Estimation of oral poliovirus vaccine effectiveness in Afghanistan, 2010-2020

Afghanistan is one of two countries with endemic wild poliovirus type 1 (WPV1). The oral poliovirus vaccine (OPV) is the predominant vaccine used for polio eradication. Although OPV has been administered in routine childhood immunization and during frequent supplementary immunization activities, WPV1 continues to circulate in Afghanistan and case incidence has been increasing since 2017. We estimated the effectiveness of OPV in Afghanistan during 2010-2020.

We conducted a matched case-control analysis using acute flaccid paralysis (AFP) surveillance data from 29,370 children < 15 years with AFP onset between January 1, 2010 and December 31, 2020. We matched children with confirmed WPV1 (cases) with children with non-polio AFP (controls) by age at onset of paralysis (+/- 3 months), date of onset of paralysis (+/- 3 months), and province of residence, and compared their reported OPV vaccination history to estimate the effectiveness of OPV in preventing paralysis by WPV1 using conditional logistic regression. To account for changes in OPV formulations provided over the analysis period, we stratified the analysis based on dates of the global switch from trivalent OPV (tOPV) to bivalent OPV (bOPV) in April 2016.

Between January 1, 2010 and December 31, 2020, there were 329 WPV1 cases in Afghanistan. The per-dose estimated effectiveness of OPV against WPV1 was 19% (95% CI: 15%-22%) and of ≥ 7 doses was 94% (95% CI: 90%-97%). Before the global switch from tOPV to bOPV, the per-dose estimated effectiveness of OPV was 14% (95% CI: 11%-18%) and of ≥ 7 doses was 92% (95% CI: 85%-96%). After the switch, the per-dose estimated effectiveness of OPV against WPV1 was 32% (24%-39%) and of ≥ 7 doses was 96% (95% CI: 90%-99%).

OPV is highly effective in preventing paralysis by WPV1; these results indicate that continued WPV1 transmission in Afghanistan is due to failure to vaccinate, not failure of the vaccine. Although difficult to implement in parts of country, improving the administration of OPV in routine immunization and supplementary immunization activities will be critical for achieving polio eradication in Afghanistan.