The year 2021 was marked by the emergence of two variants of SARS-CoV-2 which were responsible for significant upsurges in the pandemic: Delta and Omicron.
The Delta variant was first identified in October 2020 in India, where the pandemic has caused the worst humanitarian tragedy since the country’s independence. Due to its greater transmissibility, Delta quickly supplanted all previous variants: Alpha, Beta, detected in South Africa, and Gamma, detected in Brazil. Delta has been found to be twice as contagious as any other variant that has appeared so far, including the original strain identified in Wuhan, China. A case infected with the Delta variant generates twice as many secondary infections as previous strains. While a person infected with the original version of the virus could transmit to two or three other people, an individual carrying Delta can in turn infect five to six people.
The Delta variant has a mutation that allows it to make copies of itself much more efficiently and quickly than the strains that came before it, which means that people who are infected with this variant have much higher viral loads.
Delta is also more virulent than the previous variants and the ancestral strain: it carries a risk of hospitalization, admission to intensive care and death two to three times higher than other versions of SARS-CoV-2. Even if two doses of an mRNA vaccine are a little less effective in preventing infections induced by the Delta variant, their administration with an eight week delay – between the two doses – as has been done in Quebec still offers an advantage. 80% protection against infection and 95% protection against hospitalization six months after the second dose.
The Delta variant was present in 99% of the cases reported and sequenced worldwide until the appearance of the Omicron variant, which was first detected in South Africa and Botswana, and which was reported to the Organization. World Health Organization (WHO) on November 24.
Omicron has a record number of mutations compared to the ancestral strain. Some of these mutations give it an even greater transmissibility than Delta. Very contagious, Omicron is spreading at breakneck speed: in barely a week, between December 12 and 18, 2021, it had become responsible for 80% of infections in Quebec.
In addition, it appears obvious that Omicron escapes the immunity generated by a previous infection with COVID-19, as well as that induced by vaccination, as shown by the multiple cases of reinfection and infections contracted by doubly vaccinated people. However, laboratory studies have shown that giving a booster dose (third dose) generates enough antibodies to neutralize Omicron. It is not known, however, how long this immunity will persist.