12.
Vaccine effectiveness was maintained for the Alpha variant that caused the
third wave, and for the Delta variant that caused the fourth wave in Ontario. For all
strains of the SARS-CoV-2 virus that were dominant in Ontario before December
2021 (the original SARS-CoV-2 virus, the Alpha and Delta variants), a vaccinated
person was therefore much less likely to transmit COVID-19 to others, including
unvaccinated children and vulnerable persons.
13.
At the end of November 2021, the Omicron variant was detected in Ontario,
and quickly became the dominant variant in the province. Omicron is extremely
transmissible and spreads much faster than the original strain and previous variants.
Unfortunately, Omicron is also better at evading the immune system and therefore
vaccine protection against infection with 2 doses of an mRNA vaccine has been
reduced significantly with Omicron. Despite this, as of March 31, 2022, the
estimated daily rate of reported cases per million amongst unvaccinated Ontarians
was 252.3, whereas the estimated daily rate of reported cases per million amongst
Ontarians vaccinated with at least two doses of a COVID-19 vaccine was 168.1. This
represents a 33.4% reduction in risk of infection associated with vaccination with at
least two doses.
18.
At least 2 doses of a COVID-19 vaccine continue to be effective in reducing
the risk of hospital admission, ICU admission and death also after Omicron became
dominant in Ontario and globally. As of March 30, 2022, the estimated number of
COVID-19 cases in hospital per million amongst unvaccinated Ontarians was 226.4,
whereas the estimated number of COVID-19 cases in hospital per million amongst
Ontarians vaccinated with at least two doses of a COVID-19 vaccine was 45.2. This
represents a 80.0% reduction in risk of hospitalization associated with vaccination
with at least two doses. The estimated number of COVID-19 cases in intensive care
per million amongst unvaccinated Ontarians was 50.8, whereas the estimated
number of COVID-19 cases in intensive care per million amongst Ontarians
vaccinated with at least two doses of a COVID-19 vaccine was 8.5. This represents
a 83.2% reduction in risk of ICU stay associated with vaccination with at least two
doses. The reductions in the risks of hospitalization and ICU stay were even higher
when the Delta variant was dominant in Ontario in August to November 2021: an
average 96% reduction in the risk of hospitalization, and an average of 98%
reduction in the risk of ICU stay. The reduction in the risk of hospital admission,
ICU admission and death is high after two doses, and is even more pronounced after
3 doses of a COVID-19 vaccine. In a recent analysis from Kaiser Permanente
Southern California, the protection against hospitalization with Omicron was 84.5%
after 2 doses of a COVID-19 vaccine, and 99.2% after 3 doses.
20.
There is an Omicron subvariant, BA.2, which has become the dominant
subvariant in Denmark and is becoming dominant in Canada and globally. This
subvariant shows increased transmissibility (spread more easily) than the original
Omicron variant. According to a recent study performed in Danish households,
increased transmissibility of BA.2, as compared with the original Omicron variant,
was only seen in unvaccinated individuals, but not in individuals who had received
2 doses of a COVID-19 vaccine, nor in individuals who had received 3 doses of a
COVID-19 vaccine. This suggests that COVID-19 vaccination will continue to be
important for the control of BA.2 in Ontario, not only after 3 doses were received,
but also after 2 doses were received.
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