Noncompliance and delay in vaccination have caused millions of preventable deaths worldwide particularly in low-and middle-income countries (LMICs). The main reasons for delay in vaccinations or dropout from vaccination programs are vaccine hesitancy, lack of knowledge, forgetting due date, lack of trust, and religious and social barriers. Recently, various mobile health (mHealth) interventions such as voice call, text message reminders have been implemented in LMICs to improve awareness of vaccine availability and provide timely reminders. However, the implication and impact assessment of mHealth interventions with behavior change message for improving vaccination coverage and timeliness is limited. In this project, a three-armed cluster randomized controlled trial (cRCT) will be conducted to assess the impact and cost-effectiveness of mHealth interventions with behavioral change approach on vaccination coverage and timeliness in Bangladesh. A total of 7800 caregivers of children aged less than 4 weeks will be recruited and randomly allocated equally to each arm: control, voice call, and short message service (SMS) reminders with behavioral change message. The primary outcomes will include coverage of full immunization among children in first year of life, and secondary outcomes will be coverage for each vaccine, timeliness, and infant mortality.

Inclusion Criteria:

1. You are the infant's caregiver (mother, father, or legal guardian).

2. The infant is less than 4 weeks (28 days) old at the time of enrollment.

3. You reside within the jurisdiction of the designated community health center and have no plans to move within the next 12 months.

4. Depending on your assigned group, you can do one of the following:

   • You can read Bengali SMS messages yourself.
   • You can receive support from a family member living with you who reads Bengali aloud.

   (Note: For the voice call group, literacy is not an issue.)

5. You understand the content of this study and have completed the required informed consent form.

Exclusion Criteria:

1. Caregivers of children aged 4 weeks or older at the time of enrollment
2. Children who have received any vaccine other than BCG or polio at birth
3. The caregiver plans to move out of the study area within the next six months.
4. The caregiver does not want to receive the vaccination at the intervention health center.
5. The caregiver (father, mother, or other family member) does not have a phone.