Pertussis in Italy: how to protect the "unprotectable"?

Abstract

Whooping cough continues to be an important public health issue despite high levels of vaccination coverage with acellular pertussis vaccine. Young unimmunized infants represent the most vulnerable group with the highest rates of complications and death. As infant-specific pertussis epidemiologic data, especially among neonates, in Italy were limited, a retrospective observational study of hospitalizations for whooping cough in Italian infants aged <12 months between 2007 and 2018 was conducted to address this knowledge gap. The temporal trend of rates, also stratified for age classes according to the expected age for the administration of vaccine doses, were analyzed by the slope of the regression line. The mean age at the time of admission was 92 d (±64). A clear seasonal pattern in the occurrence of pertussis hospitalizations with a summer peak was observed. Infants younger than 3 months old had the highest hospitalization rates (169 x 100000 infants on average), with a significant rising trend of 9 x 100000 infants on average per year. Limiting the analysis to Bordetella pertussis-related hospitalizations such trend was even more evident. In the other age classes, hospitalization rates were considerably lower and gradually decreased with increasing age. This study demonstrated that pediatric populations, too young to be protected by vaccination, had a greater risk of contracting pertussis. Thus, it is necessary to promote additional immunization strategies besides one booster dose in adolescents, including vaccination during pregnancy.

Keywords: Pertussis; infants; pregnancy; vaccine; whooping cough.

Figure 1.

Yearly frequency distribution and hospitalization rates for pertussis admissions in Italian infants aged <12 months, 2007–2018. Trend test. Number of hospitalizations: β = 8.71, p = .104; hospitalization rates: β = 3.18, p = .009

Figure 2.

A) Pertussis hospitalization rates for all etiologies stratified by age classes. Trend test. <3 months: β = 9.12, p = .002; 3–4 months: β = 0.96, p = .419; 5–10 months: β = 1.30, p = .053; >10 months: β = 0.52, p = .117. B) Bordetella pertussis-related hospitalization rates stratified by age classes. Trend test for B. pertussis hospitalization rates. <3 months: β = 18.11, p < .001; 3–4 months: β = 3.71, p = .002; 5–10 months: β = 1.54, p = .001; >10 months: β = 0.57, p = .011. dtp3 and dt_dtp3: diphtheria-tetanus-pertussis vaccines. Trend test for vaccination coverage (VC). VC(dtp3): β = −0.23, p=.003; VC(dt_dtp3): β=-0.27, p=.001