Genome evolution and early introductions of the SARS-CoV-2 Omicron Variant in Mexico

ABSTRACT A new variant of SARS-CoV-2 Omicron (Pango lineage designation B.1.1.529), was first reported to the World Health Organization (WHO) by South African health authorities on November 24, 2021. The Omicron variant possesses numerous mutations associated with increased transmissibility and immune escape properties. In November 2021, Mexican authorities reported Omicron’s presence in the country. In this study, we infer the first introductory events of Omicron and the impact that human mobility can have on the spread of the virus. We also evaluated the adaptive evolutionary processes in Mexican SARS-CoV-2 genomes during the first month of circulation of Omicron. We infer 173 introduction events of Omicron in Mexico in the first two months of detection; subsequently, of the introductions, there was an increase in the prevalence for January. This higher prevalence of the novel variant results in a peak of cases reported, on average, six weeks after a higher mobility trend was reported. The peak of cases reported is due to the BA.1.1 Omicron sub-lineage dominated, followed by BA.1 and BA.15 sub-lineages in the country from January to February 2022. Additionally, we identified the presence of diversifying natural selection in the genomes of Omicron and found mainly five non-synonymous mutations in the RDB domain of the Spike protein, all of them related to evasion of the immune response. In contrast, the other proteins in the genome are highly conserved—however, there are homoplasies mutations in non-structural proteins, indicating a parallel evolution.