The current whooping cough epidemic in Washington state is part of a larger national spike in pertussis cases in the country as a whole, according to the Centers for Disease Control and Prevention’s (CDC) latest Morbidity and Mortality Weekly Report. Currently, there are nearly 18,000 reported cases of whooping cough nationwide, more than twice as many as reported last year at this time.

“In fact,” says Anne Schuchat, MD, director for the National Center for Immunization and Respiratory Diseases at the CDC. “it’s more than we had in each of the past five years. We may be on track for record high pertussis rates this year. We may need to go back to 1959 to find a year with as many cases reported by this time so far.”

Schuchat admits that this current trend may be part of cycle—as pertussis occurs in waves, with peaks that occur about every 3-to-5 years. However, there has still been a gradual and sustained increase in pertussis in the United States since the 1970s, when the country saw historically low levels.

Among the factors that could be contributing to this slow increase is the waning immunity provided by the pertussis vaccine, which can happen over time. However, increased reporting and increased diagnosis could also account for the increase since the 1970s. Previously, 2010 was the last peak year for cases nationally with more than 27,000 reported cases and 27 deaths, 25 of which occurred in infants.

According to the latest reports received by the CDC, so far this year, nine babies have died from whooping cough. The current national wave shows the highest rates of pertussis in infants younger than 1 year of age. About half those cases are babies under 3 months of age.

High rates are also evident in children 10 years of age. By age 10, immunity can wane from the early childhood vaccines children get, according to Schuchat. The CDC is thus recommending the Tdap booster of pertussis containing vaccine at 11- to 12- years of age.

Additionally, the MMWR reports increased rates in Washington state adolescents 13 to 14 years of age. The CDC is seeing a similar increase nationally in the same age group, a trend that is different from what has been seen in previous waves of pertussis.

“The increased number of cases among 13-to-14 year olds is a concern we are looking at in detail,” says Schuchat. “There are a number of possible causes, including how long protection from the vaccine lasts, if the switch we made in the strain we use in young children, a switch we made back in 1997 from the whole cell pertussis vaccine to the acellular pertussis vaccine might have done something to impact how long the vaccines last.”

The CDC is currently investigating the spike in this age group and recommends vaccination for all children.