An Investigation of a SARS-CoV-2 Omicron Variant Cluster Linked to Nightclubs, Kalasin Province, Thailand, December 2021

Abstract

In December 2021, an outbreak of the SARS-CoV-2 Omicron variant linked to two nightclubs was reported in Kalasin Province, Thailand. This outbreak investigation aimed to understand its epidemiological characteristics, identify sources of infection, assess vaccine effectiveness, and provide control measure recommendations. This study analyzed investigation reports, interviewed all confirmed/probable cases, engaged in active case findings, assessed vaccine effectiveness among high-risk contacts, and surveyed the nightclub environment. This study found 246 confirmed and 338 probable cases, with a median age of 29 years (quartile1–quartile3 25–38 years). Over 40% (271/584) of cases had received at least two doses of vaccination. No severe cases or deaths were reported. Index cases were a Thai couple who traveled from Belgium and visited a nightclub, where the disease spread to 75% (12/16) of nightclub workers before reaching communities. High-risk contacts who received a booster vaccination significantly reduced SARS-CoV-2 infection risk compared with unvaccinated. (vaccine effectiveness 87.5%, 95% confidence interval 59.5–96.1). One of the suspected nightclubs was air-conditioned, crowded, and lacked compliance with measures, which were potential factors contributing to the outbreak. This study strongly recommends that individuals receive booster doses of the SARS-CoV-2 vaccine. Nightclub owners should adhere to COVID-19 public health measures to prevent disease transmission to the communities.

Keywords: COVID-19, SARS-CoV-2 Omicron variants, COVID-19 vaccines, disease outbreaks