A study on Covid vaccines was published in JAMA Network Open on Tuesday. The study indicates that Covid vaccines reduced the likelihood of severe illness by approximately 50% among adults during the previous fall and winter.

The study found that the vaccine lowered the odds of a Covid-related visit to the emergency room or urgent care by 50%, and lowered the odds of hospitalization for Covid by 55%. The study examined adults who visited hospitals or urgent care facilities with symptoms consistent with Covid across seven states from September to December of the previous year. Researchers calculated the odds of participants having received the 2025-26 formulation of the Covid vaccine among those who tested positive and those who tested negative.

The study was originally scheduled for release in March in the CDC's Morbidity and Mortality Weekly Report (MMWR). CDC Acting Director Jay Bhattacharya raised concerns about the study's methodology after it had undergone scientific review and received approval from MMWR editors. His methodological concern involved the study's use of a test-negative design, which compares the vaccination status of individuals who test positive for a disease to the vaccination status of individuals who test negative. The authors of the newly published study state that a test-negative design is a "convenient and efficient method" to quickly evaluate vaccines in real-world settings.

Bhattacharya stated that he views longitudinal cohort studies as "a stronger design." He wrote in an op-ed that the test-negative design disregards data about people who were never hospitalized. He also expressed concern that factors such as prior infections could skew the results of the study. "The vaccine effectiveness estimates this method yields could be an overestimate or an underestimate; it's impossible to tell," Bhattacharya said. The CDC recently hosted a public debate on the strengths and limitations of various methods for studying vaccine effectiveness.

Emily Hilliard, a spokesperson for the Health and Human Services Department, issued a statement regarding the study's withdrawal. "The CDC protects the public's health by providing accurate, transparent, and trustworthy information. This requires applying the highest standards of scientific rigor, especially when findings may influence clinical decisions such as immunization," Hilliard said. She added, "Taking time to ensure analyses are methodologically sound and clearly communicated is always preferable to risking error."

Natalie Dean, an associate professor of biostatistics and bioinformatics at Emory University's Rollins School of Public Health, commented on the delay. Dean participated in the peer review process for the JAMA Network Open study but was not involved in the research. "It just feels funny that this has become the difference between whether or not something makes it out in MMWR," Dean said.