Nearly 26% of parents with children 6 months to 18 years old say they are hesitant to have their children get a flu shot, according to research published in the journal Pediatrics.

It found that the parents’ reluctance stems not from concerns about safety but from doubts about the effectiveness of the influenza vaccine. By contrast, just 6% of parents question the effectiveness of routine childhood vaccines, such as those for measles, mumps and whooping cough.

The findings were gleaned from a panel of 2,176 parents, considered a representative sampling of the U.S. parental population. Though the effectiveness of the flu vaccine varies from year to year, depending on how well it matches with the strain of the influenza virus that is dominant that year, health experts say that the flu vaccine lessens the severity of the illness even in years when it is not a good match with the circulating virus.

Overall, flu shots reduce risk for the disease by 40% to 60%, according to the Centers for Disease Control and Prevention. An annual flu shot, considered the most important way to prevent the flu, is recommended for nearly everyone 6 months of age and older, but the research notes that, in most years, at least 40% of American youths are not vaccinated against the flu.

Public health experts are particularly concerned that people of all ages get flu shots this year, given that the flu season and an expected second wave of cases of the new coronavirus are likely to coincide this fall and winter, severely stressing hospitals and the health-care system generally.