Sequential reminders improved MenACWY coverage in unvaccinated youth and adults in Spain.

INTRODUCTION: Invasive meningococcal disease (IMD) in Spain remains rare, yet rising trends in serogroups W and Y prior to 2019 prompted the national recommendation to replace adolescent meningococcal C vaccination with the quadrivalent meningococcal ACWY (MenACWY) vaccine and to implement a catch-up campaign for individuals born between 2001 and 2007. In the Region of Murcia, the COVID-19 pandemic delayed implementation until September 2021. After an initial mass-vaccination phase and a long period without active outreach, coverage remained suboptimal, prompting a multi-phase reminder strategy. This study assessed the impact of different reminder combinations-postal letters, short message service (SMS), or both-on MenACWY vaccination coverage. METHODS: A randomized interventional study was conducted between September and October 2022 among 66,891 unvaccinated individuals. Participants were allocated into four groups based on birth month, each receiving a different reminder strategy: letter followed by SMS, simultaneous letter and SMS, SMS followed by letter, or delayed combined reminders. Coverage was analysed overall and by age group, sex, and intervention arm. RESULTS: Six weeks after the first intervention, coverage increased from 53.52% to 56.40%. After the second intervention, coverage rose to 59.98% at six weeks and 61.07% at twelve weeks-a total increase of 7.55 percentage points. Significant differences were observed across the four strategies (p < 0.001). The most effective approach was sending a postal letter followed by an SMS six weeks later, which consistently produced the highest coverage. No differences were found by sex. Coverage achieved was significantly higher among minors (<18 years) than adults (22.5% vs. 13.2%, p < 0.001). Beyond 12 weeks after each intervention, additional increases were minimal (<1%). CONCLUSIONS: This study demonstrates that sequential reminders-specifically a postal letter reinforced by an SMS-substantially improve adolescent and young adult vaccination uptake. These findings support the integration of combined reminder strategies into routine catch-up vaccination programs.