Duration of SARS-CoV-2 viral shedding in faeces as a parameter for wastewater-based epidemiology: Re-analysis of patient data using a shedding dynamics model

Abstract

Wastewater-based epidemiology (WBE) is one of the most promising approaches to effectively monitor the spread of COVID-19. The virus concentration in faeces and its temporal variations are essential information for WBE. While some clinical studies have reported SARS-CoV-2 concentrations in faeces, the value varies amongst patients and changes over time. The present study aimed to examine how the temporal variations in the concentration of virus in faeces affect the monitoring of disease incidence. We reanalysed the experimental findings of clinical studies to estimate the duration of virus shedding and the faecal virus concentration. Available experimental data as of 23 October 2020 were collected. The viral shedding kinetics was modelled, and the dynamic model was fitted to the collected data by a Bayesian framework. Using posterior distributions, the duration of viral shedding and the concentration of virus copies in faeces over time were computed. We estimated the median concentration of SARS-CoV-2 in faeces as 3.4 (95% CrI: 0.24-6.5) log copies per gram-faeces over the shedding period, and our model implied that the duration of viral shedding was 26.0 days (95% CrI: 21.7-34.9), given the current standard quantification limit (Ct = 40). With simulated incidences, our results also indicated that a one-week delay between symptom onset and wastewater sampling increased the estimation of incidence by a factor of 17.2 (i.e., 10^1.24 times higher). Our results demonstrated that the temporal variation in virus concentration in faeces affects microbial monitoring systems such as WBE. The present study also implied the need for adjusting the estimates of virus concentration in faeces by incorporating the kinetics of unobserved concentrations. The method used in this study is easily implemented in further simulations; therefore, the results of this study might contribute to enhancing disease surveillance and risk assessments that require quantities of virus to be excreted into the environment.

Keywords: COVID-19; Faeces; Mathematical modelling; SARS-CoV-2; Viral shedding; Wastewater-based epidemiology (WBE).

Graphical abstract

Fig. 1

Fitted model to the observed virus concentration in faeces. Grey plots represent observed data, the blue curve shows the posterior median and the shaded zones are the 95% credible intervals. The dashed line indicates the quantification limit threshold of qPCR (2 log copies/g-faeces). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Estimated virus concentration in faeces (A) and simulated relative incidence caused by changing the delay between wastewater sampling and symptom onset (B). (A) From the left of the panel, violin plots of estimated concentrations in faeces on the day of peak shedding, of the entire shedding period and of raw quantified experimental data reported by Wölfel et al. are visualised (Wölfel et al., 2020). (B) Relative incidences for different sewage samples are plotted, in which we assumed that all of the individuals had the same symptom onset (τ days before sampling, as shown on the x-axis). By definition, the relative incidence equals 1 when the quantitative data in Wölfel et al. are used for the virus concentration in faeces (Wölfel et al., 2020).