(HealthDay News) – Bivalent and monovalent oral poliovirus vaccines (OPVs) are similarly effective, but decreases in vaccine coverage in parts of Pakistan and southern Afghanistan are hampering efforts to eradicate poliomyelitis.

Kathleen M. O’Reilly, PhD, from Imperial College London, and colleagues investigated the reasons behind the increasing incidence of polio in Pakistan and Afghanistan despite program interventions, including the introduction of new vaccines. A matched case-control analysis was performed on a database of 46,977 0–14-year-old children with onset of flaccid paralysis from 2001–2011. The clinical effectiveness of OPV was estimated by comparing the vaccination history of children with poliomyelitis with that of children with acute flaccid paralysis due to other causes. Vaccine coverage and serotype-specific vaccine-induced population immunity was assessed for children aged 0–2 years.

The researchers identified 883 cases of serotype 1 poliomyelitis (710 and 173 in Pakistan and Afghanistan, respectively) and 272 cases of poliomyelitis serotype 3 (216 and 56, respectively). The estimated clinical per-dose effectiveness of trivalent OPV was 12.5% against serotype 1 poliomyelitis, compared with 34.5 and 23.4% for monovalent and bivalent OPV, respectively, with the bivalent vaccine noninferior to the monovalent vaccine. During 2006–2011, vaccination coverage declined in specific areas in Pakistan and southern Afghanistan, resulting in decreased vaccine-induced population immunity to poliovirus serotype 1 and increased incidence of poliomyelitis.

“These results provide an example of the need to tailor immunization programs to epidemiological circumstances, particularly where the goal is eradication,” writes the author of an accompanying editorial.

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