COVID-19 is a notifiable disease in Ireland under Infectious Diseases Regulations ( Regulations is the Infectious Diseases ( Amendment ) Regulations 2020 ( S.I. No . 53 of 2020 ) . Notifications are registered on the Computerised Infectious Disease Reporting System ( CIDR ) . Data on confirmed COVID-19 cases notified to the HSE Health Protection Surveillance Centre were obtained from covid-19.geohive.ie . The time series of COVID-19 case counts and SARS - CoV-2 wastewater viral concentrations and daily loadings were smoothed using generalised additive models ( GAMs ) ( Wood , 2011 ) . These smoothed time series were then log10 transformed , and a series of first order differences were calculated for each of them ( i.e. time series of changes from one time point to the next ) . Finally , the time series of first order differences of COVID-19 cases was related to those for viral concentration and load using Spearman rank correlations ( ρ ) . This analysis was conducted using R version 4.0.3 and GAMs were fit using the mgcv package ( R Core Team , 2020;Wood , 2011 ) . Additionally , a grid search was employed to consider a range of lead and lag times between SARS - CoV-2 wastewater and case - based data ranging from −6 to 6 weeks ( LaValle et al . , 2004 ) . For statistical analyses , wastewater samples with levels of SARS - CoV-2 N1 below the detection limit of the assay were given the concentration of 1.25 gc / reaction , one quarter of the quantification limit of this assay .