(HealthDay)—Healthy, unvaccinated adults who get COVID are unlikely to be symptom-free, according to a new study that challenges other research suggesting that asymptomatic infection is common.

“Some studies suggest that asymptomatic infection may occur as often as 50% of the time,” but the new research casts doubt on that assertion, said senior author Dr. Edward Mitre. He’s a professor of microbiology and immunology at Uniformed Services University of the Health Sciences (USUHS) in Bethesda, Md.

The new study included 263 uninfected, unvaccinated health care workers (average age: 41) at the Walter Reed National Military Medical Center in Bethesda. They were generally healthy and had normal immune systems.

Between August 2020 and February 2021, the participants took PCR tests whenever they had symptoms. They also underwent monthly antibody testing to detect any cases of COVID that were symptom-free or missed by PCR testing.

The participants were also asked to self-report any symptoms.

In all, 12 participants tested positive for SARS-CoV-2 infection and all had symptoms, according to findings published Feb. 14 in Open Forum Infectious Diseases.

If asymptomatic infection occurs as often as 50% of the time as earlier studies have suggested, Mitre said, it would have been “very unlikely” to have seen symptoms in all 12 of those who were infected in this study.

“If we compare this to flipping a coin, the likelihood that one flips tails 12 times in a row is only 0.024%,” he said in a university news release. “Even if the true rate of asymptomatic infection is 30%, then the likelihood that 12 of 12 individuals would all be symptomatic is still only 1.4%.”

Mitre noted that this study was conducted on an unvaccinated population and may not reflect rates of asymptomatic infection in vaccinated individuals.

Lead author Emilie Goguet, also from USUHS, said researchers suspect they saw a higher rate of symptomatic infection because study participants were paying close attention to signs of infection.

She said the study’s design probably also helped. Throughout the fall and winter, participants reported symptoms any day they experienced a departure from their baseline health.

As part of the study, the researchers also compared symptoms in the 12 people who were diagnosed with COVID to those in 38 participants who developed non-COVID-19 respiratory illnesses.

Runny nose, sinus pressure and sore throat occurred in more than 70% of both groups. Both also had similar rates of loss of smell or taste.

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