A study into the effectiveness of the 2025-26 COVID-19 vaccine, blocked from publication earlier this year by political appointees within the Trump administration, has finally been released — revealing that the vaccine reduced the risk of hospitalisation by 55% among immunocompetent adults.
Effectiveness findings
The research, published Tuesday in JAMA Network Open, analysed data from more than 111,000 adults across 253 emergency department and urgent care facilities and 179 hospitals in seven US states between September and December 2025. It found the vaccine also cut the likelihood of visiting an emergency department or urgent care clinic by 50%. Among adults aged 65 and older, effectiveness stood at 48% against such visits and 53% against hospitalisation. The authors said the results show the updated vaccines provide added benefit regardless of previous vaccination or infection, and are consistent with existing research on vaccine efficacy.
“It is critical that we continue to characterise and publish estimates of vaccine effectiveness in populations with changing immunity against evolving viral strains,” wrote Natalie Dean, an associate professor of biostatistics and epidemiology at Emory University’s Rollins School of Public Health, in a commentary accompanying the study. Dean, whose work focuses on evaluating interventions against emerging infectious diseases, defended the methodology used in the study as a reliable, decades-old approach not typically considered controversial.
Other studies have previously shown that mRNA COVID-19 vaccines are highly effective against hospitalisation, though effectiveness wanes over time and a third dose substantially increases protection. One analysis found that unvaccinated adults with a prior COVID-19 infection were about five times more likely to be hospitalised than those who were vaccinated.
Controversy over publication
The study was originally slated for publication in March 2026 in the Centers for Disease Control and Prevention’s flagship journal, the Morbidity and Mortality Weekly Report. It had cleared the agency’s internal scientific review and been approved by its Office of Science. However, the acting director of the CDC at the time, Jay Bhattacharya, flagged the paper for review, effectively blocking its inclusion in the journal.
According to Althea Grant-Lenzy, the CDC’s chief science officer and a biochemist with extensive experience at the agency, Bhattacharya’s decision did not prohibit publication entirely but required the authors to address his concerns. She confirmed the authors retained the freedom to submit their work to external journals, which they eventually did.
Bhattacharya, who also serves as the director of the National Institutes of Health, has been a vocal critic of the study’s methodology, calling it “crap” and “logistically ridiculous.” He argued that the design relies too heavily on assumptions and could produce skewed results due to factors such as prior infections and varying patient behaviours. Political appointees within the Trump administration had previously prevented the study’s inclusion in the CDC publication, with some sources suggesting the decision may have been influenced by Health Secretary Robert F. Kennedy Jr.’s well-known scepticism toward vaccines. Kennedy has previously referred to the COVID-19 vaccine as “the deadliest vaccine ever made.”
Bhattacharya was also a co-author of the Great Barrington Declaration, an October 2020 letter that argued against widespread lockdowns and advocated for allowing the virus to spread among low-risk populations to build herd immunity — a strategy widely criticised by public health bodies including the World Health Organization. The Trump administration reportedly supported the declaration. Democratic leaders have since demanded answers regarding the suppression of the vaccine efficacy data, citing a pattern of the Trump administration obstructing the flow of public health information.
Methodology in the spotlight
At the heart of the dispute is an observational study design known as the “test-negative design.” The method works by examining individuals who seek medical care for respiratory symptoms. Those who test positive for the pathogen of interest — in this case, SARS-CoV-2 — are classified as “cases,” while those who test negative serve as “controls.” Researchers then compare vaccination status between the two groups to calculate the vaccine’s effectiveness.
The core assumption is that the vaccine primarily affects the likelihood of being a case (having the targeted illness) and not the risk of being a control, allowing for a valid comparison. Proponents argue the design is efficient, cost-effective, and helps mitigate biases common in other observational studies — particularly differences in healthcare-seeking behaviour between vaccinated and unvaccinated individuals. The CDC has used this methodology routinely for decades to monitor influenza vaccine effectiveness, and it has been employed in previous COVID-19 vaccine effectiveness studies. Peer-reviewed papers using the test-negative design have appeared in journals such as Pediatrics and The New England Journal of Medicine.
Critics, including Bhattacharya, counter that the test-negative design is too reliant on assumptions and can be skewed by unmeasured confounding — for instance, people who get vaccinated may also behave differently in ways that affect their infection risk. They also question whether prior infection should be accounted for differently. However, supporters of the method note that it is specifically structured to account for variations in who seeks care, and that prior infection is less of a concern given widespread exposure to the coronavirus among Americans. They acknowledge that no study design is flawless, but point out that officials at the US Department of Health and Human Services have not proposed a viable alternative for obtaining real-time vaccine-effectiveness estimates.
Earlier this month, the CDC hosted a forum to debate the merits and drawbacks of such studies. The panel included Dean and other experts who predominantly highlighted the methodology’s strengths. It also featured a prominent critic: Martin Kulldorff, a biostatistician and co-author of the Great Barrington Declaration. Kulldorff was appointed by US Health Secretary Robert F. Kennedy Jr. as head of a federal vaccine advisory committee before moving to the Office of the Assistant Secretary for Planning and Evaluation at HHS, where he now serves as chief science officer. At the forum, Kulldorff argued that test-negative studies should not include individuals with different diseases and questioned why longer-term studies were not used to evaluate COVID-19 vaccines. In response, one person in the audience retorted: “We were in a pandemic! That’s why!”