Evidence of increased circulation of Bordetella pertussis in the Italian adult population from seroprevalence data (2012-2013)

Incidence data on pertussis cases in Italy do not show pertussis resurgence as recently described in other European countries. The aim of this study was to determine the seroprevalence of IgG antibodies to pertussis toxin (PT-IgG) in selected adult age groups, who can serve as a reservoir of Bordetella pertussis and be responsible for onward transmission to vulnerable infants. The seroprevalence of PT-IgG was studied in sera collected in 2012-2013 in three age groups: 20-29 years and 30-39 years (reproductive age), and >= 60 years. These data were compared to those from sera collected in similar age groups in 1996-1997. More than 80% of the adult population analysed in the 2012-2013 group presented detectable levels of PT-IgG (>5 IU ml(-1)). PT-IgG titres of 50-99 1U ml(-1), considered indicative of infection in the last few years, and PT-IgG titres of >= 100 IU ml(-1), considered indicative of recent infection (i.e. within the last year), reached 9.1 % [95% confidence interval (CI) 6.9-11.3 %; 58/639] and 5 % (95 % CI 3.3-6.7 %; 32/639) seroprevalence, respectively. Notably, the proportion of subjects with a seroprevalence indicative of recent infection increased significantly from 9.3 % (95 % CI 7.5-11.1%; 96/1037) in 1996-1997 to 14.1 % (95 % CI 11.4-16.8%; 90/639) in 2012-2013. Overall, our data clearly indicate a significant increase in the circulation of B. pertussis in adults in Italy; therefore, it is likely that the statutory notification system underestimates the real incidence of the disease. These findings have implications for preventive strategies.