Objective: The national COVID-19 vaccination campaigns in India, prioritized healthcare workers to be vaccinated with ChAdOx1 nCoV-19 (AZD1222). So, we aimed to ascertain the effectiveness of the ChAdOx1 nCoV-19 vaccine among the healthcare workers by characterizing the breakthrough infections which occurred 14 days after the second dose of the vaccine. Methods: We prospectively evaluated the Institutional Covid Vaccination Database to categorize the SARS-CoV-2 positive healthcare workers into 2 cohorts: infected vaccinated and infected unvaccinated cohort. Along with comparing the descriptive statistics of the cohorts, the relative risk (RR), relative risk reduction (RRR), and the vaccine effectiveness were calculated. The study population was compared for the significant difference by using the parametric model (binary logistic regression) for the Odds ratio and the non-parametric model (cox proportionality model) for the Hazard ratio. Results: Out of the 324 healthcare workers employed in our institution 243 (75%) were vaccinated. When 6.58% (16) of the vaccinated healthcare workers were tested positive, 43.75% (35) were infected in the unvaccinated cohort and the median time of infection in the vaccinated cohort was 65 days (range 20 - 91 days). While the cox proportionality model reveals that the age, sex, and the contact exposure status of the cohort are not significant factors for getting infected after being vaccinated, binary logistic regression proves that a significant relationship exists between the incidence of infection and the unvaccinated status (p = 0.001, OR = 4.278). Completing the two-dose of vaccination decreased the hazard of testing positive for the SARS-CoV-2 by 84.96 % when compared with the unvaccinated individuals, which is the effectiveness of the ChAdOx1 nCoV-19 vaccine. Conclusion: There is an increased risk of the third wave of COVID-19 infection in the upcoming months. So, primary infection control measures should be followed even if vaccinated, to prevent the risk of breakthrough infections.