Long-term SARS-CoV-2 surveillance in wastewater and estimation of COVID-19 cases: An application of wastewater-based epidemiology

6-plex Crystal Digital PCR® for comprehensive surveillance of respiratory and foodborne bacterial pathogens in wastewater

Bacterial wastewater surveillance (WS) is less explored area compared to viral WS despite high burden of bacterial respiratory and gastrointestinal infections worldwide. This study established a 6-plex Crystal Digital PCR® (cdPCR) system, to comprehensively monitor an acute respiratory pathogen - Group A Streptococcus (GAS) pyogenes, foodborne disease (FBD) pathogens - Clostridium perfringens, Salmonella spp., Campylobacter jejuni, and Campylobacter coli, and an indicator bacterium, Escherichia coli in wastewater. Fifty-two grab influent samples collected weekly from a wastewater treatment plant in Yamanashi Prefecture, Japan, between June 2023 and May 2024 were centrifuged, followed by DNA extraction and cdPCR. cdPCR was performed using the naica® system (Stilla Technologies). The 6-plex cdPCR assays showed strong performance. Among the 52 samples, 100 % of samples were positive for C. perfringens, 98 % for Salmonella spp., 56 % for C. jejuni, 25 % for C. coli, and 63 % for S. pyogenes, with concentrations ranging between 4.2 ± 0.3 to 7.5 ± 0.2 log10 copies/L. The concentration of C. perfringens was significantly higher than that of other pathogens (p < 0.05), indicating its dominance. Salmonella spp. had high detection rate, implying increased Salmonella infection in the population. Seasonal variation was not observed in any of FBD pathogens, except for the detection rate of C. coli. S. pyogenes concentrations were significantly higher in spring than in other seasons, agreeing with the trend of GAS pharyngitis cases in the catchment. In conclusion, the 6-plex cdPCR system is a valuable tool for comprehensive WS, offering significant implications for public health monitoring.

Evaluation of the Enterovirus serotype monitoring approach for wastewater surveillance of hand foot and mouth disease using secondary epidemiological surveillance data

The serotypes of Enterovirus A (EVA) causing hand, foot, and mouth disease (HFMD) exhibit cyclical dominance, with the dominant serotypes being EVA71, Coxsackievirus A type 6 (CVA6), CVA10, and CVA16. In this study, a quadruplex digital polymerase chain reaction (dPCR) assay was developed for serotype-level monitoring in wastewater surveillance for HFMD. The quadruplex dPCR assay performance was comparable to that of the singleplex assays. Grab influent samples (n = 122) were collected weekly from a wastewater treatment plant in Yamanashi Prefecture, Japan, from March 2022 to July 2024. The dPCR results revealed that CVA6 had a lower prevalence in 2023 than in 2022 and 2024, while the detection ratio of EVA71 was significantly higher in 2023 than in 2022 (Chi-square test, P < 0.05). Triangulation of these results with a clinical report indicated the dominance of CVA6 in 2022 and 2024, but not in 2023, while EVA71 was found to be dominant in 2023. Secondary data revealed significantly higher HFMD cases in the catchment in the years when CVA6 was dominant and significantly lower in the years when EVA71 was dominant. The comparative analysis revealed a stronger correlation between CVA6 RNA concentrations and HFMD cases (r = 0.83) than that between a broad range of Enteroviruses, Pan-Enterovirus RNA concentrations (r = 0.47). Thus, serotype-level monitoring, particularly through quadruplex dPCR, provides valuable insights into EVA serotype dynamics, accurately reflects HFMD trends, and supports the monitoring of the cyclical patterns of EVA serotypes.

Evaluation of plasmid pBI143 for its optimal concentration methods, seasonal impact, and potential as a normalization parameter in wastewater-based epidemiology

Plasmid pBI143, abundant in the human gut, is a promising human-specific fecal marker. However, studies on its optimal concentration methods, seasonal variations, and potential as a normalization parameter for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19), remain limited. Among the three concentration methods compared, polyethylene glycol (PEG) precipitation and centrifugation demonstrated comparable efficiencies (9.3 ± 0.6 and 9.2 ± 0.6 log10 copies/L, respectively; n = 8 each), outperforming membrane filtration (8.0 ± 0.6 log10 copies/L; n = 8). PEG precipitation was further applied to quantify pBI143, together with other human-specific fecal markers (crAssphage and pepper mild mottle virus (PMMoV)), in 52 wastewater samples collected weekly over a one year from a wastewater treatment plant in Yamanashi Prefecture, Japan, by quantitative polymerase chain reaction. The higher pBI143 concentrations (9.6 ± 0.5 log10 copies/L) compared to PMMoV (8.2 ± 0.2 log10 copies/L) and crAssphage (8.0 ± 0.2 log10 copies/L) highlighted its potential as a robust marker for human fecal contamination. Unlike PMMoV and crAssphage that remained stable across seasons, pBI143 showed seasonal fluctuations, especially during summer and autumn, suggesting its greater sensitivity to environmental conditions. The study evaluated the suitability of pBI143, crAssphage, and PMMoV for normalizing SARS-CoV-2 concentrations in wastewater; however, non-normalized SARS-CoV-2 concentrations showed the highest correlation with COVID-19 cases (ρ = 0.74), whereas normalization reduced this correlation (PMMoV-normalized, ρ = 0.72; crAssphage-normalized, ρ = 0.70; and pBI143-normalized, ρ = 0.50), likely due to differences in the persistence and structural properties of the markers, indicating that these markers are less effective for SARS-CoV-2 normalization. This study underscores the promising utility of pBI143 in wastewater surveillance but highlights the need for further research across diverse regions to validate its applicability.

Improved entropy-CRITIC population model based on temporal and spatial variability: Construction and application in wastewater epidemiology

Numerous factors contribute to the uncertainty inherent in conducting wastewater-based epidemiology (WBE), with shifting populations exerting a significant influence. However, traditional single- and multi-parameter population models suffer from certain limitations. This study employs an evaluation model framework to construct a model (EC model) based on data characteristics. Weight coefficients derived from 16 cities across seven regions of China are aggregated into a national model. In contrast to alternative models, the EC model exhibits a robust correlation (r2 = 0.98) with census population data, suggesting a potentially more precise depiction of population dynamics. The low variability (RSD = 9.73 %) indicates effective constraint of anomalous parameter fluctuations, yielding minimal Bias (-1.12 %) and SRMSE (14.75 %), thus ensuring reliable population estimation. The model is applied to estimate the consumption of lifestyle-related compounds and the prevalence of hypertension in China. Northern regions demonstrate higher consumption levels, alongside a significant disparity in hypertension prevalence (26.96 %) compared to the south (16.01 %). Hypertension exhibits positive correlations with lifestyle-related compounds such as alcohol and nicotine (r = 0.52, r = 0.55). Sensitivity analysis reveals that the EC model introduces an uncertainty of 24.48 % in population estimates. Through the incorporation of representative datasets and novel algorithms, this model has the potential to enhance the reliability of outcomes in WBE strategy implementation.

Assessment of environmental factors influencing SARS-CoV-2 in Vietnam's surface water across two years of clinical data

Wastewater-based epidemiology (WBE) is an effective, non-invasive method for monitoring the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) by tracking viral prevalence in water. This study aimed to investigate the presence of SARS-CoV-2 in surface water in Vietnam over two years. One-step quantitative reverse transcription polymerase chain reaction (qRT-PCR) assays were employed to quantify SARS-CoV-2 and its variant-specific mutation sites (G339D/E484A) and pepper mild mottle virus (PMMoV) from a total of 315 samples (105 samples per site) to compare with reported Coronavirus disease 2019 (COVID-19) cases and environmental factors. SARS-CoV-2 was detected in 38 % (40/105), 43 % (45/105), and 39 % (41/105) of water samples from Sites A, B, and C, respectively, with concentrations of 3.0-5.6 log10 copies/L. PMMoV concentrations were 5.1-8.9 log10 copies/L. SARS-CoV-2 levels were higher in winter compared with summer. There was a strong positive association between the mutant type and SARS-CoV-2 concentrations (Spearman's rho = 0.77, p < 0.01). The mean concentrations of mutant and nonmutant types were 2.3 and 1.8 log10 copies/L, respectively. Peaks in SARS-CoV-2 concentrations preceded reported COVID-19 cases by 2-4 weeks, with the highest association observed at a 4-week delay (Pearson's correlation coefficient: 0.46-0.53). Environmental factors, including temperature, pH, and electrical conductivity, correlated negatively with SARS-CoV-2 (Spearman's rho = -0.21, -0.28, and -0.21, respectively, p < 0.05), whereas average rainfall, humidity, and dissolved oxygen correlated positively (Spearman's rho = 0.20, 0.27, and 0.51, respectively, p < 0.05). These correlations highlight the significance of environmental variables in understanding viral prevalence in water. Our findings confirmed the utility of WBE as an early warning system for long-term monitoring. Future research should incorporate environmental factors to improve prediction accuracy for clinical cases and other waterborne diseases.

Beyond COVID-19: Wastewater-based epidemiology for multipathogen surveillance and normalization strategies

Wastewater-based epidemiology (WBE) is a critical tool for monitoring community health. Although much attention has focused on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a causative agent of coronavirus disease 2019 (COVID-19), other pathogens also pose significant health risks. This study quantified the presence of SARS-CoV-2, influenza A virus (Inf-A), and noroviruses of genogroups I (NoV-GI) and II (NoV-GII) in wastewater samples collected weekly (n = 170) from July 2023 to February 2024 from five wastewater treatment plants (WWTPs) in Yamanashi Prefecture, Japan, by quantitative PCR. Inf-A RNA exhibited localized prevalence with positive ratios of 59 %-82 % in different WWTPs, suggesting regional outbreaks within specific areas. NoV-GI (94 %, 160/170) and NoV-GII (100 %, 170/170) RNA were highly prevalent, with NoV-GII (6.1 ± 0.8 log10 copies/L) consistently exceeding NoV-GI (5.4 ± 0.7 log10 copies/L) RNA concentrations. SARS-CoV-2 RNA was detected in 100 % of the samples, with mean concentrations of 5.3 ± 0.5 log10 copies/L in WWTP E and 5.8 ± 0.4 log10 copies/L each in other WWTPs. Seasonal variability was evident, with higher concentrations of all pathogenic viruses during winter. Non-normalized and normalized virus concentrations by fecal indicator bacteria (Escherichia coli and total coliforms), an indicator virus (pepper mild mottle virus (PMMoV)), and turbidity revealed significant positive associations with the reported disease cases. Inf-A and NoV-GI + GII RNA concentrations showed strong correlations with influenza and acute gastroenteritis cases, particularly when normalized to E. coli (Spearman's ρ = 0.70-0.81) and total coliforms (ρ = 0.70-0.81), respectively. For SARS-CoV-2, non-normalized concentrations showed a correlation of 0.61, decreasing to 0.31 when normalized to PMMoV, suggesting that PMMoV is unsuitable. Turbidity normalization also yielded suboptimal results. This study underscored the importance of selecting suitable normalization parameters tailored to specific pathogens for accurate disease trend monitoring using WBE, demonstrating its utility beyond COVID-19 surveillance.

Wastewater Surveillance of SARS-CoV-2 in Zambia: An Early Warning Tool

Wastewater-based surveillance has emerged as an important method for monitoring the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). This study investigated the presence of SARS-CoV-2 in wastewater in Zambia. We conducted a longitudinal study in the Copperbelt and Eastern provinces of Zambia from October 2023 to December 2023 during which 155 wastewater samples were collected. The samples were subjected to three different concentration methods, namely bag-mediated filtration, skimmed milk flocculation, and polythene glycol-based concentration assays. Molecular detection of SARS-CoV-2 nucleic acid was conducted using real-time Polymerase Chain Reaction (PCR). Whole genome sequencing was conducted using Illumina COVIDSEQ assay. Of the 155 wastewater samples, 62 (40%) tested positive for SARS-CoV-2. Of these, 13 sequences of sufficient length to determine SARS-CoV-2 lineages were obtained and 2 sequences were phylogenetically analyzed. Various Omicron subvariants were detected in wastewater including BA.5, XBB.1.45, BA.2.86, and JN.1. Some of these subvariants have been detected in clinical cases in Zambia. Interestingly, phylogenetic analysis positioned a sequence from the Copperbelt Province in the B.1.1.529 clade, suggesting that earlier Omicron variants detected in late 2021 could still be circulating and may not have been wholly replaced by newer subvariants. This study stresses the need for integrating wastewater surveillance of SARS-CoV-2 into mainstream strategies for monitoring SARS-CoV-2 circulation in Zambia.

Wastewater surveillance for viral pathogens: A tool for public health

A focus on water quality has intensified globally, considering its critical role in sustaining life and ecosystems. Wastewater, reflecting societal development, profoundly impacts public health. Wastewater-based epidemiology (WBE) has emerged as a surveillance tool for detecting outbreaks early, monitoring infectious disease trends, and providing real-time insights, particularly in vulnerable communities. WBE aids in tracking pathogens, including viruses, in sewage, offering a comprehensive understanding of community health and lifestyle habits. With the rise in global COVID-19 cases, WBE has gained prominence, aiding in monitoring SARS-CoV-2 levels worldwide. Despite advancements in water treatment, poorly treated wastewater discharge remains a threat, amplifying the spread of water-, sanitation-, and hygiene (WaSH)-related diseases. WBE, serving as complementary surveillance, is pivotal for monitoring community-level viral infections. However, there is untapped potential for WBE to expand its role in public health surveillance. This review emphasizes the importance of WBE in understanding the link between viral surveillance in wastewater and public health, highlighting the need for its further integration into public health management.

Group A Streptococcus pyogenes in wastewater: Applicability of wastewater-based epidemiology for monitoring the prevalence of GAS pharyngitis during the late COVID-19 pandemic phase

Streptococcus pyogenes, Group A Streptococcus (GAS), is a human pathogen that causes a spectrum of diseases from mild to severe, including GAS pharyngitis, a common acute respiratory disease in developed countries. Although wastewater-based epidemiology (WBE) has been extensively used to monitor viral pathogens such as severe acute respiratory syndrome coronavirus 2, its applicability to S. pyogenes remains unexplored. This study was conducted to investigate the feasibility of detecting and quantifying S. pyogenes in wastewater by quantitative polymerase chain reaction (qPCR) and evaluate the applicability of WBE for monitoring the prevalence of GAS pharyngitis. A total of 52 grab influent samples were collected from a wastewater treatment plant in Japan once a week between March 2023 and February 2024. The samples were centrifuged, followed by nucleic acid extraction and qPCR for the S. pyogenes-specific genes speB and spy1258. Of the 52 samples, 90 % and 81 % were positive for speB and spy1258 genes, respectively, indicating the feasibility of S. pyogenes for wastewater surveillance. However, the percentage of quantifiable samples for speB gene was significantly higher in winter than in spring and summer. Similarly, the concentrations of both genes in wastewater samples were significantly higher in winter (speB, 4.1 ± 0.27 log10 copies/L; spy1258, 4.1 ± 0.28 log10 copies/L; One-way ANOVA, p < 0.01) than in spring and summer. Higher concentrations and detection ratios of S. pyogenes genes were observed during increased GAS pharyngitis cases in the catchment. Significant moderate correlations were observed between target gene concentrations and reported GAS pharyngitis cases. This study enhances the understanding role of WBE in monitoring and managing infectious diseases within communities.

Evaluating acute gastroenteritis-causing pathogen reduction in wastewater and the applicability of river water for wastewater-based epidemiology in the Kathmandu Valley, Nepal

Rapid urbanization and population growth without the implementation of proper waste management are capable of contaminating water sources, which can lead to acute gastroenteritis. This study examined the detection and reduction of five gastroenteritis-causing enteropathogens, Salmonella, Campylobacter coli, Campylobacter jejuni, Clostridium perfringens, and genogroup IV norovirus, and one respiratory pathogen, influenza A virus, in two municipal wastewater treatment plants (WWTP) using an oxidation ditch system (WWTP A; n = 20) and a stabilization pond system (WWTP B; n = 18) in the Kathmandu Valley, Nepal, collected between August 2017 and August 2019. All enteropathogens were detected in wastewater via quantitative PCR. The concentrations of the pathogens ranged from 5.7 to 7.9 log10 copies/L in WWTP A and from 4.9 to 8.1 log10 copies/L in WWTP B. The log10 reduction values of the pathogens ranged from 0.3 to 1.0 in WWTP A and from -0.1 to 0.2 in WWTP B. The association between the pathogen concentrations and the number of clinical cases in the corresponding week could not be evaluated; however, the consistent detection of pathogens in the wastewater despite low number of case reports suggested the use of wastewater-based epidemiology (WBE) for early warning of acute gastroenteritis (AGE) in the Kathmandu Valley. The pathogens were also detected in river water at approximately 7.0 log10 copies/L and exhibited no significant difference in concentration compared to wastewater, suggesting the applicability of river water for WBE of AGE. Insufficient treatment of all pathogens in the wastewater was observed, suggesting the need for full rehabilitation of the treatment plants. However, the influent may be utilized for early detection of AGE-causing pathogens in the city, whereas the river water may serve as an alternative in areas without connection to the WWTPs.

Detection of SARS-CoV-2 and Omicron variant RNA in wastewater samples from Manila, Philippines

Manila, a highly urbanized city, is listed as one of the top cities with the highest recorded number of coronavirus disease 2019 (COVID-19) cases in the Philippines. This study aimed to detect and quantify the RNA of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the Omicron variant in 51 wastewater samples collected from three locations in Manila, namely Estero de Santa Clara, Estero de Pandacan, which are open drainages, and a sewage treatment plant (STP) at De La Salle University-Manila, between July 2022 and February 2023. Using one-step reverse transcription-quantitative polymerase chain reaction, SARS-CoV-2 and Omicron variant RNA were detected in 78 % (40/51; 4.9 ± 0.5 log10 copies/L) and 60 % (24/40; 4.4 ± 0.3 log10 copies/L) of wastewater samples collected from all sampling sites, respectively. SARS-CoV-2 RNA was detected frequently at Estero de Santa Clara (88 %, 15/17); its highest concentration was at the STP (6.3 log10 copies/L). The Omicron variant RNA was present in the samples collected (4.4 ± 0.3 log10 copies/L) from all sampling sites, with the highest concentration at the STP (4.9 log10 copies/L). Regardless of normalization, using concentrations of pepper mild mottle virus RNA, SARS-CoV-2 RNA concentrations exhibited the highest positive correlation with COVID-19 reported cases in Manila 5 days after the clinical report. These findings revealed that wastewater-based epidemiology may aid in identifying and monitoring of the presence of pathogens in open drainages and STPs in the Philippines. This paper provides the first documentation on SARS-CoV-2 and the Omicron variant in wastewater from Manila.

Optimization of the 5-plex digital PCR workflow for simultaneous monitoring of SARS-CoV-2 and other pathogenic viruses in wastewater

Wastewater-based epidemiology is a valuable tool for monitoring pathogenic viruses in the environment, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19). While quantitative polymerase chain reaction (qPCR) is widely used for pathogen surveillance in wastewater, it can be affected by inhibition and is limited to relative quantification. Digital PCR (dPCR) offers potential solutions to these limitations. In this study, a 5-plex dPCR workflow was optimized for the simultaneous detection of SARS-CoV-2, influenza A virus, enteroviruses (EnV), and noroviruses of genogroups I (NoV-GI) and GII (NoV-GII) in wastewater samples. Wastewater samples (n = 36) were collected from a wastewater treatment plant in Japan between August and October 2022. The optimization included the evaluation of singleplex and 5-plex dPCR assays, and two different concentration methods, extraction kits, and dPCR approaches. The performance of singleplex and 5-plex dPCR assays showed comparable linearity and reliability, with the 5-plex assays showing greater efficiency. The polyethylene glycol (PEG) precipitation method showed better performance over the centrifugation method, two-step reverse transcription (RT)-dPCR over the one-step RT-dPCR, and AllPrep PowerViral DNA/RNA Kit showed better performance than the QIAamp Viral RNA Mini Kit. The optimal workflow therefore included PEG precipitation, the AllPrep PowerViral DNA/RNA Kit, and two-step RT-dPCR. This workflow was selected to monitor the presence of SARS-CoV-2 and other pathogenic viruses in wastewater samples in a 5-plex dPCR approach, yielding promising results. SARS-CoV-2 RNA was detected in the majority of samples, with NoV-GI, NoV-GII, and EnV also being detected. The successful optimization and application of the 5-plex dPCR assay for pathogen surveillance in wastewater offers significant benefits, including enhanced community health assessment and more effective responses to public health threats.

Detection of enteroviruses related to hand foot and mouth disease in wastewater of Asian communities

Hand, foot, and mouth disease (HFMD) is contagious and predominantly affects children below the age of five. HFMD-associated serotypes of Enterovirus A (EVA) family include EVA71, Coxsackievirus A type 6 (CVA6), 10 (CVA10), and 16 (CVA16). Although prevalent in numerous Asian countries, studies on HFMD-causing agents in wastewater are scarce. This study aimed to conduct wastewater surveillance in various Asian communities to detect and quantify serotypes of EVA associated with HFMD. In total, 77 wastewater samples were collected from Indonesia, the Philippines, Thailand, and Vietnam from March 2022 to February 2023. The detection ratio for CVA6 RNA in samples from Vietnam was 40 % (8/20). The detection ratio for CVA6 and EVA71 RNA each was 25 % (5/20) for the Indonesian samples, indicating the need for clinical surveillance of CVA6, as clinical reports have been limited. For the Philippines, 12 % (2/17) of the samples were positive for CVA6 and EVA71 RNA each, with only one quantifiable sample each. Samples from Thailand had a lower detection ratio (1/20) for CVA6 RNA, and the concentration was unquantifiable. Conversely, CVA10 and CVA16 RNAs were not detected in any of the samples. The minimum and maximum concentrations of CVA6 RNA were 2.7 and 3.9 log10 copies/L and those for EVA71 RNA were 2.5 and 4.9 log10 copies/L, respectively. This study underscores the importance of wastewater surveillance in understanding the epidemiology of HFMD-associated EVA serotypes in Asian communities. Long-term wastewater surveillance is recommended to monitor changes in dominant serotypes, understand seasonality, and develop effective prevention and control strategies for HFMD.

Estimation of Norovirus infections in Japan: An application of wastewater-based epidemiology for enteric disease assessment

Noroviruses of genogroup I (NoV GI) and NoV GII are the primary causes of acute gastroenteritis (AGE) in developed countries. However, asymptomatic and untested NoV infections lead to an underestimation of AGE cases, and the lack of mandatory viral identification in clinical cases hinders precise estimation of NoV infections. Back estimation of NoV infections in the community using a wastewater-based epidemiology (WBE) approach can provide valuable insights into the disease's extent, progression, and epidemiology, aiding in developing effective control strategies. This study employed a one-step reverse transcription-quantitative PCR to quantify NoVs GI and GII in wastewater samples (n = 83) collected twice a week from June 2022 to March 2023 in Japan. All samples from the Winter-Spring (n = 27) tested positive for NoV GI and GII RNA, while 73 % and 88 % of samples from the Summer-Autumn (n = 56) were positive for NoV GI and NoV GII RNA, respectively. Significantly higher concentrations of NoV GI/GII RNA were found in the Winter-Spring season compared to the Summer-Autumn season. NoV RNA was consistently detected in wastewater throughout the year, demonstrating the persistence of AGE cases in the catchment, suggesting an endemic NoV infection. Estimates of NoV infection incorporated viral RNA concentrations, wastewater parameters, and signal persistence in a mass balance equation using Monte Carlo Simulation. The median estimated NoV GI infections per 100,000 population for Summer-Autumn was 133 and for the Winter-Spring season, it was 881. Estimated NoV GII infections were 1357 for Summer-Autumn and 11,997 for the Winter-Spring season per 100,000 population. The estimated NoV infections exceeded by 3.2 and 23.9 folds than the reported AGE cases in Summer-Autumn and Winter-Spring seasons, respectively. The seasonal trend of estimated NoV infections closely matched that of AGE cases, highlighting the utility of WBE in understanding the epidemiology of enteric infections.

Quantitative analysis of SARS-CoV-2 RNA in wastewater and evaluation of sampling frequency during the downward period of a COVID-19 wave in Japan

Wastewater-based epidemiology (WBE) is a practical approach for detecting the presence of SARS-CoV-2 infections and assessing the epidemic trend of the coronavirus disease 2019 (COVID-19). The purpose of this study was to evaluate the minimum sampling frequency required to properly identify the COVID-19 trend during the downward epidemic period when using a highly sensitive RNA detection method. WBE was conducted using the Efficient and Practical virus Identification System with ENhanced Sensitivity for Solids (EPISENS-S), a highly sensitive SARS-CoV-2 RNA detection method, at nine neighboring wastewater treatment plants (WWTPs). These WWTPs were in the same prefecture in Japan, and they had different sewer types, sampling methods, and sampling frequencies. The overall detection rate of SARS-CoV-2 RNA was 97.8 % during the entire study period when the geometric means of new COVID-19 cases per 100,000 inhabitants were between 3.3 and 7.7 in each WWTP. The maximum SARS-CoV-2 RNA concentration in wastewater was 2.14 × 104 copies/L, which corresponded to pepper mild mottle virus (PMMoV)-normalized concentrations of 6.54 × 10-3. We evaluated the effect of sampling frequencies on the probability of a significant correlation with the number of newly reported COVID-19 cases by hypothetically reducing the sampling frequency in the same dataset. When the wastewater sampling frequency occurred 5, 3, 2, and 1 times per week, these results exhibited significant correlations of 100 % (5/5), 89 % (8/9), 85 % (23/27), and 48 % (13/27), respectively. To achieve significant correlation with a high probability of over 85 %, a minimum sampling frequency of twice per week is required, even if sampling methods and sewer types are different. WBE using the EPISENS-S method and a sampling frequency of more than twice a week can be used to properly monitor COVID-19 wave epidemic trends, even during downward periods.