The incidence of pertussis among U.S. adults 50 and older may be greatly under-reported and under-recognized, according to findings from a study conducted by GlaxoSmithKline researchers who analyzed approximately 48 million cases of cough-related illness in the U.S. between 2006 and 2010. The findings were released at the 2013 Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC).
Two GSK researchers, using a multiple linear regression model, estimated that the actual number of pertussis cases was approximately 520,000 versus the 8,764 medically-attended cases among U.S. adults ages 50 to 64, and approximately 465,000 versus 6,359 medically-attended pertussis cases among adults 65 and older in the same database.
That equates to an incidence on average of 202 per 100,000 in adults 50-64, and 257 per 100,000 among adults 65 and older. These estimated incidences were about 42 to 105 times higher than the medically-attended pertussis cases documented in the same database during the years 2006-2010. In 2010, the estimated incidence was 94 and 264 times higher than nationally reported incidences for individuals aged 50-64 and 65 and older, respectively.
The GSK researchers who led the study, Cristina Masseria, PhD, and Girishanthy Krishnarajah, MPH, MBA, MS, utilized data from the IMS private practice database that included more than 80 million claims per year. The commercial laboratory testing database represents approximately 40 percent of respiratory-laboratory testing that took place in the U.S. during the years looked at in the study.
The CDC, other public health authorities and infectious disease experts have long suspected that pertussis cases in adults go undetected or are misdiagnosed as other respiratory ailments, says Leonard Friedland, MD, vice president and director of scientific affairs and public health for GSK Vaccines. To our knowledge, this is the first attempt to quantify the incidence of cough illness attributed to B. pertussis via regression modeling among those greater than 50 years old. The authors plan to share their research methods and welcome other researchers to further examine and build upon the findings of this study. These findings suggest a major need for healthcare providers to consider the possibility of pertussis in older patients they see who have respiratory symptoms.
According to the CDC, there were 48,277 cases of pertussis reported in the U.S. in 2012. This was the most cases reported in the U.S. since 1955 when nearly 63,000 cases were reported. In 2011, 18,719 cases were reported. The CDC has developed and published a comprehensive pertussis guide that can be accessed at: http://www.cdc.gov/pertussis/outbreaks/about.html.
The results of this study should be interpreted in the context of its limitations. Claims data are inherently limiting, because they are collected for billing and reimbursement purposes rather than for research. Only patients seeking medical attention from private practitioners and only positive B. pertussis laboratory tests were included. Lastly, the incidence of cough attributed to B. pertussis was based on mathematical modeling. Models try to mimic the reality and are subject to numerous limitations and assumptions.