Background World Health Organization declared COVID 19 as pandemic in March 11,2020. SARS-CoV-2 is different from other known viruses due to multiple mutations on the sites of nonstructural proteins. Since the healthcare workers (HCWs) are at high risk for COVID 19 infection, Government of India gives priority for HCWs for vaccination against COVID 19. Aims and objectives: The present study aims and objectives are to estimate the prevalence and severity of COVID 19 infection in vaccinated HCWs and to determine the severity, course and outcome of COVID 19 infection in vaccinated HCW and to determine the effectiveness of vaccines among HCWs Materials and Methods: Cross-sectional questionnaire-based study conducted between October 2021 and December 2021 among 262 Health CareWorkers (Doctors, CRRIs, Nurses, Paramedical staff) in Government and Private hospitals in South Tamilnadu. Results: Prevalence of covid-19 in vaccinated HCWs was 12.9%. ADR was seen in 19.2% HCWs, mostly mild ADRs like fever, myalgia, headache, etc. SpO2 was more than 93 in 94% HCWs infected with covid, 6% had in between 90-93. Chest CT was taken in 17 HCWs infected with covid, 10 persons had a normal study, 3 persons showed 8% lung involvement, 1person had 32% lung involvement based on CT study. Conclusion: Covid-19 infection was found in 12.9% of the vaccinated people. Vaccination is effective in preventing Covid infection