior to their procedures, all adults underwent routine screening for COVID-19 symptoms via a standardized telephone or electronic questionnaire. Upon arrival at the hospital or clinic, they again underwent screening and received polymerase chain reaction tests for SARS-CoV-2.
Between December 17, 2020 and February 8, 2021, 39,156 people who were apparently asymptomatic underwent a total of 48,333 tests for the virus.
The researchers chose December 17, 2020 as their start date because it was the first day of vaccination of COVID-19 in this health system.
Out of 3,006 tests in people vaccinated against COVID-19, 42 (1.4%) were positive. In contrast, 1436 (3.2%) were positive of 45,457 tests in people who were not vaccinated.
The researchers adjusted their findings to take into account other factors affecting the risk of COVID-19. They say that these factors include ‘age, gender and race / ethnicity’.
After making these adjustments, they found that people who had received a single dose of the vaccine more than ten days earlier were 79% less likely to test positive than people who had not been vaccinated.
People who received a second dose of one of the vaccines were 80% less likely to test positive than people who were not vaccinated.
Most vaccinated people (94%) received the Pfizer-BioNTech mRNA vaccine. The rest received the Moderna mRNA vaccine.
In their paper, the researchers conclude:
“We observed a significant decrease in asymptomatic infection, consistent with timing and extent observed with clinical trials to evaluate the prevention of symptomatic infection after vaccination with mRNA vaccine.”
The authors note some limitations in their study. For example, most 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 had mild, unrecorded symptoms of COVID-19, or that they developed symptoms after selection.
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