Genomic Epidemiology of SARS-CoV-2 in Seychelles, 2020-2021.
Morobe JM., Pool B., Marie L., Didon D., Lambisia AW., Makori T., Mohammed KS., de Laurent ZR., Ndwiga L., Mburu MW., Moraa E., Murunga N., Musyoki J., Mwacharo J., Nyamako L., Riako D., Ephnatus P., Gambo F., Naimani J., Namulondo J., Tembo SZ., Ogendi E., Balde T., Dratibi FA., Yahaya AA., Gumede N., Achilla RA., Borus PK., Wanjohi DW., Tessema SK., Mwangangi J., Bejon P., Nokes DJ., Ochola-Oyier LI., Githinji G., Biscornet L., Agoti CN.
Seychelles, an archipelago of 155 islands in the Indian Ocean, had confirmed 24,788 cases of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) by the 31st of December 2021. The first SARS-CoV-2 cases in Seychelles were reported on the 14th of March 2020, but cases remained low until January 2021, when a surge was observed. Here, we investigated the potential drivers of the surge by genomic analysis of 1056 SARS-CoV-2 positive samples collected in Seychelles between 14 March 2020 and 31 December 2021. The Seychelles genomes were classified into 32 Pango lineages, 1042 of which fell within four variants of concern, i.e., Alpha, Beta, Delta and Omicron. Sporadic cases of SARS-CoV-2 detected in Seychelles in 2020 were mainly of lineage B.1 (lineage predominantly observed in Europe) but this lineage was rapidly replaced by Beta variant starting January 2021, and which was also subsequently replaced by the Delta variant in May 2021 that dominated till November 2021 when Omicron cases were identified. Using the ancestral state reconstruction approach, we estimated that at least 78 independent SARS-CoV-2 introduction events occurred in Seychelles during the study period. The majority of viral introductions into Seychelles occurred in 2021, despite substantial COVID-19 restrictions in place during this period. We conclude that the surge of SARS-CoV-2 cases in Seychelles in January 2021 was primarily due to the introduction of more transmissible SARS-CoV-2 variants into the islands.