At the start of an unprecedented meeting of vaccine committee of the US Centers for Disease Control and Prevention, the agency scientists who normally generate and interpret data to inform this group’s votes were in an unusual position: answering questions and countering statements that cast doubt on the safety and efficacy of approved vaccines.
In Wednesday’s meeting, the agency experts answered polite but atypical queries about Covid-19 illnesses and vaccines, many focused on whether data could have been biased or misinterpreted, and whether patients counted as going to the hospital for Covid-19, might actually have had mild infections, but really been hospitalized for something else.
Thursday’s agenda for the Advisory Committee on Immunization Practices includes two planned presentations on topics that have been fodder for vaccine skeptics for decades: one on a form of mercury used as a preservative in vaccines and another on the risks of rare febrile seizures after of some kinds of combination measles vaccines.
The presentations will not be given by CDC staffers or members of working groups that have met for months to discuss the topics, but by a newly appointed chair of the committee and a former leader of the anti-vaccine organization Children’s Health Defense.
On Tuesday, CDC experts posted evidence reviews, labeled as background briefing material, on both topics online. By Wednesday, one of the documents – on the vaccine preservative thimerosal – had been taken down because it had not gone through the “appropriate process to be posted,” a US Department of Health and Human Services spokesperson said.
Agency scientists often pull together available research in advance of these meetings to help brief ACIP members, and it’s common for presentation slides to be posted ahead of time. But the posting of the CDC scientists’ detailed briefing documents — and with no authors identified —was unusual, according to a former CDC official with knowledge of the group’s processes.
Officials said it was a quiet sign of distress that career scientists are feeling as US Health and Human Services Secretary Robert F. Kennedy Jr. seeks to remake the process that informs vaccine recommendations.
“I find it very interesting that these documents were posted,” said the former CDC official, who asked not to be named for fear of retaliation.
A different orientation around vaccines
It was clear from the questions asked by the new committee members on Wednesday that some are distrustful of evidence showing the safety and effectiveness of Covid-19 vaccines.
Following an announcement last month by HHS Secretary Robert F. Kennedy Jr. that Covid-19 vaccines were no longer going to be recommended for healthy children and pregnant women, the CDC showed data noting the continuing high rates of hospitalization for children younger than age 2.
The CDC noted that hospitalization rates for Covid-19 in babies and toddlers were roughly the same as for older adults last year.
Committee member Dr. Cody Meissner, a professor of pediatrics at Dartmouth University, pointed out that based on the CDC’s latest data, roughly 1.6 of every 100,000 babies younger than 6 months were hospitalized for their infections during the week ending May 31.
“This is a very rare illness in young children as well as in adults now,” Meissner said.
Dr. Christopher Taylor, an expert on the CDC’s respiratory disease data, responded that although the data on its website shows the current rates by week, the scientists used cumulative numbers over a year for their presentation.
In absolute terms, since July 2024, CDC data shows that 268 of every 100,000 babies under 6 months old were hospitalized for Covid-19; for adults ages 65 through 74, the same rate was 266 for every 100,000.
Children ages 6 to 23 months were hospitalized at rates on par with adults ages 50 to 64, about 100 for every 100,000.
“In cumulative numbers, this does remain a substantial burden for the youngest and oldest age groups,” Taylor said.
Healthy children can still receive Covid vaccines after consultation with a health care provider, a contingency known as shared clinical decision making. Though the recommendations for pregnant women has been dropped from the vaccine schedule, pregnancy remains a condition that puts people at higher risk of severe illness from the infection.
Meissner also asked whether Covid-19 hospitalizations reported by the CDC represented people who were severely ill from Covid-19 infections or whether those patients might be in the hospital for other reasons and happened to test positive for Covid-19 while they were there.
“People may have been hospitalized because of appendicitis, for example, and happen to have a positive [test] result, because we know that asymptomatic colonization is quite common with this virus,” he said.
But Taylor said that while it’s true that the definition the researchers used depended on a lab-confirmed Covid-19 infection, their clinical data goes through another layer of scrutiny to see whether patients had symptoms consistent with Covid-19 or received medication consistent with a Covid infection.
Taylor added that early in the pandemic, many hospitals required all patients to be screened for Covid-19 during their stay — a practice that picked up infected people who initially came to the hospital for other reasons. Since then, however, most hospitals have dropped that practice.
Committee member Dr. Retsef Levi, a professor of operations management at Massachusetts Institute of Technology, asked about potential confounding in the CDC’s studies, which he thought might be showing the opposite conclusions of what the scientists were presenting on efficacy of the current Covid-19 vaccines. He speculated that the control groups of the studies, which consisted of people who had other kinds of respiratory infections, were actually people damaged by Covid-19 vaccinations that put them at greater risk for those respiratory infections.
“In that case, you will assume that the vaccine is protective,” Levi said, “but an alternative explanation would be that the vaccine is actually making you more vulnerable for multiple viruses.”
Dr. Ruth Link-Gelles, a CDC epidemiologist, said that control subjects were chosen to closely match the hospitalized patients with Covid-19 in terms of age and symptoms.
“We think that the controls here are exactly what we want to understand the relative impact of the vaccination,” she said.
Dr. Tracy Beth Hoeg, a senior adviser at the US Food and Drug Administration who participated in the meeting as liaison member, said she too had concerns about confounding in the CDC’s studies and would have preferred to see data from randomized controlled trials.
“I think I share a lot of people on this panel’s desire to see randomized control trials to minimize these types of bias so we aren’t sitting here saying, ‘Are we being misled by these data?’ ” Hoeg said.
CDC experts say the study designs were chosen to generate timely results in a cost-effective way, to better understand the impact of vaccination on emergency department visits, hospitalizations and deaths in real time. CDC works with study sites to ensure that cases and controls are well-matched and minimize bias, they said.
“As for the question about clinical trials, I would actually defer that back to you all at FDA,” Link-Gelles said.
“Yeah, we’re working on that,” Hoeg said.
CDC’s take on thimerosal
Anti-vaccine groups have long blamed thimerosal for causing neurodevelopmental problems such as autism in children, despite numerous studies showing that it is not linked to those conditions.
Thursday’s presentation on the topic will be given by Lyn Redwood, a nurse practitioner who was president of the World Mercury Project, a group that later “expanded its mission” to become Children’s Health Defense, which lists Kennedy as its founder and former chairman. Kennedy has described Redwood as one of the original “mercury moms” who recruited him to join their fight against thimerosal in vaccines.
Redwood’s presentation highlights data that she says shows that thimerosal is not effective at preventing bacterial growth in vaccines and isn’t safe. She then proposes a recommendation that pregnant women, infants and children in the US get only thimerosal-free vaccines.
At least one of the studies referenced in Redwood’s slides originally posted for the meeting didn’t appear to exist. Those slides were later replaced with a version that didn’t include the citation.
Before its removal, the CDC briefing document pointed out that since 2001, all childhood vaccines licensed and recommended in the US have been thimerosal-free, except some flu vaccines drawn from multidose vials, which are used infrequently. In the 2024-25 flu season, 96% of flu vaccines given in the US and 98% of flu vaccines given through the CDC’s Vaccines for Children program were thimerosal-free, the CDC said in the document.
Dr. Peter Hotez, a pediatrician who directs the Center for Vaccine Development at Texas Children’s Hospital, said that although Redwood’s proposed recommendation may seem relatively toothless – most flu vaccines given to kids already are thimerosal-free – the basis for it is troubling.
What’s more, Hotez said, some low- and middle- income countries have to rely on multidose vaccine vials, and these formulations depend on the use of antibacterial preservative like thimerosal. If the US casts doubt on its safety, it could have ripple effects on vaccination programs around the world.
The CDC’s evidence review said science shows that thimerosal doesn’t harm children’s brains.
“Considering the breadth of evidence and consistency in results from multiple population-based studies conducted in several countries with various study designs, the evidence does not support an association between thimerosal-containing vaccines and autism spectrum disorder or other neurodevelopmental disorders,” the briefing document said.
Rare seizures after some measles shots
Another presentation on the agenda, to be made by Dr. Martin Kulldorff, on the combination MMRV vaccine in kids under 5, had not been posted to the ACIP website as of Wednesday night.
The CDC briefing document explains that since 2009, ACIP’s vaccine schedule has recommended giving the measles, mumps and rubella (MMR) vaccine and the varicella vaccine separately in children who are 1 to 2 years old.
Giving the shots separately cuts a very small additional risk of fevers with seizures that can happen when all the vaccine components are combined. The added risk amounts to 1 case of febrile seizure for every 2,300 to 2,600 vaccinated kids. Giving the vaccines separately avoids it.
The review notes that even when these seizures happen, they resolve without long-term consequences.
“Although rare cases of encephalitis and death have been reported after MMRV vaccination, no direct link between these events and the vaccine has been established in persons with healthy immune systems,” the review said.
Kulldorff announced Wednesday that a new CDC work group will look at vaccines for measles, mumps, rubella and chickenpox, or varicella. It may reevaluate the combination vaccine recommendation for 1-year-olds, the optimal timing of the vaccine and potential alternatives, such as one used in Japan.