RANDOMIZED CONTROLLED TRIAL OF THE USE OF TEXT MESSAGE REMINDERS TO INCREASE PNEUMOCOCCAL VACCINATION COVERAGE IN ADULTS AGED 65 YEARS IN CHILE.

Background and Rationale Streptococcus pneumoniae is a globally distributed pathogen responsible for a wide spectrum of diseases, with the highest burden concentrated in children under 5 years and adults over 50 years. In Chile, invasive pneumococcal disease represents a growing public health threat among older adults: between epidemiological weeks 1 and 30 of 2024, reported cases increased by 9% compared to the previous year, with adults aged 60 years and older accounting for 47.7% of all registered isolates.

Despite the availability of the 23-valent pneumococcal polysaccharide vaccine (PPV23) free of charge through Chile's National Immunization Program (Programa Nacional de Inmunizaciones, PNI) since 2007, vaccination coverage among adults turning 65 years has remained consistently insufficient. The cohort born in 1957 reached 25.6% coverage in 2022; the 1958 cohort reached 25.0% in 2023; and the 1959 cohort reached 30.1% in 2024. These figures fall well below established public health targets and reflect a structural gap that conventional communication strategies have failed to close.

Text message reminders have emerged as a promising intervention to improve vaccination coverage. They are low-cost, scalable, rapidly deployable, and applicable at the population level. The most recent systematic review and meta-analysis of randomized controlled trials on this topic demonstrated that text message reminders significantly increase vaccination coverage by 9% (95% CI: 6-13%), an effect that is consistent regardless of message content, country income level, or target population age. Chile offers particularly favorable conditions for this type of intervention, with over 30 million active mobile phones, 94.1% internet access, and 75% social media users, positioning it as one of the most digitally connected countries in Latin America.

Study Design Is an open-label, parallel-group, randomized controlled trial conducted within the operational framework of Chile's National Immunization Program. Adults born in 1961 who turn 65 years of age in 2026, have a valid mobile phone number registered in the National Immunization Registry (Registro Nacional de Inmunizaciones, RNI), and have no prior record of pneumococcal vaccination will be eligible for inclusion. Individuals identified as deceased in the RNI will be excluded.

From an estimated base cohort of 175,000 adults born in 1961, approximately 43,000 individuals are expected to meet the eligibility criteria and constitute the study population. All eligible individuals will be included by convenience sampling, with no additional active selection process. Participants will be randomized 1:1 to the intervention or control group using stratified randomization by sex and region of Chile.