According to NBC News, scientists believe that the vaccine efficacy for whooping cough is diminished due to mutation in the pertussis bacteria.

Researchers at the Centers for Disease Control and Prevention analyzed lab samples from patients with whooping cough between 2000 and 2013 and found Bordetella pertussis, which causes whooping cough, has gone through genetic changes over time.

That means the current vaccine is not a perfect match to the bacteria. Researchers hope the new data published Wednesday in the journal “Emerging Infectious Diseases” will help change that.