Report of a pertussis outbreak in a low coverage booster vaccination group of otherwise healthy children in Italy

BACKGROUND: The introduction of universal pertussis immunization and the high coverage achieved in most developed countries have largely changed the epidemiology of the disease. Although vaccination rates are high in the first year of life, the rates of booster doses are unsatisfactory and lead to the onset of outbreaks. This report describes an outbreak of pertussis affecting school students already immunized in a town of Puglia (Italy), detected at the end of April 2009. METHODS: Vaccine effectiveness is measured by calculating the incidence rates (attack rates- AR) of disease among vaccinated (ARV) and unvaccinated (ARU) people and determining the percentage reduction in the incidence rate of disease among vaccinated people compared to unvaccinated people. RESULTS: The index case was a healthy child, female, 9-years-old who attended a local elementary school and developed pertussis on 27 April 2009. The secondary cases were the aunt and the cousin of the index case who developed a cough on 10 May 2009. In the elementary class of the index case, a cluster occurred. The overall AR was 15.8%, in particular 20% in children who did not receive the booster doses at 5--6 years old (ARU) and 14.3% in children receiving the booster (ARV). The VE of booster dose in this setting was 28.5%. Moreover, only the index case developed a persistent cough; the VE against moderate to severe pertussis was 100%. A cluster was detected in the middle school class that the cousin of the index case attended; AR was 44.4% (12/27); ARU was 50% (10/20) and ARV 28.6% (2/7). VE in this setting was 42.8%. CONCLUSIONS: Our results confirm the need to administer booster doses; failure the booster is the principal determinant for the outbreak onset.