Preliminary results from an ongoing multisite case-control study of healthcare personnel (HCP) in 25 states indicate the two mRNA COVID-19 vaccines are 94% effective in real-world conditions involving work and the community, the CDC reported.1 The study is underway at 33 sites, with 75% of enrolled healthcare workers employed at acute care hospitals.

“This study adds to the growing body of evidence that confirms mRNA COVID-19 vaccines’ real-world effectiveness,” says lead investigator Tamara Pilishvili, PhD, an epidemiologist at the CDC National Center for Immunization and Respiratory Diseases. “Our study evaluated effectiveness against symptomatic illness among healthcare personnel who are at increased risk for exposure to SARS-CoV-2 both at the workplace and in the community.”

The CDC study revealed “a single dose of Pfizer-BioNTech or Moderna COVID-19 vaccines to be 82% effective against symptomatic COVID-19, and two doses to be 94% effective.”

Case-patients and control participants were identified through routine employee testing performed based on site-specific occupational health practices.

“HCP with a positive SARS-CoV-2 polymerase chain reaction (PCR) or antigen-based test result and at least one COVID-19-like illness symptom were enrolled as case-patients, and HCP with a negative SARS-CoV-2 PCR test result, regardless of symptoms, were eligible for enrollment as controls.”

Vaccination records, including dates and type of COVID-19 vaccine received, were obtained from occupational health at the sites. “As of March 18, 2021, 623 case-patients and 1,220 controls had been enrolled,” the CDC reported. “The median ages of case-patients and controls were 38 years (range = 19-69 years) and 37 years (range = 19-76 years), respectively. The majority of HCP (60% of case-patients and 64% of controls) were women who worked in occupational categories with substantial anticipated direct patient contact.”

Some of these variables, such as the predominantly white and relatively young age of healthcare workers, may limit extrapolating the vaccine efficacy reported to the general population, the CDC said.

Hospital Employee Health asked Pilishvili for further comment in the following interview, which has been edited for length and clarity.

HEH: COVID-19 symptomatic illness was reduced by 94% among those vaccinated, according to the assessment. Does that mean the “breakthrough” infection rate is 6%? Can you elaborate on this issue of immunization and breakthrough infections?

Pilishvili: Effectiveness of 94% can be interpreted in simplified terms as a 94% reduction in risk of symptomatic illness among those vaccinated with two doses of mRNA vaccine compared to risk among unvaccinated. The rate of breakthrough infection is related to vaccine effectiveness but could not be estimated using this study design. Breakthrough infection (defined in our study as symptomatic HCP testing positive for SARS-CoV-2 more than seven days after the receipt of the second dose of the vaccine) were rare. During the study period (January through March 2021), we detected 19 breakthrough infections among healthcare personnel out of a total of 623 case-patients enrolled.

HEH: What symptoms did vaccinated HCP experience?

Pilishvili: Nineteen infections among healthcare personnel who received two doses of mRNA vaccine were detected. None of these case-patients were hospitalized for their illness. A smaller proportion of fully vaccinated cases compared to partially vaccinated or unvaccinated case-patients experienced fever, cough, shortness of breath, and other symptoms of COVID-like illness (e.g., 11% reported fever compared to 40% of all case patients; 21% had cough compared to 56% of all case-patients, and 11% had shortness of breath, compared to 26% of all case-patients).

HEH: Do you think this proof of high vaccine efficacy can be used to increase immunization rate among HCP who have declined COVID-19 shots?

Pilishvili: The results of this study add to the growing body of evidence demonstrating the effectiveness of mRNA vaccines in a real-world setting and should help reassure the public on the benefits of vaccination. We are encouraged by the results and hope they help improve vaccination rates among healthcare personnel and general population.


  1. Pilishvili T, Fleming-Dutra KE, Farrar JL, et al. Interim estimates of vaccine effectiveness of Pfizer-BioNTech and Moderna COVID-19 vaccines among health care personnel — 33 U.S. sites, January-March 2021. MMWR Morb Mortal Wkly Rep 2021;70: 753-758.