Questionnaire surveys
The methodology of 2013 and 2018 studies was identical to that of the previous surveys of 1978, 1991, 1998, 2003 and 20086,7,19. Standard asthma parental questionnaires were distributed during January and February 2013 and 2018 to third- and fourth-grade schoolchildren (i.e. aged 8-9 years, range 7-10 years) from 44 out of 50 public primary schools of Patras, a coastal city of Western Greece with a metropolitan area of approximately 214.000 inhabitants. The target population represented approximately 89.8% of the local population within the respective age range in 2013, and 91.2% in 2018 (National Census 2011 data). The same 44 schools were surveyed in 1998, 2003 and 2008.
The exact wording of the questionnaire and the definitions used for wheeze and asthma have remained constant since 1991 and are presented in E-table 1. Briefly, the questionnaire consisted of two questions on current (i.e. within two years from the survey year) physician-diagnosed wheeze (two or more episodes) and asthma, and two questions on non-current (i.e. prior to but not within two years of the survey) physician-diagnosed wheeze and asthma. A positive answer to any of these questions was termed ‘lifetime wheeze/asthma’. Physician-diagnosed wheeze with or without asthma within the last two years but not earlier, was termed ‘recent-onset’ wheeze/asthma, while wheeze/asthma that occurred both prior to and within the last two years was termed ‘persistent wheeze/asthma’. A positive answer to a question on asthma premised a positive answer to the respective (current or non-current) question on wheeze. Conflicting answers were resolved by telephone call; elsewise, they were not included in the analysis.
The methodology used in the distribution and collection of the questionnaires has been described previously6,7,19; a relevant scheme is presented in E-figure 1. All positive responders (at least one positive response) and an approximately equal number of randomly selected negative responders were approached by telephone contact to confirm their answers (validation process).