Patients without COVID-19 symptoms are far less likely to test positive, and unknowingly spread COVID-19 ten days after receiving a second dose of a messenger RNA vaccine, compared to those who have not been vaccinated, a study says.

Researchers from Mayo Clinic in the US noted that several vaccines are now clinically available under emergency use authorisation and have demonstrated efficacy against symptomatic COVID-19.

However, the impact of vaccines on asymptomatic SARS-CoV-2 infection is largely unknown, they said.
The Pfizer-BioNTech and Moderna messenger RNA (mRNA) vaccines for COVID-19 are authorised for emergency use in many countries.

An mRNA vaccine is based on the segments of the genetic material, or RNA, of the novel coronavirus. With two doses of a mRNA COVID-19 vaccine, people with no symptoms showed an 80 per cent lower adjusted risk of testing positive for the disease after their last dose, according to the researchers.

The findings, published in the journal Clinical Infectious Diseases, underscore the efficacy of mRNA vaccines for COVID-19 to significantly limit the spread of COVID-19 by people with no symptoms who may unknowingly spread the infection to others.

“We found that those patients without symptoms receiving at least one dose of the first authorised mRNA COVID-19 vaccine, Pfizer-BioNTech, 10 days or more prior to screening were 72 per cent less likely to test positive,” said Aaron Tande, a Mayo Clinic infectious diseases specialist and co-first author of the paper.

“Those receiving two doses were 73 per cent less likely, compared to the unvaccinated group,” Tande said.
After adjusting for a range of factors, researchers found an 80 per cent risk reduction of testing positive for COVID-19 among those with two doses of an mRNA COVID-19 vaccine.

The researchers retrospectively looked at a cohort of 39,000 patients who underwent pre-procedural molecular screening tests for COVID-19.

Over 48,000 screening tests were performed, including 3,000 on patients who had received at least one dose of an mRNA COVID-19 vaccine, they said.

According to the researchers, these screening tests were part of routine COVID-19 testing prior to treatments not related to COVID-19, such as surgeries and other procedures. Patients in the vaccinated group had received at least one dose of a messenger RNA COVID-19 vaccine, they added.