Decline of pertussis in hospitalised children following the introduction of immunisation in pregnancy - results from a nationwide, prospective surveillance study, 2013-2020.

Surveillance of pertussis at a national level is important to collect information on the epidemiology of the disease and to design optimal immunisation strategies. After an initial surveillance period for pertussis in hospitalised children in Switzerland from 2006 to 2010, new recommendations (including immunisation in pregnancy) were implemented in the national pertussis immunisation schedule in 2013. To monitor its effects, surveillance was resumed in 2013 and concluded in 2020. From 1 January 2013 to 31 December 2020, hospitalised children under the age of 16 years with suspected or proven Bordetella pertussis infection were reported to the Swiss Paediatric Surveillance Unit. We analysed epidemiological and clinical characteristics for all patients who fulfilled the clinical case definition of pertussis (physician diagnosis, cough lasting ≥14 days in combination with at least one of the following symptoms: paroxysmal cough, whooping or post-tussive vomiting, or apnoea not otherwise explained in patients under the age of 12 months) with and without laboratory confirmation (polymerase chain reaction, culture or serology). 220 of the 306 reported cases met the inclusion criteria. Of these, 214 (97%) were laboratory-confirmed B. pertussis infections: 209 by polymerase chain reaction and 5 by culture from nasopharyngeal specimens. 172/220 (78%) patients were infants under six months of age at the day of hospitalisation. The mean annual hospitalisation rate for pertussis was 27.9 per 100,000 children for infants and 2.1 per 100,000 in all children <16 years of age. Of the 115 cases with precise records of immunisation, 50 (43%) were unimmunised, 3 (3%) were incompletely immunised and 62 (54%) were up to date with their immunisation status according to their age, including 10 (9%) whose next doses were due. However, most patients with an up-to-date immunisation status (85%) were still too young to have completed their primary series, leaving only eight cases of vaccine failure. Only 5 of the 172 infants <6 months of age had mothers who had been vaccinated during pregnancy. After the introduction of immunisation in pregnancy in Switzerland, hospitalisation rates in infants declined. However, the remaining cases call for increased efforts towards more complete and timely immunisation of children, those in close contact with children, and pregnant women.