The increase since the 1980s is due to Immunity waning in previously vaccinated adolescents and adults Parents refusing to vaccinate their children (see Vaccine Hesitancy) Such unprotected patients may become ill; furthermore, unprotected adolescents and adults are an important reservoir for B pertussis and are thus often the source of infection for unprotected infants < 1 year (who have had the highest increase in annual incidence and the highest case fatality rate).