An outbreak investigation of COVID–19 and vaccine efficacy assessment among health care workers in a hospital, Phra Nakhon Sri Ayutthaya Province, Thailand, 27 May–28 June 2021

Abstract

Background: During the Coronavirus 2019 (COVID–19) pandemic, a new wave of viral infections emerged among the staff of one main hospital. The Division of Disease Control in Emergencies, Department of Disease Control, Thailand was notified to initiate a joint investigation. The objectives were to confirm the outbreak and diagnosis, describe its characteristics, identify its risk factors and source, and provide recommendations for further prevention.
Methods: An ambidirectional cohort study was conducted. Medical records and situation outbreaks were reviewed to collect descriptive data and identify associated factors for the outbreak and potential correlated environmental factors. Additionally, prospective data collection involved a self–assessment survey, interviews with confirmed cases, and laboratory studies to identify viral serotypes.
Results: A total of 28 COVID–19 cases were confirmed among hospital staff, which accounted for 1.65% of the healthcare workers (HCW) among these cases, 22 were asymptomatic (78.57%). Multivariable analysis indicates that working with multiple departments significantly increased the risk of disease transmission (Adjusted Odds Ratio 44.77, 95%CI 12.83–156.26), In contrast, receiving 2 doses of the vaccine within a period of less than 4 week was found to be a significant protective factor (Adjusted Odds Ratio 0.15, 95%CI 0.05–0.48). The protective factor yielded an 85% vaccine effectiveness (95%CI 52%–95%). Moreover, it was observed that the lunch break and joint lunch periods were associated with the highest frequency of social distancing and personal protective protocol violations.
Conclusions: A cluster of COVID–19 outbreak among hospital staff was identified. The root causes of the epidemic were found to be associated with environmental factors and working behavior. The double–boost COVID-19 vaccine was identified as a protective factor against disease transmission. Hospitals may consider implementing a vaccine-boost protocol to prevent in–hospital outbreaks.