Emergence and Persistent Dominance of SARS-CoV-2 Omicron BA.2.3.7 Variant, Taiwan

T he COVID-19 pandemic, caused by SARS-CoV-2, originated in China in late 2019, probably in the city of Wuhan (1,2).The outbreak of this unusual respiratory disease led to a wide variety of responses by various countries across the world (3-6).The response in Taiwan was rapid and based on both its proximity to China and its experiences during the SARS pandemic ≈2 decades earlier (5,7,8).The introduction of strict travel restrictions on incoming air and sea passengers, long compulsory quarantine periods for the few residents allowed to enter Taiwan, and a vast public acceptance of safety measures (e.g., social distancing, temperature checks, mask wearing) resulted in a delay in the emergence of the COVID-19 pandemic in Taiwan compared with other countries (5,9,10).Until April 2022, there were only limited outbreaks, all of which were quickly contained.Taiwan therefore provides a unique opportunity to explore what happened when the Omicron variant finally evaded the controls put in place by the Taiwan government and began to spread through the country's population.Residents of Taiwan had not been exposed, on a large scale, to any of the virus variants before Omicron.By the time SARS-CoV-2 began to spread widely in Taiwan April 2022, there had been around 17,000 recorded cases of COVID-19 in the country, and most of them were linked to the Alpha variant (almost all cases in our study had not been infected with SARS-CoV-2 before).Vaccination rates of Taiwan's population at that time were 82.7% having received 1 dose, 78% having received 2 doses, and 59.1% having received 3 doses.The vaccines used in Taiwan before May 2022 were the Oxford-AstraZeneca vaccine (https://www.astrazeneca.com),the Pfizer-BioN-Tech vaccine (https://www.pfizer.com), the Moderna vaccine (https://www.modernatx.com), the Johnson & Johnson/Janssen vaccine (https://www.jandj.com), and The Median vaccine (a protein subunit COVID-19 vaccine made in Taiwan).Most residents of Taiwan received doses of the first 3 vaccines.