Increase of pertussis incidence in 2010 to 2012 after 12 years of low circulation in Spain.

In Spain, whole cell pertussis vaccination started in 1975, with three doses before the age of 6-7 months. Doses at 15-18 months and 4-6 years were introduced in 1996 and 2001, respectively. Spain switched to an acellular vaccine in 2005. From 1998 to 2009, pertussis incidence rates remained ≤1.5 cases/100,000 inhabitants but increased from 2010 to 7.5 cases/100,000 in 2012. Data from 1998 to 2012 were analysed to assess disease trends and susceptible populations. We defined four epidemic periods: 1998-2001 (reference), 2002-05, 2006-09 and 2010-12. In 2002-05, the incidence rate increased in individuals aged 15-49 years (IRR: 1.41 (95% CI: 1.11-1.78)) and ≥50 years (IRR: 2.78 (95% CI: 1.78-4.33)) and in 2006-09 increased also in infants aged <3 months (IRR: 1.83 (95% CI: 1.60-2.09)). In 2010-12, the incidence rate increased notably in all age groups, with IRRs ranging between 2.5 (95% CI: 2.3-2.8) in 5-9 year-olds and 36.0 (95% CI: 19.4-66.8) in 20-29 year-olds. These results, consistent with the country's vaccination history, suggest a progressive accumulation of susceptible individuals due to waning immunity after years of low incidence. Further vaccination strategies should be assessed and implemented to prevent pertussis in pre-vaccinated infants, in whom the disease is more severe.