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Observational epidemiologic data on 132 California kids who caught pertussis (of 15,000 in a cohort), presented at the American Society for Microbiology meeting last week, suggested that protection from pertussis vaccination may wane as soon as 3 years after the last dose. Kids age 8-12 were most at risk (the last dose is given at age ~4-6, with a booster dose advised at ~11-12).
The CDC's estimates of the drop-off in protection are lower, but no one doubts that pertussis immunity wanes relatively quickly compared to other childhood vaccinations. There were 27,000 cases of reported pertussis in 2010. But since partial immunity persists for decades and reduces severity of subsequent pertussis infection, probably tens of thousands more experienced mild infection (indistinguishable from the common cold) that went unreported.
For individual protection, to enhance herd immunity, and because pertussis can be deadly to an incompletely vaccinated infant coughed on by an infected family member, CDC recommends an extra Tdap dose in adulthood, given at any age up to 65, or older in select cases. Azithromycin, clarithromycin, or erythromycin cure pertussis easily and can be used as prophylaxis for infants or others with an infected family member.