Federal health officials are warning that the strength of the whooping cough vaccine administered to children may be waning, leaving some children vulnerable to the disease.

A research article released by the Centers for Disease Control and Prevention sheds light on the potential waning strength of the current vaccination recommended for protection against pertussis, or whooping cough, Fox News reports.

The article, published in the journal Emerging Infectious Diseases, reports on a 2013 case involving a Tallahassee, Fla., preschool with 26 children, two staff members, and 11 family members who contracted pertussis  – even though many were vaccinated against the virus.

The CDC article noted that in one classroom in which all students received the pertussis vaccine, the infection rate of the virus reached 50 percent.

“This outbreak raises concerns about vaccine effectiveness in this preschool age group,” the authors write in the article, “and reinforces the idea that recent pertussis vaccination should not dissuade physicians from diagnosing, testing or treating persons with compatible illness for pertussis.”

According to the CDC, pertussis cases increased six-fold from 7,867 to 48,277 from 2000 to 2012— and one contributing factor for that growth may be waning immunity after vaccination, the article noted.