Abstract
SummaryBackgroundTrivalent oral polio vaccine (tOPV) was replaced worldwide from April, 2016, by bivalent types 1 and 3 oral polio vaccine (bOPV) and one dose of inactivated polio vaccine (IPV) where available. The risk of transmission of type 2 poliovirus or Sabin 2 virus on re-introduction or resurgence of type 2 poliovirus after this switch is not understood completely. We aimed to assess the risk of Sabin 2 transmission after a polio vaccination campaign with a monovalent type 2 oral polio vaccine (mOPV2).MethodsWe did an open-label cluster-randomised trial in villages in the Matlab region of Bangladesh. We randomly allocated villages (clusters) to either: tOPV at age 6 weeks, 10 weeks, and 14 weeks; or bOPV at age 6 weeks, 10 weeks, and 14 weeks and either one dose of IPV at age 14 weeks or two doses of IPV at age 14 weeks and 18 weeks. After completion of enrolment, we implemented an mOPV2 vaccination campaign that targeted 40% of children younger than 5 years, regardless of enrolment status. The primary outcome was Sabin 2 incidence in the 10 weeks after the campaign in per-protocol infants who did not receive mOPV2, as assessed by faecal shedding of Sabin 2 by reverse transcriptase quantitative PCR (RT-qPCR). The effect of previous immunity on incidence was also investigated with a dynamical model of poliovirus transmission to observe prevalence and incidence of Sabin 2 virus. This trial is registered at ClinicalTrials.gov, number NCT02477046.FindingsBetween April 30, 2015, and Jan 14, 2016, individuals from 67 villages were enrolled to the study. 22 villages (300 infants) were randomly assigned tOPV, 23 villages (310 infants) were allocated bOPV and one dose of IPV, and 22 villages (329 infants) were assigned bOPV and two doses of IPV. Faecal shedding of Sabin 2 in infants who did not receive the mOPV2 challenge did not differ between children immunised with bOPV and one or two doses of IPV and those who received tOPV (15 of 252 [6%] vs six of 122 [4%]; odds ratio [OR] 1·29, 95% CI 0·45–3·72; p=0·310). However, faecal shedding of Sabin 2 in household contacts was increased significantly with bOPV and one or two doses of IPV compared with tOPV (17 of 751 [2%] vs three of 353 [1%]; OR 3·60, 95% CI 0·82–15·9; p=0·045). Dynamical modelling of within-household incidence showed that immunity in household contacts limited transmission.InterpretationIn this study, simulating 1 year of tOPV cessation, Sabin 2 transmission was higher in household contacts of mOPV2 recipients in villages receiving bOPV and either one or two doses of IPV, but transmission was not increased in the community as a whole as shown by the non-significant difference in incidence among infants. Dynamical modelling indicates that transmission risk will be higher with more time since cessation.FundingBill & Melinda Gates Foundation.
Highlights
In anticipation of the eradication of poliomyelitis, WHO convened a committee in 1998, which recommended that vaccination with oral polio vaccine should stop when there was sufficient confidence in global eradication, suitable containment of poliovirus stocks, and sufficient evidence that Sabin vaccine strains from the oral polio vaccine would not transmit indefinitely in the postvaccination era
Because wild type 2 poliovirus has been eradicated, and the Sabin 2 vaccine strain causes most circulating vaccine-derived poliovirus cases and roughly a third of all vaccine-associated poliomyelitis cases, WHO recomm en ded in 2015 the removal of Sabin 2 from the trivalent oral polio vaccine used for routine polio immunisation and mass campaigns in more than 100 countries, replacing tOPV with bivalent types 1 and 3 oral polio vaccine with at least one dose of inactivated polio vaccine (IPV)
Findings of clinical trials in which participants were given challenge doses of monovalent type 2 oral polio vaccine showed that mixed immunisation schedules of bivalent types 1 and 3 oral polio vaccine and inactivated polio vaccine (IPV) provide superior protection from infection relative to IPV alone, but the protection is inferior to that produced by a full course of tOPV
Summary
Evidence before this study We searched PubMed for all papers published up to March, 2017, with the terms: (“polio” OR “poliomyelitis” OR “poliovirus”) AND (“communicability” OR “transmission” OR “transmissibility”) AND (“oral”) AND (“vaccine” OR “poliovaccine” OR “Sabin”). This search identified 511 reports, of which only 81 were transmission studies. Findings of clinical trials in which participants were given challenge doses of monovalent type 2 oral polio vaccine (mOPV2) showed that mixed immunisation schedules of bivalent types 1 and 3 oral polio vaccine (bOPV) and inactivated polio vaccine (IPV) provide superior protection from infection relative to IPV alone, but the protection is inferior to that produced by a full course of tOPV. We do not know of any published studies that describe Sabin 2 transmission in the context of mixed tOPV and bOPV and IPV population immunity
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