Assessing the individual risk of fecal poliovirus shedding among vaccinated and non-vaccinated subjects following national health weeks in Mexico

BACKGROUND Mexico introduced inactivated polio vaccine (IPV) into its routine immunization (RI) schedule in 2007 but continued to give trivalent oral polio vaccine (tOPV) twice a year during national health weeks (NHW) through 2015. OBJECTIVES To evaluate individual variables associated with poliovirus (PV) shedding among children with IPV-induced immunity after vaccination with tOPV and their household contacts. MATERIALS AND METHODS We recruited 72 children (both genders, ≤30 months, vaccinated with at least two doses of IPV) and 144 household contacts (both genders, 2 per household, children and adults) between 08/2010 and 09/2010 in Orizaba, Veracruz. Three NHW took place (one before and two after enrollment). We collected fecal samples monthly for 12 months, and tested 2500 samples for polioviruses types 1, 2 and 3 with three serotype-specific singleplex real-time RT-PCR (rRT-PCR) assays. In order to increase the specificity for OPV virus, all positive and 112 negative samples were also processed with a two-step, OPV serotype-specific multiplex rRT-PCR. ANALYSIS We estimated adjusted hazard ratios (HR) and 95% CI using Cox proportional hazards regression for recurrent events models accounting for individual clustering to assess the association of individual variables with the shedding of any poliovirus for all participants and stratifying according to whether the participant had received tOPV in the month of sample collection. RESULTS 216 participants were included. Of the 2500 collected samples, using the singleplex rRT-PCR assay, PV was detected in 5.7% (n = 142); PV1 in 1.2% (n = 29), PV2 in 4.1% (n = 103), and PV3 in 1.9% (n = 48). Of the 256 samples processed by multiplex rRT-PCR, PV was detected in 106 (PV1 in 16.41% (n = 42), PV2 in 21.09% (n = 54), and PV3 in 23.05% (n = 59). Both using singleplex and multiplex assays, shedding of OPV among non-vaccinated children and subjects older than 5 years of age living in the same household was associated with shedding of PV2 by a household contact. All models were adjusted by sex, age, IPV vaccination and OPV shedding by the same individual during the previous month of sample collection. CONCLUSION Our results provide important evidence regarding the circulation of poliovirus in a mixed vaccination context (IPV+OPV) which mimics the "transitional phase" that occurs when countries use both vaccines simultaneously. Shedding of OPV2 by household contacts was most likely the source of infection of non-vaccinated children and subjects older than 5 years of age living in the same household.