A new study has come to some surprising conclusions about an important vaccine.

The study found that the Tdap booster, a booster vacine meant to fight whooping cough that is often required for middle school-aged children, isn’t providing long-lasting protection against the disease, according to a CNN report.

The Tdap booster protects against not only whooping cough — or pertussis — but also tetanus and diphtheria. But this study found that while it protected 69 percent of adolescents against whooping cought in the first year, it dropped to 57 percent by the second year, to 25 percent in the third year, and as low as just 9 percent in the fourth year.

The researchers based this on a study of 280,000 10-year-old children in 2009 and then revisited them in 2015. California requires the booster for seventh graders, so nearly everyone had received a Tdap by 11 or 12 years of age.

It’s causing experts to start to question the current strategy of using vaccines. Fortunately, the cases of pertussis tended to be mild or moderate, but whooping cough can still cause problems for children and result in a lot of missed days at school. Scientists may examine figuring out a new way to effectively target vaccinations, according to the report.

For example, children could get the Tdap only if there is a growing possibility of a pertussis outbreak, a strategy that would require local health authorities to be vigilant. Or, Tdap boosters could be given to adolescents of all ages every few years.