This is a summary, written by members of the CITF Secretariat, of:

D’Aoust PM, Tian X, Towhid ST, Xiao A, Mercier E, Hegazy N, Jia JJ, Wan S, Kabir MP, Fang W, Fuzzen M, Hasing M, Yang MI, Sun J, Plaza-Diaz J, Zhang Z, Cowan A, Eid W, Stephenson S, Servos MR, Wade MJ, MacKenzie AE, Peng H, Edwards EA, Pang XL, Alm EJ, Graber TE, Delatolla R. Wastewater to clinical case (WC) ratio of COVID-19 identifies insufficient clinical testing, onset of new variants of concern and population immunity in urban communities. Sci Total Environ. 2022 Dec 20;853:158547. doi: 10.1016/j.scitotenv.2022.158547.

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

A CITF-funded study, published in Science of the Total Environment, demonstrated that the ratio of SARS-CoV-2 wastewater signals to clinical cases (WC ratio) would add more value to wastewater-based surveillance (WBS). The WC ratio could serve as an additional diagnostic tool during the COVID-19 pandemic and help in future pandemics. This study strongly suggests that regular, daily monitoring of the WC ratio can reveal and detect the onset of changes in disease transmission patterns, and the arrival/onset and waning of more infectious variants or mutations of a pathogen or disease. The authors suggest the WC ratio should be an additional monitoring metric to identify important epidemiological occurrences, complementing clinical case counts and wastewater signals. It was led by Dr. Robert Delatolla (University of Ottawa) with CITF funding directed towards CITF-funded researcher Dr. Sharon Strauss (University of Toronto) and was done in collaboration with Dr. Xiaoli Pang (University of Alberta and Alberta Precision Laboratories).

The SARS-CoV-2 wastewater signal to clinical cases (WC) ratio was investigated across seven cities in Canada over periods ranging from eight to 21 months.

Key findings:

  • The study demonstrated that:
    • Significant increases in the WC ratio occurred when disease burden increased in the presence of constrained access to clinical testing. For example, when acute infection PCR testing became by appointment-only in 5 of the 7 cities, there was a marked decrease in new detected clinical cases due to decreased access to PCR testing, at a time when wastewater signals were increasing;
    • The WC ratio decreased significantly in six of the seven studied locations, serving as a potential signal of the emergence of the Alpha variant of concern (VOC) in a relatively non-immunized community (40–60% allelic proportionThe allelic proportions/percentage of different SARS-CoV-2 VOCs were calculated by dividing measured VOC-associated viral copies/g by the universal/non-mutated equivalent genomic region viral copies/g.);
    • A more muted decrease in the WC ratio signaled the emergence of the Delta VOC in a relatively well-immunized community (40–60 % allelic proportion);
    • A significant decrease in the WC ratio signaled the emergence of the Omicron VOC, leading to a significant number of new reported clinical cases, even when community immunity was high.
  • The WC ratio, used as an additional monitoring metric, could complement the individual metrics of clinical case counts and wastewater signals. The WC ratio has the potential ability to identify important epidemiological occurrences, adding value as an additional diagnostic tool during the COVID-19 pandemic that could also serve in future pandemics.