[The different phases of molecular and antigenic evolution of SARS-CoV-2 viruses during the 20 months following its emergence].

From its emergence in December 2019 and until the end of the fourth pandemic wave in October 2021, SARS-CoV-2 circulation has been associated with significant molecular evolutions of the virus. These were linked to mutations that have led to new virus linages with replication advantages as a result of increased transmission, or partial immune escape in the context of progressively increasing global immunisation. The pandemic context with large scale epidemics massive outbreaks observed in highly populated areas has favoured this emergence of "variants". During the 20 months period, at least three evolutionary phases have been observed, leading to the situation observed in October 2021. For the first time, an unprecedented worldwide surveillance effort has been conducted to monitor the circulation of the emerging virus, with rapid data sharing. This molecular surveillance system has provided an accurate description of the circulating viruses, and their evolution. The implementation of these tools and skills able to provide SARS-CoV-2 molecular epidemiological data has upgraded the global capacity for surveillance worldwide, and may allow us to be better prepared for a future pandemic episode.