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A new study suggests there may be a link between aluminum used in vaccines that are given to young children in the first two years of life and the risk of developing asthma before age 5. The findings are preliminary and both the authors and others who have studied the data urge caution in the interpretation of the results.

Based on the findings, the possible association needs to be further explored, said the authors, who nonetheless are worried that in an era of rampant vaccine misinformation and expanding hesitancy on the part of some parents, results that should be seen as the start of a scientific exploration may instead be viewed as the conclusion of it.

Matthew Daley, the first author of the study, published Tuesday in the journal Academic Pediatrics, said the results will not change how he practices medicine.

“I practice as a pediatrician and … I need to feel comfortable that I’m doing best by the kids that I’m vaccinating,” Daley, a senior investigator at the Institute for Health Research at Kaiser Permanente Colorado, told STAT in an interview. “And I’m still going to advocate for vaccines as strongly as I did before we had these findings.”

The Centers for Disease Control and Prevention, which funded the study, said it will not alter its recommendations on vaccines that should be given to children based on this one study, noting in a statement that previous studies have not found a link between childhood vaccines and asthma. The CDC, the Food and Drug Administration, and the National Institutes of Health are in discussions about what additional studies should be conducted to further investigate the issue.

In an emailed statement, the CDC said an increase in asthma among young children predated the addition of aluminum to vaccines, which occurred in 1991. The increase in asthma, based on data from the National Health Interview Survey, was first observed in 1980. The agency noted that many factors that couldn’t be measured in this study might increase a child’s risk of developing asthma.

“We hope that additional studies can quickly provide more clarity, but at least from the national trend data, it appears that addition of aluminum-adjuvanted vaccines do not account for the overall trends that we see,” the agency said.

Daley and colleagues undertook the work after a 2013 report on vaccine safety from the Institute of Medicine — since renamed the National Academy of Medicine — urged additional study of whether there might be a link between exposure to aluminum in vaccines and the development of asthma.

“There’s a theoretical possibility that aluminum could increase allergy risk. But I’d highlight that it’s theoretical and it’s based on fairly limited animal data,” Daley said.

Aluminum is used in a number of vaccines as an adjuvant — a way to boost the immune response the product triggers. The practice has been used for decades. Vaccines that young children receive that contain aluminum include: diphtheria, tetanus, and acellular pertussis; hepatitis B; some formulations of Haemophilus influenzae type B or Hib vaccine; and pneumococcal conjugate vaccines.

Covid-19 vaccines and flu shots do not contain aluminum, the CDC said in its statement.

All young children are also exposed to other sources of aluminum, which is found in breast milk, infant formula, and foods. That fact is one of a number that complicates investigating whether there is a link between aluminum in childhood vaccinations and asthma, the study said, though the authors noted that a 2019 report said it appears that little or no aluminum that is ingested from dietary sources is absorbed through the gastro-intestinal tract.

The new study is based on a large group of children — nearly 327,000 — which is a point in its favor. But this is not a clinical trial, where some children received an intervention and the others received a placebo. Here the researchers conducted what is called a retrospective observational study, scouring the medical records of the children involved.