[Monitoring of a whooping cough epidemic 1994/95 in Switzerland using the sentinel notification system. Sentinella Registry].Schweiz Med Wochenschr. 1996 Aug 24; 126(34):1423-32.SM
Since June 1991 pertussis cases have been reported in the Swiss Sentinel Network (Sentinella). A total of 150-200 general practitioners, physicians specialized in internal medicine, and pediatricians participate in this system on a voluntary basis. Of the three specialties involved, this non-randomized sample represents 3.0%-3.5% of all physicians registered in Switzerland. The objective of this surveillance system is to monitor clinical pertussis over time. The case definition included all patients with a cough illness lasting at least 14 days with one of the following: paroxysms of cough, inspiratory "whoop", post-tussive vomiting (sporadic cases), or an epidemiological link to a pertussis case (epidemic cases). A laboratory diagnosis based on the polymerase chain reaction technique (PCR) was available for 82.7% of cases reported in 1994 and 1995. Of these, 27.7% had a positive PCR result. Reports of epidemic pertussis tested for Bordetella pertussis by PCR were confirmed by the laboratory in 46.5% of cases. The laboratory confirmation rate was more than twice as high among epidemic cases than among sporadic cases (20.7%). The crude incidence rate of whooping cough was 70 cases per 100,000 population per year in 1992 and 1993. Compared to previous years, pertussis incidence was significantly higher in 1994 and 1995 (370 cases per 100,000 population and 280 cases per 100,000 population respectively). The increase in reports was especially marked between July and October 1994 and whooping cough became epidemic in the third trimester of 1994 and at the beginning of 1995. In these 2 years, Switzerland experienced an estimated 40,000 clinical pertussis cases. Based on the proportion of PCR-positive pertussis cases in the sentinel sample, 12,500 of these would have been laboratory-confirmed. Most cases were observed in infants and in children up to 6 years of age. Assuming a vaccination coverage of 90%, the global efficacy of vaccination (3 or more doses versus less than 3) for 1994 and 1995 among children aged 12 to 47 months and not born before 1991 was 0.74 (0.59 and 0.88 for a vaccination coverage of 85% and 95% respectively). Vaccine efficacy was higher in PCR-positive cases (0.87; 0.79; 0.94) than in PCR-negative cases (0.54; 0.27; 0.78). Vaccination efficacy estimates on the basis of surveillance data are certainly less precise than those inferred from clinical trials. However, our results indicate that the efficacy of vaccination in children significantly declined with increasing age. Whooping cough still has the potential to cause epidemics in Switzerland in spite of a high vaccination coverage. With the introduction of acellular pertussis vaccines and new vaccination schemes in Switzerland, the Swiss Sentinel Network fulfills an important task as a monitoring system and contributes to the evaluation of new vaccination strategies.