Pertussis in the Netherlands 2001-2002

To gain insight into the incidence and severity of pertussis in the Netherlands in 2001 and 2002, surveillance data based on notifications, laboratory data, hospitalisations and deaths were analysed for these two years and compared to that for 1989-2000. The decreasing coverage of serologic data from the LIS-RIVM compared with 1996 was taken into account. According to various surveillance sources pertussis is still endemic with epidemic peaks every two to three years (in 1996, 1999 and 2001). The reported incidence of notified cases was highest in 2001 (50.2/100,000) and decreased in 2002 (28.0). The incidence/100,000 in 2001 calculated from positive two-point serology amounted 4.4 (corrected for decreasing coverage 8.0) and positive one-point serology 30.7 (corrected 55.8) and hospital admissions 2.5. The incidence calculated from these surveillance sources was again lower in 2002: incidence/100,000 positive two-point serology 2.1 (corrected 4.1), positive one-point serology 15.4 (corrected 29.9) and hospitalisations 1.6. Highest incidence of hospitalisations was reported among infants less than 1 year (especially those aged < 3 months). In 2002 the first year an effect of the booster vaccination for four-year-olds might be visible- all surveillance sources showed a decrease in the incidence of cases aged 3 and 4 year old compared with previous years. Besides, small increases in the number of patients older than 5 years were seen. Estimations of vaccine efficacy based on surveillance data showed a slight improvement in vaccine-efficacy for the 1-year-olds in the period 1998-2002 compared with 1996-1997. Still, pertussis is endemic with peaks every 2 to 3 years and with a higher incidence compared to the period prior to the epidemic in 1996-1997. The introduction of the acellular booster-vaccination for 4-year-olds in 2001 has caused a decrease in the incidence of pertussis among the target-population. Long-term surveillance will be necessary to provide insight into the possible effect among the population at large. Estimations of vaccine-efficacy have improved in 1998-2002 compared to 1996-1997, probably as a result of the introduction of a 'stronger' pertussis vaccine in 1997. Pertussis is still most severe among young unvaccinated infants. The latter should be taken into account with the development of future vaccination-strategies (e.g. boostering of parents/care givers).