Rethinking seasonal SMS vaccination reminders: quasi-experimental evaluation in Portugal, 2023–2026

Background

Seasonal influenza and COVID-19 vaccination coverage in older adults remains below WHO and ECDC targets. SMS reminders are increasingly deployed as low-cost strategies to improve uptake, yet robust real-world population-scale evidence is scarce and existing estimates may be biased by campaign-time confounding. We assessed the short-term effect of nationally delivered SMS reminders on influenza and COVID-19 vaccination uptake in older adults in Portugal.

Methods

We conducted a quasi-experimental study using daily national vaccination records for adults aged 60+ years during the 2023/24, 2024/25 and 2025/26 campaigns. Short-term effects (days 0–7) were estimated using interrupted time series models. Reminder-specific estimates were synthesised by campaign, and, for 2025/26, results were compared with staggered models including date fixed effects.

Results

For influenza, pooled IRR was 0.90 (95%CI 0.84–0.96; I²=42.4%), ranging from 0.80 (0.56–1.14) in 2023/24 to 1.23 (1.13–1.34) in 2025/26. For COVID-19, pooled IRR was 1.00(95%CI 0.86-1.17; I²=92.7%), ranging from 0.88 (0.83–0.93) in 2023/24 to 1.13 (1.05–1.23) in 2025/26. Critically, in 2025/26, staggered models adjusting for date fixed effects reversed the apparent positive association, yielding IRRs of 0.77 (0.62–0.96) for influenza and 0.75 (0.62–0.91) for COVID-19 - suggesting the naïve positive effects reflected concurrent campaign dynamics rather than the reminders themselves.

Conclusions

In this national scale evaluation, SMS reminders were not consistently associated with increased seasonal vaccination uptake in older adults, and apparent positive effects disappeared under more robust model specifications. These findings challenge the assumption that digital reminders are a reliable stand-alone lever to improve vaccination coverage and underscore the need for rigorous quasi-experimental evaluation of digital public health interventions across European vaccination programmes.

Key messages

• Text message reminders did not consistently increase influenza and COVID-19 vaccination uptake.

• Policymakers should consider personalized or other strategies to improve uptake in Portugal.