A study examining the effectiveness of the updated COVID-19 vaccine has been published in the peer-reviewed journal JAMA Network Open, following its blockage from a federal government report. The research, conducted by scientists from the CDC and several state health departments, focused on the 2025-26 vaccine's impact on adults without weakened immune systems during the fall and winter of 2025.
The study estimated that the updated vaccines reduced the risk of COVID-19 hospitalization by 55% and emergency department or urgent care visits by 50% compared to individuals who did not receive the shot. The authors noted that the findings did not account for prior COVID-19 infections or previous vaccinations, meaning the results may reflect the additional protection offered by the 2025-26 vaccine on top of existing immunity.
These findings align with previous research indicating that COVID-19 vaccines can reduce the risk of severe illness. The study was originally slated for inclusion in the Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report in March.
Concerns over the study's methodology, which employed a test-negative design, were raised by Dr. Jay Bhattacharya, acting Director of the CDC and leader at the National Institutes of Health, in an April op-ed for The Washington Post. The test-negative design is an observational method used to assess real-world vaccine effectiveness by comparing the vaccination status of patients with symptoms who test positive for a disease against those who test negative.
Authors of the study acknowledged limitations, including potential inaccuracies in hospital admission reasons, incomplete vaccine records, and reduced analytical power due to low vaccination and hospitalization numbers. A spokesperson for the Department of Health and Human Services stated that methodological questions are addressed directly, emphasizing that "Responsible science requires careful review. Taking time to ensure analyses are methodologically sound and clearly communicated is always preferable to risking error."
Despite these acknowledged limitations, some public health experts have defended the test-negative design, noting its utility in overcoming challenges such as the absence of a centralized healthcare database for monitoring vaccine effectiveness. Dr. Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia, stated that the test-negative design has been used for years to answer such questions. He suggested that critiques of the methodology should be detailed and potentially debated publicly rather than leading to the withholding of information.
The CDC operates under the purview of HHS Secretary Robert F. Kennedy Jr., who has previously expressed skepticism about COVID-19 vaccines. Health officials maintain that COVID-19 vaccines are safe and effective, citing extensive clinical trials and their role in saving millions of lives. Studies dating back to 2020 have consistently shown the vaccines' safety and effectiveness, particularly in preventing severe outcomes like hospitalization, ICU admission, and death.