HealthDay News — The number of pertussis cases reported thus far in 2012 in the ongoing Washington state pertussis epidemic is the highest number of cases reported in the state since 1942, according to CDC health officials.

Since January, Washington state has experienced 2,520 pertussis cases —  a 1,300% increase compared with the first six months of 2011, the CDC reported in Morbidity & Mortality Weekly Report, raising concerns that the nation as whole may experience record levels of the disease in 2012.

In April, Washington’s Secretary of Health officially declared the pertussis outbreak an epidemic. Subsequently, the CDC’s Chas Debolt, MPH, and colleagues have assessed clinical, epidemiologic and laboratory factors associated with the rise in pertussis cases to better understand the outbreak.

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Disease rates were highest in infants and children aged younger than 10 years, a trend consistent with case reports on a national level, the researchers found. However, pertussis rates were also increased in 13- to 14-year-old adolescents who had received the tetanus toxoid, reduced diphtheria toxoid and acellular pertussis (Tdap) vaccine, supporting the hypothesis that immunity may wane soon after Tdap vaccination with acellular vaccines. These findings are similar to trends observed in the 2010 California pertussis outbreak

“Investigation of the Washington epidemic demonstrates multiple [Bordetella] pertussis strains causing infection, primarily in vaccinated persons. Given the high transmissibility of B. pertussis, a proportion of vaccinated persons remains susceptible and can become infected during a pertussis outbreak,” the researchers wrote.

In fact, only 4.5% of Washington pertussis cases occurred in schoolchildren who were exempted from pertussis vaccine, study data reveal, which represents only a small proportion of those at risk for pertussis in the state.

Despite concerns about waning immunity, “vaccination remains the single most effective strategy for prevention of infection,” Debolt and colleagues concluded.

They emphasized the importance of focusing pertussis prevention and control efforts at protecting infants and others at greatest risk for severe disease, as well as improving vaccination coverage in adolescents and adults, specifically pregnant women.

Centers for Disease Control and Prevention. Morb Mortal Wkly Rep. 2012 Jul; 61(28):517-522.