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New research has found the flu vaccine may weaken some children’s immune systems to other influenza viruses. While experts do not recommend halting flu vaccines, they do recommend further research to eradicate adverse side effects, according to a study published in the Journal of Virology.

Researchers from the Erasmus Medical Center in Rotterdam, the Netherlands, collected blood samples from 27 healthy, unvaccinated children with an average age of 6 years old, and 14 children with cystic fibrosis who received an annual flu shot. Children with chronic illnesses like cystic fibrosis are required to get flu shots in the Netherlands.

Children who were not vaccinated built up more antibodies across a wider variety of influenza strains than kids who were vaccinated, the small study found.

“Annual vaccination against influenza is effective but may have potential drawbacks that have previously been underappreciated and that are also a matter of debate,” lead author Rogier Bodewes said in a statement.

While the current vaccine, which has been around for more than 60 years, is not a perfect one, Dr. Andy Pavia, chair of the Infectious Diseases Society of America’s Pandemic Influenza Task Force, said people should not be discouraged against getting the vaccine from this study.

“For kids with cystic fibrosis, their lungs are already bombarded with other infectious agents,” said Pavia. “We’d really like to see whether the unvaccinated or vaccinated kids do better when faced with a new strain of influenza. The ideal study would compare healthy kids to healthy kids to really see if the results are true.”

The Centers for Disease Control and Prevention currently recommends that everyone older than 6 months of age receive the flu vaccine. Pregnant women, children under 5, health care workers, those over 50 and people with chronic medical conditions are at especially high risk of flu-related complications, and should receive the shot as soon as it becomes available each year.

“We’re seeing a lot of work on a wide variety of promising vaccines,” said Pavia. “It isn’t perfect, and the study does point out how vaccines could get even better in preventing viruses.”