The trial will investigate to what extent SMS reminders for influenza vaccination increases coverage of influenza vaccination among elderly from the majority population, and among elderly from immigrant groups with low vaccination coverage

Background: The coverage of influenza vaccination among elderly in Norway is insufficient, especially in some immigrant groups. To improve public health, there is a need for an intervention that can increase influenza vaccination coverage. Further, interventions tailored to reduce potential barriers among immigrants can reduce health inequities.

Objective: InfluSMS aims to determine if SMS nudging increases vaccination coverage among those aged 65 or older (i) in Norway's general population; (ii) among immigrants born in Poland; (iii) among immigrants born in Ukraine; and to evaluate the impact of SMS nudging in Norwegian versus in the official language of the native country of immigrants born in Poland or Ukraine.

Methods: InfluSMS is a pragmatic randomized controlled trial conducted among people aged 65 or older residing in Norway. Influenza vaccination coverage is the main outcome, measured in control and intervention arms for each of the three populations listed above. In all three populations, the control arm is standard care, i.e., no individual reminder for influenza vaccination. All populations have an intervention arm that will receive an SMS nudge in the Norwegian language. In addition, the Polish and Ukrainian immigrant populations include a second intervention arm that will receive an SMS nudge in Polish or Ukrainian, respectively. Allocation to study arm is randomized, and the intervention will take place at the start of the 2025/2026 influenza season. All eligible individuals will be passively followed up through the National Immunisation Registry SYSVAK, from which individual influenza vaccination status three months after the SMS nudge will be extracted.

Results: Inclusion of participants will start in the third quarter of 2025, and the registry data will be available in the first quarter of 2026. Coverage rates of each strategy, and coverage differences between strategies will be presented.

Implications: SMS nudging could be integrated into the national influenza vaccination program if the trial shows it effectively increases influenza vaccination coverage among the elderly. Further, the trial will establish whether language is a barrier for influenza vaccination uptake among recent immigrant groups that have low influenza vaccination coverage, and to what extent this potential barrier can be diminished by SMS nudging in the official language of their native country.

Inclusion Criteria:

• Aged 65+ (age at the end of 2025, i.e., born 1960 or earlier)
• Resident in Norway and have a valid ID number on 1 September 2025
• Have a registered mobile phone number

Exclusion Criteria:

• Have received the 2025 influenza vaccine prior to the SMS nudge dispatch date
• Have emigrated or died before SMS nudge dispatch date

The project is not allowed to share the data. Data access requires legal permissions and applications for data must be directed to the registries involved

The outcome measure of the trial is based solely on routinely collected data and those reporting/registering this information will not be aware of whether a vaccinated individual has received an SMS nudge. Thus, the trial is blinded to the outcomes assessor. Moreover, treatment allocation is randomized and computer-generated, there is no interaction between the InfluSMS project team and study participants, and the InfluSMS researchers work with de-identified data and do not know the identity of any participant in the dataset. Due to the inherent characteristics of the intervention, treatment allocation cannot be blinded for participants.