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Two studies announced Thursday based on so-called real world data suggested that mRNA COVID-19 vaccines reduced the risk of developing asymptomatic infection.

One of these, from researchers at the Mayo Clinic, was published in a journal; the other was from Pfizer and BioNTech, described only in a press release with a promise of full publication at a later date.

And because neither was a randomized trial, they left plenty of room for questions and skepticism.

The first, appearing in , was a retrospective review of some 48,000 asymptomatic patients in Mayo Clinic facilities in Minnesota, Wisconsin, and Arizona who underwent COVID-19 testing prior to undergoing surgical procedures, including about 3,000 who had received at least one dose of either the Pfizer or Moderna mRNA vaccine.

As reported by Aaron Tande, MD, of the Mayo Clinic in Rochester, Minnesota, and colleagues, 3.2% of the unvaccinated patients tested positive, compared to 1.4% of those who had received a vaccine. After adjusting for certain confounding variables, they calculated the relative risk of asymptomatic infection was 0.35 (95% CI 0.26-0.47) with vaccination.

The study's primary outcome was relative risk of a positive SARS-CoV-2 molecular test. Patients seen in the Mayo system from Dec. 17 to Feb. 8 were considered eligible; those tested due to presence of symptoms or known exposure were excluded.

Many Limitations

The authors characterized their findings as a "real-world study," though the population was not representative of the general public: it took place during a time in which vaccination was limited mainly to healthcare workers and nursing home residents. Mean ages of the two groups -- 55 among those who had not been vaccinated and 47 for those who had -- suggest that many were, in fact, Mayo employees, and that the proportion was greater in the vaccinated cohort. Tande and colleagues did not provide a breakdown, however; their statistical adjustments were limited to age, sex, race/ethnicity, and proximity to Mayo facilities.

Moreover, the cohort was largely white, non-Hispanic, and under age 65, and patients were not followed to assess for subsequent development of symptoms.

Tande and colleagues argued that their population would be expected to have an increased risk of COVID-19 exposure, but did not take account of the countervailing effect of personal protective equipment commonly worn by healthcare workers, as well as their knowledge of risk wearing protective equipment.

In the end, they concluded, "the fact that lower positivity rates were seen in the vaccinated cohort supports a significant mitigating effect of vaccination."

Pfizer Cites Data from Israel

In a , Pfizer and BioNTech claimed vaccine effectiveness of 94% against asymptomatic SARS-CoV-2 infections.

The source was de-identified aggregate surveillance data from the Israeli Ministry of Health from January 17 to March 6. Vaccine effectiveness was measured from 2 weeks following the second dose.

While the 94% figure is impressive, the press release did not include many details, including confidence intervals, absolute numbers of individuals tested or infected, or criteria for performing tests.

"Detailed findings from this collaboration [with Israel's Ministry of Health] will... be submitted to a peer-reviewed journal," the release stated.

Previous real-world Israeli data were published in the New England Journal of Medicine with "documented asymptomatic infection" as the endpoint, which, the investigators acknowledged, may not be comprehensive. It was unclear whether the new data cited by Pfizer relied on the same endpoint.

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