Molecular Epidemiology of Wild Poliovirus Type 1 and Shift in the Historically Reservoir Areas of Pakistan During 2019-2022

Background: Pakistan is one of the two countries endemic for wild poliovirus type 1 (WPV1). Active clinical and environmental wastewater surveillance along with laboratory investigation is an integral and primary component of the polio eradication strategies. The current study is mainly focused on the virological data to understand the current epidemiology of WPV1 in Pakistan during 2019-2022.

Methods: 141,037 stool specimens of patients reported with acute flaccid paralysis (AFP) and 3,171 wastewater samples were tested for poliovirus detection using cell culture and PCR. Phylogenetic analysis of WPV1 was performed using MEGA and Nextstrain.

Results: Poliovirus isolates were classified into 15 distinct genetic clusters with multiple transmission lineages. Spatio-temporal trends indicated a significant decline in the incidence of poliomyelitis reported in 58 districts in 2019 to just 3 in 2022. The historical reservoirs in Peshawar, Quetta, and Karachi successfully eliminated the indigenous transmission chains of wild poliovirus active there for years.

Conclusions: Our findings reinforce the evolving epidemiology of poliovirus in Pakistan which is now confined to South KP. All historically known reservoirs in Peshawar, Karachi and Quetta blocs are now free of poliovirus. Intensified clinical and environmental surveillance should be maintained to eliminate the very few remaining transmission lineages and certify the poliovirus eradication by 2026.