Since the COVID-19 vaccination program began on December 14, 2020 and up to June 26, 2021,
a total of 9,864,684 individuals in Ontario received at least one dose of vaccine (Table 2).
Of the almost 9.9 million vaccinated individuals only 0.16% (15,592 individuals including both
symptomatic and asymptomatic cases) became infected when they were partially vaccinated
and only 0.02% (1,635 individuals) became infected when they were fully vaccinated; as these
were fully vaccinated individuals, these are considered breakthrough cases (Table 2).
The majority (51.7%) of post-vaccination cases identified were not yet protected from
vaccination when they acquired their infection, as their symptom onset date was within 0 to <14
days following dose 1 administration (Table 1).
The number of post-vaccination cases declines dramatically as time from vaccination increases.
The number of post-vaccination cases appears to decrease at about 10 days after dose 1. A
marked decrease in post-vaccination cases is observed 28 or more days after dose 1 and very
few cases are reported following dose 2 (Figure 1).
Only 4.6% of cases post-vaccination occurred 14 or more days after dose 2 administration
and are considered breakthrough cases (Table 1).
Trends in partially vaccinated and breakthrough cases are a reflection of both trends in vaccine
administration (increasing number of doses administered over time) and trends in COVID-19
incidence (Figure 2a and 2b).