Between 2011 and 2012 the United States experienced a pertussis epidemic that reached 49 states. Washington State was hit hardest, with cases occurring at a rate nearly five times the national average. Authors of a recent study hypothesized that the epidemic would have spurred pertussis vaccination rates in the state; sadly, their theory proved to be incorrect.

The before-after study looked at how many infants were up to date with pertussis vaccinations at three critical time points. The epidemic was considered to have started on October 1, 2011, and to have lasted through December 31, 2012. The three data collection points were September 30, 2011 (the day before the official start of the epidemic), September 30, 2012 (during the epidemic), and September 30, 2013 (nine months after the epidemic ended).

The study cohort consisted of infants between three and eight months of age at the data collection points. Infants were deemed up to date with vaccinations if they had received one or more doses of the pertussis vaccine by age three months, two or more doses by five months, or three or more doses by seven months. (Editor's note: Three is the recommended number of doses by seven months; booster doses are begun later.)

The vaccination data were extracted from the Washington State Immunization Information System, which also provided county information, which was analyzed to determine whether specific county characteristics affected the results.

A total of 129,341 infant records were initially retrieved, although some didn't contain county or vaccine data. The percentages of infants with listed county data (n = 121,596) whose vaccinations were up to date before, during, and after the epidemic were 67.4%, 69.5%, and 67.6%, respectively. This represented an insignificant 2.1% absolute difference between the preepidemic and intraepidemic time points and a 0.2% difference between the preepidemic and postepidemic time points.

Large variations were observed in vaccination status across the most populous counties. Two counties (King and Pierce) had increases in pertussis vaccination rates between the preepidemic and intraepidemic time points (King County saw an increase of 7.4%, and Pierce County saw a 1.9% increase), whereas three other populous counties (Spokane, Clark, and Yakima) showed declines of 4.4%, 7.2%, and 9.9%, respectively. The authors attribute some of this variability to the media attention given to the epidemic in certain geographic areas within the state.-AK

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