The risk of vaccinated children developing pertussis increases with the time since their last dose, in research suggesting that waning effectiveness between injections is a significant contributor to outbreaks.

Fully vaccinated children make up four out of five cases of pertussis in parts of California, according to a study in Pediatrics.

“Children who were further away from their last dose were at increased risk of pertussis, even after controlling for undervaccination,” the researchers wrote.

They looked at the electronic health records of almost 470,000 children aged under 11 who were members of Kaiser Permanente Northern California, a large US healthcare provider.