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BACKGROUND: In late 2011, there was an increase in the number of pertussis cases being reported to the Washington State Department of Health (DOH). By February 2012, a multi-divisional DOH workgroup was convened to monitor pertussis surveillance information and develop appropriate response strategies to the marked increase in cases being reported from multiple local health jurisdictions (LHJs). A pertussis epidemic was declared in the state on April 2, with 608 cases reported compared to 94 during the same time period in 2011. In early May an Epi Aid team was dispatched from the Centers for Disease Control and Prevention (CDC) to assist DOH epidemiologists with an in depth assessment of the epidemic.
METHODS: Cases were defined using the Council of State and Territorial Epidemiologists’ surveillance definitions for confirmed and probable pertussis. Suspect cases were defined as a positive polymerase chain reaction (PCR) test and cough of <2 weeks or unknown duration. Reported cases were analyzed by geographic area, age, reported vaccination status, and clinical severity.
RESULTS: There were 4783 confirmed and probable and 582 suspect cases reported in Washington State during 2012. Urban parts of Western Washington reported cases to LHJs earlier in the epidemic and had the highest rates of disease. Eastern Washington counties, which are more rural, saw increased reporting later in the year. School-aged children (5-18 years) represented 57% of cases. Rates were unexpectedly high in 11-18 year olds, with 10 and 13 year olds having the highest rates among adolescents (380 and 335 cases /100,000, respectively). Infants under 1 year had the highest rates of disease among all age groups (413/100,000). Valid vaccination history was available for 93% of case-patients aged 3 months to 18 years. Seventy one percent of cases aged 3 months to 10 years and 76% of 11-12 year olds were up-to-date for the childhood series. Receipt of tetanus and diphtheria toxoid and acellular pertussis (Tdap) vaccine was documented in 48% of 11-12 year olds and 75% of 13-18 year olds. Ninety nine patients were hospitalized, 72% of whom (n=71) were under one year of age; one death in a young infant occurred in week 50.
CONCLUSIONS: The majority of cases in this pertussis epidemic were in school aged children who had been fully or partially vaccinated according to the recommended schedule, leading to concerns about the effectiveness and duration of protection of the acellular pertussis vaccines currently in use.