BACKGROUND፡ When the whole world is fighting in an unprecedented pace against COVID-19 pandemic, the breakthrough COVID infections poise to dampen the rapid control of the same. We carried out this project with two objectives; first, to estimate the proportion of breakthrough COVID-19 infection among completely vaccinated individuals and second, to study the clinico-epidemiological profile of breakthrough COVID-19 infections among them.METHODS: This cross-sectional analytical study was conducted among 2703 fully vaccinated individuals from AIIMS, Patna COVID Vaccination Centre (CVC), Bihar, India. The participants were selected randomly using a systematic sampling technique from the list of beneficiaries maintained at the CVC. Telephonic interviews were made to collect the information by trained data collectors.RESULTS: A total of 274 fully vaccinated beneficiaries [10.1% (95% CI: 9.1%, 11.4%)] were diagnosed with breakthrough COVID-19 infection. The infections were more among males (10.4%) and the individuals aged ≤29 years (12.5%). The beneficiary categories, the healthcare-worker and the frontline-worker, were identified as predictors of the breakthrough COVID infections. Only one in three participants had adopted adequate COVID appropriate behaviour following the full vaccination. The majority of the breakthrough infections occurred during the second wave of COVID-19. The majority of the individuals with breakthrough infections were asymptomatic and no death was reported among them.CONCLUSION: One in every ten fully vaccinated individuals can get the breakthrough COVID infections. The healthcare-worker and the frontline-worker had independent risk of getting the breakthrough infections. Very few with breakthrough infections were serious and no death was reported among them.KEYWORDS: SARS CoV-2, Breakthrough COVID infections, Vaccination, COVID-19, Covaxin, India