Background <p>Close monitoring of side effects and their time-to-onset following COVID-19 vaccination is essential to ensure vaccine safety and maintain public confidence. The study aims to find the incidence, time of side effects, and factors responsible for the COVID-19 vaccine.</p> Methods <p>A cross-sectional design was used to collect data from 716 recipients of the first COVID-19 vaccine dose at Sikkim Manipal University, Sikkim, using a validated, self-administered questionnaire. Survival analysis was applied to find the incidence, time, and associated factors of side effects.</p> Result <p>Of the participants, 79.5% experienced at least one side effect, with pain at the vaccination site (73.8%) and fever (65.2%) being the most frequently reported. Maximum side effects occurred within 12&#xa0;h of vaccination. Among the survival models evaluated, the log-logistic model demonstrated the best fit for characterizing the time to symptom onset. The analysis revealed that age (Hazard Ratio [HR] = 1.51), education level (HR = 2.28), and use of prior medication (HR = 1.48) were significant risk factors for post-vaccination side effects.</p> Conclusion <p>This study highlights the importance of early monitoring of side effects following COVID-19 vaccination to ensure vaccine safety and maintain public confidence. The finding indicated that most side effects occurred within 12&#xa0;h of vaccination. Among the models assessed, the log-logistic survival model demonstrated the best fit for modelling the time to onset of vaccine side effects. Older age, lower education, and use of prior medication were identified as significant predictors of the post-vaccination side effects. These findings support the early monitoring and personalized care to ensure vaccine safety, health, and sustain public confidence.</p>