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Frequency of hospitalization for pertussis in Italy stratified by age groups (2002–2016 time-frame). Trend test. <1 year: β coefficient = 1.37, p < 0.001; 1–4 years: β coefficient = 0.11; p = 0.178; 5–14 years: β coefficient = −1.20; p < 0.001; ≥15 years: β coefficient = −0.28; p = 0.045.
