Recovery of enteroviruses and poliovirus in Harare sewage using the bag-mediated filtration system at the introduction of the inactivated polio vaccine in Zimbabwe

Environmental surveillance is a sensitive method for detecting circulating virus in the absence of clinical cases and is important for monitoring progress for poliovirus (PV) eradication. This study used the bag-mediated filtration system (BMFS) to determine PV and enterovirus (EV) prevalence in sewage at the transition from oral polio vaccine type 2 (OPV2) use to inactivated polio vaccine (IPV) use in Zimbabwe, and examined the correlation between environmental surveillance results and vaccination coverage of OPV. A total of 18 BMFS samples from 6 sampling sites were analysed for the presence of EV and PV via direct RT-qPCR, direct ITD (intratypic differentiation), and the WHO algorithm. EV prevalence in Harare wastewater was 88.9% (16/18) using direct RT-PCR, 61.1% (11/18) using direct ITD, and 77.8% (14/18) using the WHO algorithm. Of the 18 samples analysed using the WHO algorithm, 10 samples (55.6%) were positive for Sabin-like PV type 3 (SL3). Of these 10 samples, 2 were also positive for non-polio enteroviruses (NPEV), resulting in a total of 6 (33.3%) samples positive for NPEV and 4 negative. The sensitivity of isolation in detecting EVs in sewage was 92.9% when comparing direct RT-qPCR results to the WHO algorithm. Using direct ITD, two high-density, low-income sampling sites were negative for SL3 and one low-density, high-income sampling point was negative for SL3 using the WHO algorithm. There was a strong association between relative EV concentration and the number of OPV3 vaccine recipients (r = 0.8590; p = 0.0284) in sampled areas. This study demonstrated the ability of BMFS to detect PVs circulating in Harare wastewater at the beginning of the OPV-IPV switch and can be used to monitor potential reintroduction of wild PV or vaccine-derived PVs from endemic areas.