COVID-19: PHAC Modelling Group Report MARCH 24, 2022

Methods The PHAC-McMaster model is used to forecast cases by considering the effect of public health measures in mid- late December (prior to limitations on surveillance data due to the holiday season and changes in testing), ongoing vaccination efforts on SARS-CoV-2 transmission, and the expansion of the Omicron variant. [...] RT-qPCR has been performed on wastewater samples to evaluate the concentration of SARS-CoV-2, as a proxy for the prevalence of infection in the communities included in the catchment areas of the wastewater treatment plants (WWTP). [...] The method, which is described in the annex, uses the reported number of COVID-19 tests and the temporal changes in the susceptible population size due to increasing cumulative case counts and vaccination rates to estimate the actual number of cases for each country. [...] The resulting paucity of data could result in an underestimation of the model results during the Omicron wave, while the country-level underestimation of the number of people previously infected with Omicron (i.e. [...] Given the current limitations and the need for caution in the interpretation of case surveillance data, this report focuses on the experiences of other countries in re-implementation, and now in some instances, the subsequent easing of public health measures, increasing vaccine coverage and expanding booster dose eligibility.