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CDC Acting Director Delays, Then Blocks Publication of COVID-19 Vaccine Effectiveness Study; Study Later Published in External Journal

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A Study Delayed: Inside the CDC's Scientific Standoff Over COVID-19 Vaccine Effectiveness

A study on the effectiveness of the 2025-26 COVID-19 vaccine—initially delayed and subsequently blocked from publication by the CDC's acting director—was later published in an independent peer-reviewed journal. The incident has highlighted a methodological debate within federal health agencies and raised concerns about political influence over scientific processes.

Study Findings

The research, conducted by scientists from the CDC and multiple states, found that the 2025-26 COVID-19 vaccine formulation significantly reduced the likelihood of severe illness among adults.

  • Vaccine Effectiveness: A preliminary summary of the study's results indicated that the vaccine reduced the likelihood of severe illness by approximately half among adults.
  • Specific Outcomes (unpublished data from Washington Post): According to reporting by the Washington Post, the study found that, between September and December 2025, vaccinated healthy adults had a 55% lower likelihood of COVID-19-related hospitalization and a 50% lower likelihood of emergency or urgent care visits compared to unvaccinated individuals.
  • Published Results: The study was later published in JAMA Network Open, estimating the vaccine reduced the risk of COVID-19 hospitalization by 55% and emergency department or urgent care visits by 50% among adults 18+ without weakened immune systems.

Bottom Line: The vaccine reduced the risk of severe outcomes by approximately half—a finding consistent across both preliminary and final analyses.

Publication Timeline and Delays

The study's path to publication was marked by a series of unexpected delays and a dramatic change of venue.

  • Original Plan: The study was scheduled for publication in the CDC's flagship scientific journal, the Morbidity and Mortality Weekly Report (MMWR), on March 19.
  • Initial Delay: CDC Acting Director Dr. Jay Bhattacharya delayed the publication, citing concerns about the study's methodology.
  • Cancellation: After a delay of several weeks, the publication of the study in the MMWR was canceled entirely.
  • External Publication: The study was subsequently published in JAMA Network Open, an independent, peer-reviewed medical journal.

Methodology and Rationale for Delay

The central point of contention was the study's methodology—a debate that has divided scientists and raised questions about scientific independence.

Methodology Used:
The study employed a "test-negative design." This observational method compares the vaccination status of people who test positive for a disease (e.g., COVID-19) to those who test negative for the same disease, among individuals seeking medical care for similar symptoms.

Commonality of the Design:
Current and former CDC employees, as well as other public health experts, stated that the test-negative design is a common, well-established, and widely accepted method for evaluating the real-world effectiveness of vaccines against respiratory viruses. It has been used for decades in studies on influenza and COVID-19 vaccines. A separate MMWR report on flu vaccine effectiveness using the same methodology was published the week prior to the delayed study.

Concerns Raised:
Dr. Bhattacharya expressed concerns about the observational method used to calculate vaccine effectiveness. In an op-ed, he argued that the test-negative design "disregards data on people never hospitalized" and that prior infections could skew results, making it difficult to determine if estimates are over- or under-estimates. He stated a preference for longitudinal cohort studies, which track vaccinated and unvaccinated individuals over time.

Expert Rebuttals:
Other experts, including biostatisticians and former CDC officials, defended the test-negative design. They noted that it is an important and practical approach for real-time vaccine effectiveness estimates, and that alternative methods like cohort studies or randomized controlled trials can be more expensive, less timely, or pose ethical concerns for already-approved vaccines. They also stated that the design is intended to account for differences in care-seeking behavior.

Core Disagreement: Dr. Bhattacharya argued the method was flawed; career scientists countered that it is the gold standard for rapid, real-world vaccine monitoring.

Official Statements and Reactions

Various officials and organizations provided statements regarding the delay and subsequent cancellation.

  • HHS Spokesperson Andrew Nixon: Stated that it is routine for CDC leadership to review MMWR papers before publication. He said Dr. Bhattacharya "expressed concerns about the observational method used in this study to calculate vaccine effectiveness" and wanted to ensure the paper used the most appropriate methodology. Nixon added that "taking time to ensure analyses are methodologically sound and clearly communicated is always preferable to risking error."

  • HHS Spokesperson Emily Hilliard: After the study was blocked, she stated, "The CDC protects the public's health by providing accurate, transparent, and trustworthy information. This requires applying the highest standards of scientific rigor."

  • Former CDC Medical Epidemiologist Fiona Havers: Described the delay as "a new level of political interference into CDC's scientific process," noting the methodology is well-established.

  • U.S. Senator Dick Durbin (D-Ill.): Expressed concern, stating that "health care professionals rely on the MMWR for timely, objective and fact-based information."

  • Medical Experts: Some public health experts viewed the withdrawal of the study as political interference, given that HHS Secretary Robert F. Kennedy Jr. has a history of anti-vaccine activism.

Key Figures and Background

  • Dr. Jay Bhattacharya: A political appointee serving as the acting director of the CDC since February. During the 2020 pandemic, he opposed lockdowns and mask mandates.
  • Robert F. Kennedy Jr.: The Health and Human Services Secretary who oversees the CDC. He has a history of anti-vaccine activism and has publicly criticized COVID-19 vaccines.
  • Previous MMWR Pauses: Publication of the MMWR journal has been paused previously, including in January 2025 during a temporary communication freeze and during a government shutdown.