ROCHESTER, Minn. — New research shows if you get vaccinated against COVID-19, chances are good you can't get infected with the virus or spread it to others.

That's the takeaway of a large Mayo Clinic study published Thursday, March 11 in the journal Clinical Infectious Diseases.

The review of over 48,000 consecutive electronic tests within Mayo found the mRNA vaccines manufactured by Pfizer-BioNTech and Moderna are not only effective at preventing symptomatic illness, they greatly reduce the risk of contracting it in the first place. The vast majority of vaccine recipients studied (95%) received the shots made by Pfizer-BioNTech.

"We observed a really remarkable decrease in test positivity," said lead author and Mayo Clinic infectious disease specialist Dr. Aaron Tande. "After we accounted for some differences in age and other demographic features of the vaccinated and unvaccinated, we saw about 79% reduction (in risk of asymptomatic infection) 10 days after the first vaccine dose, and 80% reduction (in risk of asymptomatic infection) after a second dose."

“This is incredibly encouraging news," said Minnesota Department of Health Director for Infectious Disease Kris Ehresmann in a statement to Forum News Service. "The vaccines were shown to be efficacious in clinical trials against clinical disease and this study from Mayo provides encouraging information on the role of the vaccines (even a single dose) in preventing asymptomatic illness. We look forward to additional studies that support this same finding.”

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The results build on a Israeli finding published in the New England Journal of Medicine in late February showing that a first dose of the vaccine reduced the rate of infection by 46%, and a second dose by 92%. Early Thursday morning, Pfizer announced an updating of that data, showing 94% reduction of asymptomatic infection after two doses.

Though the Mayo finding supports a much higher level of effectiveness after one dose, Tande expressed caution about making comparisons between the two studies, given that they were different in size and design. He said that when it came to measuring effectiveness in preventing infection after one dose, the truth likely lay somewhere in between the two studies.

While the Mayo finding is lesser-strength observational data and had the limitation of having evaluated a homogenous sample largely made up of vaccinated health care workers and nursing home residents, the findings provide a measure of relief for public health officials now considering the question of whether each person vaccinated can safely be assumed to signify one less person capable of spreading the illness.

That question was not studied in the trials that gained the drugs an emergency use authorization. In an effort to speed the transformative medicines to market, those clinical trials only studied their ability to safely reduce the risk of moderate illness, with the assumption that this was a stand-in for serious illness, hospitalization and death.

Both manufacturers established excellent safety data and prevention of mild illness, but none knew if that meant the vaccines could also prevent the vaccinated from contracting and spreading the illness without developing symptoms.

"We just didn't know because the clinical trials didn't by and large assess for that," Tande says. "Certainly it's what everybody hoped, so it's nice to see in our study... that it does seem to decrease the likelihood of asymptomatic infection."

The data were collected from the start of vaccine availability on December 17 of 2020, until February 8 of 2021. The new study utilized COVID-19 screening records of nearly 40,000 asymptomatic patients seeking to obtain medical procedures at Mayo Clinic Midwest and in Arizona during this time, then compared COVID-19 prevalence among the vaccinated and unvaccinated.

Patients are routinely screened for asymptomatic or presymptomatic COVID-19 at Mayo upon seeking care for any services generating aerosols. Symptomatic patients would not have been scheduled for non-COVID-19 care.

There were over 3,000 vaccinated and over 45,000 unvaccinated patients during that period.

Upon analyzing this data, investigators learned that 3.2% of unvaccinated patients had asymptomatic COVID-19, compared to just 1.4% of all vaccinated individuals treated.

"The fact that we observed this decrease suggests from a public health standpoint, it will help to decrease those transmission events," Tande says, "and hopefully help us get control of this pandemic."

More than half of all transmission of the virus is believed to occur from individuals with no symptoms. The potential for asymptomatic spread of the illness by the vaccinated has been an ongoing concern as health officials weigh how quickly to lift mitigation measures as a reward for rising vaccination rates.

Gov. Tim Walz said on Thursday he plans on Friday to announce his biggest relaxing of measures yet.

The researchers adjusted their findings to account for variables like age, sex, race or ethnicity, proximity to the hospital, repeated screenings and region of residence.