Late in November 2021, World Health Organization (WHO) designated the B.1.1.529 lineage of SARS-CoV-2 as Variant of Concern (VOC) Omicron. The swift rise of Omicron and displacement of previous lineages of SARS-CoV-2 across the world, particularly Delta, was suggestive that Omicron not only had a fitness advantage over Delta but also had the ability to infect people who had prior immunity to Delta or any previous lineages of the virus.
Three sister lineages
During its designation in 2021, VOC Omicron initially comprised three sister lineages: BA.1, BA.2 and BA.3, although each lineage differs significantly from the other in terms of its mutations. While lineage BA.3 remained relatively rare, lineages BA.1 and BA.2 were primarily responsible for the upsurge in COVID-19 cases across different countries, and also causing infections in fully vaccinated individuals — otherwise known as vaccine breakthrough infections. Over the time, BA.2 became the predominant Omicron lineage worldwide.
