Delay in CDC Vaccine Effectiveness Study Release
Last month, a CDC study was anticipated to demonstrate that Covid vaccines can significantly lower the risk of severe illness. However, Dr. Jay Bhattacharya, the acting director of the agency, postponed its release, citing concerns regarding the study’s methodology.
This study was supposed to be featured in the CDC’s primary scientific journal, the Morbidity and Mortality Weekly Report (MMWR). Andrew Nixon, a spokesperson for the Department of Health and Human Services, indicated in an email that Bhattacharya “expressed concerns about the observational method used in this study to calculate vaccine effectiveness.”
Nixon elaborated, noting that it’s standard for CDC leadership to evaluate potential issues related to the methodology of MMWR papers before publication. “Dr. Bhattacharya wants to ensure that the most appropriate methodology is being used,” he stated.
In recent times, high-ranking officials in the Trump administration have openly criticized traditional methods of vaccine research. This criticism has led some public health experts to see it as part of a larger campaign aimed at fostering distrust in vaccine safety and access.
While the Trump administration has pledged to restore “gold-standard science” to health agencies, some argue this is more about discrediting studies that don’t validate the administration’s stance on vaccines. Health Secretary Robert F. Kennedy Jr., who oversees these agencies, has a noted history of promoting anti-vaccine views.
Bhattacharya, appointed in February to lead the CDC until a permanent director is named, has not responded to inquiries regarding the matter.
A preliminary summary of the study, reviewed by NBC News, revealed that the Covid vaccine formulation for 2025-26 appeared to reduce severe illness risk by around half for adults.
An epidemiologist from the CDC’s National Center for Immunization and Respiratory Diseases—who requested anonymity for job security reasons—stated the paper was set to be released on March 19.
The Washington Post first reported the delay in publication.
Both current and former CDC employees remarked that it’s quite unusual for a political appointee, like Bhattacharya, to halt an MMWR paper that had already passed scientific review and received editorial approval.
Although there have been instances in the past where MMWR publication was paused under the Trump administration, it is rare for a specific paper to be withdrawn entirely. The CDC previously delayed certain MMWR papers in January 2025 during a temporary communication freeze and again in the fall amid a government shutdown.
Fiona Havers, a former medical epidemiologist at the CDC, noted that stopping this report seems to represent a new level of political interference in the agency’s scientific processes. She resigned from the CDC in protest in June after Kennedy appointed a new vaccine advisory committee, which is seen as skeptical towards vaccines.
The issue with the Covid paper centered around a methodology known as test-negative design. According to insiders, this approach involves comparing the vaccination status of individuals who tested positive for Covid against those who tested negative, indicating that vaccinated individuals had lower hospitalization rates. Attempts to reach several editors and authors of the paper for comments were unsuccessful.
Current and former employees asserted that this method is commonly used to assess vaccine effectiveness, with many previous MMWR papers employing the same framework. An official from HHS indicated that Bhattacharya had not taken up his role in time to review an earlier flu vaccine paper but would have raised similar concerns.
Alternative methods often require larger budgets and extended timelines to yield results. For example, a cohort study, which tracks vaccinated and unvaccinated individuals over time, is much more expensive and time-consuming. Some federal officials have called for more randomized controlled trials, but ethical dilemmas arise, particularly in withholding approved vaccines from trial participants.
Havers remarked, “It’s not feasible to conduct randomized controlled trials every year once a vaccine has been licensed.”
The CDC epidemiologist mentioned that staff presented the test-negative design methodology at Bhattacharya’s request in March but that no decision has been made regarding the paper’s publication status.
If it doesn’t get published in the MMWR, its authors might still submit it to an independent medical journal.
Nixon asserted that the CDC is actively engaged in scientific discussions on measuring vaccine effectiveness. He emphasized the importance of ensuring that analyses are sound and clearly communicated, rather than risking errors through hurried publication.
He concluded by affirming the agency’s commitment to transparency and timely publication of findings.
