METHODS: This two-arm quasi-experimental study was conducted in two primary healthcare facilities selected using simple random sampling with opaque envelopes. A total of 524 participants were enrolled using purposive sampling. Data were statistically analysed using the Statistical Package for the Social Sciences version 28 with repeated-measures logistic regression analysis, the Z test for Poisson rates and the Wilcoxon signed-rank test.

RESULTS: The parental SMS reminders significantly improved the immunisation coverage, as reported by 69 (26.3%) (P=0.001, 95% confidence interval [CI] = 12.2-40.5), 117 (44.7%) (P=0.001, 95% CI=32.6-56.9) and 116 (44.3%) participants (P=0.001, 95% CI=34.2-54.4) for the 6th-, 10th- and 14th-week schedules, respectively. Compared to mothers' involvement, fathers' involvement did not significantly affect the immunisation coverage (B=0.158, P=0.311, 95% CI=-0.148-0.464). With regard to the immunisation timeliness, the parental SMS reminders yielded a significant effect for the 6th-, 10th- and 14th-week schedules (P=0.001, 95% CI=25.9-46.7; P=0.001, 95% CI=24.2-43.0; and P=0.001, 95% CI=21.1-36.9, respectively). Compared to mothers' involvement, fathers' involvement significantly influenced the immunisation timeliness (B=0.298, P=0.038, 95% CI=0.016-0.579). Lastly, the parental SMS reminders significantly reduced the barriers to immunisation, with a P-value of 0.001.