Challenges of interpreting epidemiologic surveillance pertussis data with changing diagnostic and immunization practices: the case of the state of São Paulo, Brazil.

Background: A significant increase in pertussis incidence occurred in Brazil, from 2011 to 2014, despite high coverage of childhood immunization with whole-cell-pertussis (wP) containing vaccines. This study presents pertussis surveillance data from São Paulo state and discusses the challenges to interpret them considering pertussis cyclic epidemic behavior, the introduction of new diagnostic techniques and new vaccination strategies, and enhanced disease awareness during epidemics.
Methods: Observational study including pertussis cases reported to the Surveillance System in São Paulo state, from January 2001 to December 2015. Pertussis cases data were retrieved from the National Notifiable Diseases Information System (SINAN) website and from São Paulo state Epidemiological Surveillance Center (CVE/SP) database. Vaccination coverage and homogeneity data were collected from the Unified Health System Department of Informatics (DATASUS). We presented cases distribution by year, age group and diagnostic criteria and calculated pertussis incidence rates. The proportions of cases among different age groups were compared using chi-square test for trend.
Results: Infants less than 1 year of age were the most affected during the whole period, but the proportions of cases in this age group had a significant decreasing trend, with significant increase in the proportions of cases reported among older age groups (1-4, 5-10 and ≥20 years). Cases among infants aged less than 6 months represented ≥90% of all cases in children less than 1 year of age in all but 2 years (2012 and 2015). A non-significant decrease in the proportion of cases among infants aged < 2 months was observed in parallel to a significant increase in the proportion of cases in infants aged 6-11 months.
Conclusions: A pertussis outbreak has occurred in a state with universal use of wP vaccine. The disease cyclic behavior has probably had a major role in the increased incidence rates registered in São Paulo state, from 2011 to 2014, as well as in the decreased incidence in 2015. Maternal vaccination cannot explain the drop in the number of cases among all age groups, in 2015, as herd protection is not expected, but may have had an impact on the number of cases in infants aged < 2 months.