More than 300 UI Health Care employees participated in a large observational study of U.S. health care workers that found that both FDA-authorized mRNA-based COVID-19 vaccines (made by Pfizer-BioNTech and Moderna) are highly effective at preventing symptomatic COVID-19 in real-world clinical settings.
Moreover, the vaccines appeared to work just as well for older individuals (over age 50); people from racial or ethnic groups that have been disproportionately affected by COVID-19; people with underlying medical conditions; and people who had higher levels of exposure to patients with COVID-19. Vaccine effectiveness was lower in immunocompromised people.
The study, which was published Sept. 22 in the New England Journal of Medicine, found that the effectiveness of a two-dose regimen was 89% for the Pfizer-BioNTech vaccine and 96% for the Moderna vaccine.
“Our effectiveness findings are in line with what was shown in the clinical trials for these vaccines. But the value of our study is that it shows how well these vaccines work in the real world, rather than in a clinical trial setting,” says Nick Mohr, MD, MS, UI professor of emergency medicine, anesthesia critical care, and epidemiology, and co-principal investigator of the PReventing Emerging Infections through Vaccine EffectiveNess Testing (Project PREVENT) study.
Measuring vaccine effectiveness in health care workers
Project PREVENT is a CDC-funded study led by Mohr and David Talan, MD, professor of emergency medicine and infectious diseases at the The study aims to evaluate COVID-19 vaccine effectiveness. Tamara Pilishvili, PhD, at the CDC, was lead author of the new study, which collected data for about 5,000 health care workers who were tested for COVID-19 between December 2020 and May 2021.
The study made use of employee testing protocols for COVID-19 at 33 participating medical centers across the U.S. to identify and enroll 1482 cases and 3449 controls. Participants who were symptomatic and tested positive for COVID-19 (cases) were compared to people with COVID-like symptoms but who tested negative for COVID-19 (controls). All study participants completed a survey that provided demographic information, risk factors for severe disease from COVID-19, and job type, as well as their vaccination status.
The study showed that the effectiveness of complete vaccination (seven or more days after the second dose) was high for both vaccines, but slightly better for Moderna (96%) than for Pfizer-BioNTech (89%). Vaccine effectiveness for partial vaccination (14 days after the first dose through six days after the second dose) was 77.6% for Pfizer-BioNTech and 88.9% for Moderna.
Because of the relatively short time period of the study, the findings don’t provide an answer to how long vaccines continue to provide protection against COVID-19. In addition, the study data were collected before the emergence of the delta variant, so vaccine effectiveness may have changed compared to effectiveness against earlier variants.
“We will continue to study how the vaccines perform as the pandemic continues to evolve,” Mohr says. “Keeping track of changes in vaccine effectiveness and duration of protection is critical to our ability to protect health care workers and keep our communities safer.”