A majority of pertussis cases in the United States may go undetected in people under the age of 50, particularly in adults, results of a retrospective database cohort study suggest.

“The incidence of pertussis in adolescents and adults is very difficult to quantify,” wrote Chi-Chang Chen, MD, of IMS Health, Plymouth Meeting, Pa., and associates. Symptoms may be misdiagnosed as other respiratory illnesses, infected individuals may not seek treatment, and pertussis may not be considered as a possible diagnosis in adults, they noted.

To project the possible range of pertussis incidence in this population, the investigator used three different models to analyze information from private insurance and laboratory databases as well as data from the Centers for Disease Control and Prevention for a 6-year period. The first method, which used medical claims for ICD-9 diagnosed pertussis, found an annual incidence rate of 9/100,000 population. The second used a proxy pertussis model that was based on symptoms that could indicate undiagnosed pertussis, showing an incidence rate of 21/100,000. The third method used pathogen data to estimate the fraction of cough illness statistically attributable to pertussis, resulting in an incidence rate of 649/100,000 population, which is 58-93 times higher than the ICD-9 estimated incidence.

These estimates “highlight the need for improved preventive measures – such as increased vaccination – against pertussis,” the investigators said, noting that immunization recommendations for additional age groups and research involving strategies to reduce waning immunity after vaccination should be considered.

The study was funded by GlaxoSmithKline Vaccines.

Read the full study in Human Vaccines & Immunotherapeutics (2016 May. doi: 10.1080/21645515.2016.1186313 ).