Table 3:

Period-specific seroprevalence and surveillance underascertainment ratio estimates between the sixth and seventh and the seventh and eighth serosurveys

VariableAge group, yr
0–45–910–1920–2930–3940–4950–5960–6970–79≥ 80All ages
General population estimates (9)*
2021142 779149 664314 910473 049495 620419 465444 663382 294243 627137 6723 203 743
2022142 825150 102317 492472 965507 929425 441442 950392 127254 142143 1043 249 077
Between sixth and seventh serosurveys (September–October 2021 to March 2022)
Case reports, Sept. 26, 2021–Mar. 12, 202252655296766715 58717 09713 50210 33967833545385688 944
Seroprevalence §, % (95% CrI)49.6 (41.6–57.4)54.7 (47.4–61.9)44.7 (37.1–52.2)40.8 (33.4–48.2)43.9 (36.5–51.1)34.4 (27.2–42.1)23.5 (16.7–30.5)19.1 (12.9–25.4)6.1 (0.7–11.8)9.7 (4.4–15.5)33.1 (30.5–35.8)
SUAR*§ (95% CrI)13.4 (11.3–15.5)15.5 (13.3–17.6)19.3 (15.8–22.6)13.2 (10.7–15.7)13.2 (11.0–15.3)10.8 (8.4–13.4)10.8 (7.7–14.1)10.7 (7.3–14.4)5.1 (1.9–9.2)3.4 (1.7–5.3)12.1 (11.0–13.2)
Between seventh and eighth serosurveys (March to July–August 2022)
Case reports, March 13–July 30, 2022824135246120617451181134115342082379014 085
Seroprevalence §, % (95% CrI)9.8 (0.2–19.4)4.53 (4.2–13.3)20.1 (11.6–28.6)19.8 (10.5–29.1)10.2 (0.9–19.4)19.6 (10.4–28.9)26.8 (17.9–35.8)17.4 (8.7–25.9)28.8 (20.7–36.5)22.5 (14.6–30.5)18.6 (15.4–21.9)
SUAR*§, (95% CrI)23.9 (10.6–39.7)115.8 (25.2–261.7)313.1 (192.6–434.1)101.2 (60.8–146.9)39.6 (17.0–68.8)78.4 (44.6–113.6)95.4 (63.5–128.3)44.7 (25.1–64.9)33.1 (23.6–42.1)8.6 (5.8–11.8)91.9 (75.2–110.2)
  • Note: ALF = assisted-living facility, CrI = credible interval, FHA = Fraser Health Authority, ILF = independent-living facility, LTCF = long-term care facility, SUAR = surveillance underascertainment ratio, VCHA = Vancouver Coastal Health Authority.

  • See Appendix 1, Supplementary Material 4 for methodological details related to SUAR estimation.

  • * Population census estimates for FHA and VCHA combined. Population census estimates include LTCF and ALF or ILF residents, whereas serosurvey sampling and surveillance case report tallies excluded these individuals. An estimated 25 000 Lower Mainland adults aged 65 years and older may reside in these settings (Appendix 1, Supplementary Material 4, available at www.cmaj.ca/lookup/doi/10.1503/cmaj.221335/tab-related-content). Prisoners were included in both population census estimates and case reports but represent a small proportion of the BC population overall.

  • Surveillance case reports from FHA and VCHA combined, according to episode date, hierarchically defined by onset date or, if not available, by specimen collection date or, if not available, then test result date. Excludes out-of-province cases. Cases identified as residents of LTCFs, ALFs or ILFs were also excluded, but this may have been incomplete, especially for the last period, owing to variation in surveillance processes.

  • Infection-induced seroprevalence estimates based on dual-assay positivity, of which at least 1 positive assay must include anti-nucleocapsid protein detection. Period-specific seroprevalence estimates represent the new infection rate between specified serosurveys, assuming no waning or reinfections.

  • § Assuming no waning or reinfections during the specified analysis period.

  • Bayesian estimates standardized for age, sex and health authority.