Pertussis in the Netherlands, is the current vaccination strategy sufficient to reduce disease burden in young infants?

Background: Pertussis has resurged in the Netherlands since 1996. Several measures, i.e. acceleration of the schedule, introduction of a preschool acellular pertussis booster and change from an infant whole cell to an acellular pertussis combination vaccine were implemented in the National Immunisation Programme to decrease disease burden, in particular among very young infants who have the highest morbidity and mortality of pertussis. Nevertheless, a large outbreak occurred in 2011-2012.

Methods: 1996-2010 was divided in 3-year-periods to assess the impact of the measures taken, using notifications and hospitalisations. These results were compared with 2011-2012. Mean Incidence rates (IRs) per 100,000 were calculated.

Results: Although the measures taken resulted in decreased IRs among the targeted age groups after implementation, overall mean IRs of notifications increased from 32 (1996-2004) to 37 (2005-2010) and 63 (2011-2012). Young infants, not yet vaccinated, did not benefit; during the 2011-2012 outbreak, IR in 0-2-month-olds amounted to 259.6. IR among persons over 9 years of age increased from 6.8 (1996-1999) to 59.1 (2011-2012) For hospitalisations overall mean IRs decreased from 1.95 per 100,000 (1997-2004) to 0.88 (2005-2010) and 0.76 (2011).

Conclusion: The measures taken reduced IRs of notifications and hospitalisations among groups eligible for vaccination, but had no effect on the increasing IRs in adolescents and adults. This trend is also observed in other countries. The high IRs in 2012 in adolescents and adults probably resulted in increased transmission to infants, who are at risk for contracting severe pertussis. Therefore, additional measures to protect this group should be considered.