Background and Objective:
Pertussis is an emerging disease in countries with long-established vaccination programmes and high vaccination coverage. Immunity waning has led to a worrying upsurge in severe cases in unvaccinated infants. In Spain, since 2006, pertussis has emerged in several epidemic waves reaching a peak in 2015, which introduced the pertussis vaccination in pregnant women in 2016. The objective is to estimate the influence of this vaccination on the incidence and hospitalizations in children under one year of age.

Methods:
We studied pertussis cases reported to the national surveillance system and hospitalisations in children under 1 year of age (<1y) between 2006-2019. We analysed the evolution of the incidence rate (IR) and hospitalisation rate (HR) in children under 1 year of age and by subgroups: <3 months (<3m) and 3-11 months (3-11m). We calculated both rates for the pre-vaccination (P1: 2006-2015) and post-vaccination (P2: 2016-2019) periods as well as the incidence rate ratio (IRR) and hospitalisation rate ratio (HRR) using Poisson regression.

Results:
Between 2006-2019, 9107 pertussis cases were reported in infants (IR: 146.6/105). IR in <1y slightly increased (P1: 145.2/105 and P2: 153.6/105) with IRR: 1.06 (95%CI: 1.59-1.65); by subgroups, in <3m IR decreased (IRR: 0.61 (95%CI: 0.58-0.67) by 39% but IR increased (IRR 1.74 (95%CI: 1.64-1.84) in the 3-11m group. The overall HR 2006-2019 was 142.5/105. HT in <1y was reduced in the post-vaccination period (P1: 154.9/105 and P2: 107.7/105) with HRR: 0.70 (95%CI: 0.66-0.74); the reduction is maintained for the two subgroups: <3 months (HRR: 0.61 (95%CI: 0.58-0.67), 39% reduction and between 3-11m (HRR: 0.85 (95%CI: 0.78-0.92) 15% reduction.

Conclusion:
In Spain, pertussis vaccination in pregnant women has reduced severe pertussis in children under one year of age, especially in the under-3 months group. High vaccination coverage should be maintained as an effective measure to prevent complications.