Following the outbreak of covid-19 in the state of Indiana, the American Centre for Disease Control (CDC) instigated a monitoring program to detect levels of the virus in sewage waste water in 4 neighbouring states (Ohio, Kentucky, Michigan and Illinois) to check the direction of virus spread. In the early stages of the surveillance, wastewater samples of 100 mL were collected from the wastewater treatment plants of 10 major cities within each state, on the same dates over a period of 3 separate weeks, to estimate the viral load for each state. The cities selected for waste water monitoring were randomly selected from a list of all the townships within each state with a population of over 25,000. The virus concentration in each test sample was measured as RNA copies per mL waste water, using real time-PCR. At the same time the CDC monitored the number of cumulative cases of daily covid-19 infection reported in each city (standardized as cases per 100,000 people to account for city population differences). The data for both virus concentration and number of cases were transformed to logarithm (base10) values as the data were skewed (refer to Section 2.1.7 of the textbook).
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