COVID Vaccine Study Previously Blocked By CDC Is Out — Here's What It Found

• A COVID-19 vaccine efficacy study that had previously been blocked by the CDC was published in a scientific journal.
• Acting CDC Director Jay Bhattacharya blocked the study from appearing in the CDC’s Morbidity and Mortality Weekly Report (MMWR), prompting months of criticism.
• The study affirmed that COVID-19 vaccines reduced the risk of hospitalization and severe illness among older adults, reinforcing the safety and effectiveness of vaccination against the disease.

A study on COVID-19 vaccines that was delayed and ultimately blocked from publication by the acting director of the Centers for Disease Control and Prevention (CDC) was published this week in a medical journal.

The study, originally slated for publication in an edition of the CDC’s Morbidity and Mortality Weekly Report (MMWR), concluded that COVID-19 vaccines reduced the likelihood of COVID-19-related hospitalization and severe illness among adults by about half.

The research was published on June 23 in JAMA Network Open. 

The study’s publication arrives months after the Washington Post first reported that it had been blocked in April. Acting CDC Director Jay Bhattacharya questioned the study’s methodology after it had already completed scientific review and received editorial approval. 

The controversy around the study has extended beyond the findings. Rather than centering on whether the updated vaccine provided protection, much of the debate has focused on the research method used to measure vaccine effectiveness.

However, experts say that nothing in the study, nor its design, was significant enough to warrant its being pulled.

“There are legitimate reasons to pause — a serious error, a data problem, conclusions that go further than the evidence supports,” said Robert Glatter, MD, attending physician in the Department of Emergency Medicine at Lenox Hill Hospital in New York City, and assistant professor of Emergency Medicine at Zucker School of Medicine at Hofstra/ Northwell. Glatter wasn’t involved in the study.

“But the fix for all of those is the open scientific process: revise, respond to reviewers, replicate. Quietly halting accepted research without a transparent scientific reason falls outside how regulators are supposed to operate,” he told Healthline.

Blocked COVID study reaffirms vaccine efficacy

The newly published study examined adults who sought medical care for COVID-19-like illness across hospitals and emergency departments in seven states.

Researchers found that adults who received the updated 2025-2026 COVID-19 vaccine were substantially less likely to require emergency, urgent, or inpatient care for COVID-19 than comparable adults who had not received that season’s vaccine.

It also addresses whether seasonal COVID-19 vaccines provide meaningful benefit after previous vaccinations and infections.

“It really reinforces the message that COVID is still with us, and the vaccine provides substantial, not perfect, but substantial protection against more serious illness, and most importantly, reduces your risk of hospitalization,” said William Schaffner, MD, professor of preventive medicine and infectious diseases in the Department of Health Policy at Vanderbilt University Medical Center. Schaffner wasn’t involved in the study.

Glatter said the study’s design makes the findings especially relevant because it reflects the reality that most adults already have some degree of immunity.

“The real message isn’t ‘the vaccine is 50% effective, period.’ It’s that even if you’ve had COVID in the near or distant past, and been vaccinated before, the current vaccine still measurably reduces your odds of getting sick enough to require medical attention,” he said.

Updated COVID vaccines still offer protection

The study analyzed data from the CDC-funded Virtual SARS-CoV-2, Influenza, and Other Respiratory Viruses (VISION) Network, which links electronic health records and vaccination records across seven states.

Researchers evaluated adults ages 18 years and older with healthy immune functioning who sought medical care for COVID-19-like illness between September and December 2025.

The study used a test-negative case-control design, comparing patients who tested positive for SARS-CoV-2, the virus that causes COVID-19, with people who sought care for similar respiratory symptoms but tested negative.

Among 85,725 emergency department and urgent care encounters, researchers identified 3,941 COVID-19 cases and 81,784 controls.

Among those who tested positive, 5% had received the updated vaccine, compared with 12% among those who tested negative. After adjusting for factors including age, sex, and ethnicity, the researchers estimated the updated vaccine reduced the risk of COVID-19-associated emergency department or urgent care visits by 50%.

The vaccine also reduced the risk of COVID-19-related hospitalization by an estimated 55%. 

In older adults (65 and older), who continue to face the greatest risk of severe COVID-19, estimated vaccine effectiveness was 48% against emergency department or urgent care visits and 53% against hospitalization.

The study’s findings support the benefit of COVID vaccines, even in those who have previously been exposed to the virus or vaccinated.

“The question people actually ask is: ‘I’ve had COVID twice, and I’m vaccinated — why bother?’ This study answers it with data rather than reassurance,” Glatter said.

Concerns over blocking vaccine safety studies

The publication of the COVID vaccine study followed months of scrutiny over why it was excluded from the CDC’s Morbidity and Mortality Weekly Report.

Acting CDC Director Jay Bhattacharya has been critical of test-negative study design, and explained his rationale in an op-ed for the Washington Post.

Bhattacharya has argued that longitudinal cohort studies — which follow vaccinated and unvaccinated groups over time — represent a stronger approach for evaluating vaccine effectiveness. 

“The CDC protects the public’s health by providing accurate, transparent, and trustworthy information,” Emily G. Hilliard, Press Secretary for the Department of Health and Human Services (HHS), said in a statement shared with Healthline.

“This requires applying the highest standards of scientific rigor, especially when findings may influence clinical decisions such as immunization. We engage in scientific discourse on how best to measure vaccine effectiveness across all CDC publications,” Hilliard continued.

The blocked study is the latest to emerge against concerns over vaccine research within federal health agencies.

In May, two COVID-19 vaccine studies and two studies evaluating the shingles vaccine Shingrix were reportedly blocked from publication by the Food and Drug Administration (FDA). 

Experts say recent developments have raised concerns about where legitimate public debate ends and the suppression of research begins. 

“The normal way to handle a study you disagree with is to critique it in the open: write a rebuttal, demand a correction, run your own study. Stopping a finished, peer-reviewed paper from ever seeing daylight is a different act entirely,” Glatter said.

Why is ‘test-negative’ so controversial?

Much of the controversy surrounding the study centered on the research method itself.

The test-negative design has been used for roughly two decades to evaluate seasonal vaccines against respiratory viruses such as influenza and COVID-19. 

Natalie Dean, PhD, associate professor of biostatistics and bioinformatics at the Rollins School of Public Health at Emory University, addresses the benefits and limitations of test-negative design in an accompanying editorial in JAMA Network Open. 

She described it as a “pragmatic solution to the tricky problem of estimating vaccine effectiveness.”

The design offers practical advantages because it compares patients with similar health care-seeking behavior rather than comparing people who may differ substantially in whether they seek medical attention.

Rather than following large groups of vaccinated and unvaccinated people over months or years, which is both expensive and impractical, researchers begin with people who have already sought medical care for similar symptoms.

Like any observational research method, however, it has limitations.

“It’s the standard tool health agencies use to track vaccines season to season. But it has real limits, and honesty requires acknowledging them. It can’t rule out every hidden factor. It can be thrown off if people’s vaccination records are wrong,” Glatter said.

For example, some patients may have sought emergency or hospital care for reasons unrelated to COVID-19 despite having COVID-like symptoms. Vaccination records could have been incomplete or incorrectly classified.

Dean told Healthline that criticizing study methodology is a completely normal part of epidemiologic research.

“These debates and discussions are happening all the time, as we’re always trying to figure out the best approach. What balances practicality, budget, accuracy, and timeliness?” she said.

But, she added, “It’s easy to criticize, but it’s harder to do. If people are going to critique the design, then they need to present a fully fleshed-out alternative.”

For Glatter, the larger concern is how public disputes between federal health agencies and scientific researchers might affect public confidence.

“The deeper issue is what suppression does to trust. Public confidence in vaccines isn’t a fixed thing — it erodes bit by bit, and it erodes fastest when people sense that information is being managed rather than shared,” he said.