Key Takeaway:
- A CDC-delayed Covid-19 vaccine study was published in JAMA Network Open and found the updated vaccine reduced Covid-related hospitalizations by 55% and ER/urgent care visits by 50%.
- Acting CDC Director Jay Bhattacharya blocked the study’s original publication, citing concerns about its test-negative research design and calling for stronger methodologies.
- Independent experts said the study’s limitations are well known but do not undermine its conclusions, supporting the continued use of the test-negative approach for vaccine research.
A CDC-blocked Covid vaccine study was published Tuesday in JAMA Network Open, finding vaccines reduced severe illness among adults by about half during the 2025-26 respiratory virus season.
The peer-reviewed CDC-blocked Covid vaccine study found that the updated COVID-19 vaccine lowered the risk of COVID-19-related emergency room or urgent care visits by 50% and reduced hospitalization risk by 55%. Researchers analyzed data from adults who sought medical care with Covid-like symptoms in seven states between September and December 2025.
The CDC-blocked Covid vaccine study had been scheduled for publication in March in the CDC’s Morbidity and Mortality Weekly Report, a leading agency publication. However, Acting CDC Director Jay Bhattacharya halted its release after raising concerns about the research design, according to current and former CDC employees cited by NBC News.
Study finds vaccines cut serious covid outcomes
Researchers used a test-negative design, a method that compares vaccination rates among people who test positive for a disease with those who test negative. The approach is widely used to evaluate vaccine performance in real-world settings.
The authors said the method provides a “convenient and efficient” way to assess vaccine effectiveness quickly. The study concluded that the updated vaccine provided meaningful protection against severe Covid outcomes during the fall and winter respiratory virus season.
The findings were published in JAMA Network Open, a peer-reviewed journal of the American Medical Association, after undergoing external scientific review.
CDC director questions research method
Bhattacharya argued that longitudinal cohort studies, which follow vaccinated and unvaccinated groups over time, provide stronger evidence than test-negative studies.
In a Washington Post opinion article, Bhattacharya wrote that the method excludes data from people who were never hospitalized and may be affected by factors such as prior infections.
“The vaccine effectiveness estimates this method yields could be an overestimate or an underestimate; it’s impossible to tell,” Bhattacharya wrote.
Emily Hilliard, a spokesperson for the Department of Health and Human Services, defended the agency’s review process.
“The CDC protects the public’s health by providing accurate, transparent, and trustworthy information,” Hilliard said in a statement. “Taking time to ensure analyses are methodologically sound and clearly communicated is always preferable to risking error.”
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Experts defend design despite limitations
Public health experts said the test-negative design has recognized limitations but remains a valuable tool for measuring vaccine effectiveness.
Natalie Dean, an associate professor of biostatistics and bioinformatics at Emory University’s Rollins School of Public Health, said researchers have understood the method’s weaknesses for decades and generally do not view them as severe enough to invalidate findings.
“It just feels funny that this has become the difference between whether or not something makes it out in MMWR,” Dean said.
Dean, who participated in the study’s peer-review process but was not involved in the research, said the quality of the paper remained unchanged after the CDC review. In an accompanying editorial, she described the test-negative design as “an important and practical approach” for evaluating vaccine effectiveness.
The study acknowledged potential limitations, including the possibility that people seeking hospital or urgent care treatment may be more likely to trust medical institutions and receive vaccinations than the broader population. Researchers also noted that participants who tested negative for COVID often had other respiratory illnesses.
The publication of the CDC-blocked Covid vaccine study comes amid continuing debate over how federal health agencies evaluate and communicate vaccine research, with the CDC recently hosting discussions on different methods for studying vaccine effectiveness.
