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Switch from tOPV to bOPV, IPV facilitates poliovirus 2 transmission
Researchers who implemented a mass vaccination campaign with the monovalent type 2 oral polio vaccine in Bangladesh found that the incidence of type 2 poliovirus, or Sabin 2 virus, was higher among household contacts of infants who received a bivalent oral polio vaccine and inactivated polio vaccine than of those who received a trivalent oral polio vaccine.
However, Mami Taniuchi, PhD, of the division of infectious diseases and international health at the University of Virginia, and colleagues reported in the Lancet Infectious Diseases the switch from the trivalent oral polio vaccine (tOPV) to bivalent oral polio vaccine (bOPV) and inactivated polio vaccine (IPV) did not have a significant impact on the community as a whole.
“The risk of epidemic Sabin 2 transmission remains small in well-immunized communities within 1 year of tOPV cessation, but the increased quantity of virus shed in populations with birth cohorts receiving bOPV and IPV relative to those receiving tOPV indicates that the transmission potential of Sabin 2 will increase as the birth cohort receiving bOPV and IPV grows,” the researchers wrote. “This risk should be mitigated by maintaining high population immunity with IPV, improved environmental surveillance for vaccine-derived poliovirus transmission, and maintenance of mOPV2 stockpiles and protocols to respond to vaccine-derived poliovirus outbreaks.”
The researchers investigated transmission patterns during an open-label, cluster-randomized trial conducted in 67 villages in the Matlab region of Bangladesh. Between April 30, 2015 and Jan.14, 2016, Taniuchi and colleagues randomly assigned villages to the following vaccine schedules:
- tOPV at 6 weeks, 10 weeks and 14 weeks (n = 22 villages; 300 infants);
- bOPV at 6 weeks, 10 weeks and 14 weeks plus one dose of IPV at 14 weeks (n = 23 villages; 310 infants); or
- bOPV at 6 weeks, 10 weeks and 14 weeks plus two doses of IPV at 14 weeks and 18 weeks (n = 22 villages; 329 infants).
The researchers then launched a mOPV2 vaccination campaign from Jan. 25 to Jan. 29, 2016, that targeted 40% of participants younger than 5 years. The primary outcome was the incidence of Sabin 2 among per-protocol infants who did not receive mOPV2 during the first 10 weeks after the campaign. The researchers hypothesized that Sabin 2 incidence would be higher among bOPV and IPV recipients than tOPV recipients.
The results showed that among infants who did not receive mOPV2, fecal shedding of Sabin 2 virus was similar between children who received bOPV and IPV and those who received tOPV (6% vs. 4%; OR = 1.29; 95% CI, 0.45-3.72). Among household contacts, however, fecal shedding of Sabin 2 was significantly higher with bOPV and IPV vs. tOPV (2% vs. 1%; OR = 3.60; 95% CI, 0.82-15.9). Additional analyses revealed that two doses of IPV did not offer better mucosal immunity than one dose of IPV.
“Although the small observed increase in transmission does not pose an immediate threat of uncontrolled transmission of type 2 poliovirus in our well-vaccinated community, principled extrapolation from a biologically plausible polio transmission model showed that, as population immunity to Sabin 2 declines, reintroduction will be capable of seeding poliovirus transmission at intensities comparable with those seen during wild poliovirus transmission,” Taniuchi and colleagues wrote. “This intrinsically high transmission potential of Sabin 2 has much greater effect in communities in which population immunity is low.”
In a related editorial, Walter A. Orenstein, MD, an infectious diseases professor at Emory University and associate director of the Emory Vaccine Center in Atlanta, and colleagues said that the study raises a “major question” about whether IPV — which they suggested is clearly inferior to tOPV — can reduce the transmission of type 2 polioviruses in the community. According to the authors, the Strategic Advisory Group of Experts on Immunization recommend that countries should include at least two IPV doses in their routine immunization schedule after global withdrawal of trivalent, bivalent and monovalent oral poliovirus vaccines.
“It is clear IPV provides protection against paralytic disease, and to a lesser degree against emergence from leftover tOPV or against community transmission,” they wrote. “Therefore, until the risk of reintroduction is either eliminated or reduced to virtually zero, IPV should continue to be used. Of course, for any vaccine policy to have most effect in securing complete polio eradication, achieving and maintaining high immunization coverage is essential.” – by Stephanie Viguers
Disclosure: Orenstein reports receiving funding from the Bill & Melinda Gates Foundation. Please see the full study and related editorial for a list of all other authors’ relevant financial disclosures.