Polio in Pakistan, January 2022–June 2023: Cases Confined to One Corner, but Virus in Karachi's Sewage

The Global Polio Eradication Initiative (GPEI) began in 1988. Since then, wild poliovirus type 1 (WPV1) has never stopped circulating in two countries: Pakistan and Afghanistan. They share a long border crossed by highly mobile people, so they're treated as one epidemiologic block. GPEI's 2022–2026 strategy aimed to interrupt all WPV1 transmission worldwide by the end of 2023 — a goal Pakistan's continued circulation could put at risk.

Cases

WPV1 and cVDPV2 cases by month, January 2019–June 2023. CDC

All 20 cases in 2022 were in three districts of south Khyber Pakhtunkhwa: North Waziristan (17), Lakki Marwat (2) and South Waziristan (1) — an area with serious security problems and a history of vaccine hesitancy. The one 2023 case, in Bannu district of the same province, was paralyzed on February 20, 2023.

Location of WPV1 cases by period, January 2022–June 2023. CDC

Of the 21 children with WPV1 over the whole period (ages 3 months to 16 years, median 15 months), 17 (81%) had never received oral polio vaccine through routine immunization; the other four had had 1–3 doses.

Vaccine-derived type 2 polio (cVDPV2) caused 165 cases from July 2019 to April 2021, and none since April 23, 2021.

Surveillance

Paralysis surveillance is strong on paper. Pakistan found 18.9 cases of non-polio acute flaccid paralysis per 100,000 children under 15 in 2022 (provinces 10.7–28.6), far above the benchmark of 2, and stool samples met the adequacy target everywhere. In the seven districts of south Khyber Pakhtunkhwa, the rate rose from 17.7 in 2021 to 24.9 in 2022.

Sewage testing at 114 sites in 80 districts tells a more worrying story:

What the genes show

All the case viruses belong to one cluster, YB3C, endemic in Pakistan. Of the 11 sewage viruses sequenced, 3 were YB3C and 8 were YB3A — a cluster circulating in eastern Afghanistan. Viruses in Lahore, Peshawar and Hangu linked to Afghan strains: as long as polio circulates in Afghanistan, it can cross the border.

The Karachi virus, also YB3A, differed by 5.3% from its closest relative, found in Karachi in January 2021 — far beyond the 1.5% that marks an "orphan" virus. That means one lineage had been spreading undetected in Karachi for a long time, missed by both paralysis and sewage surveillance. Overall, the genetic diversity of WPV1 has narrowed from 10 clusters in 2019–2020 to two.

Vaccination

Routine immunization

For 2021, WHO and UNICEF estimated national coverage at 83% for both 3 doses of oral polio vaccine (OPV) and 1 dose of inactivated polio vaccine (IPV) by age 12 months. A Gavi-sponsored survey found 3-dose OPV coverage among children 12–23 months ranging from 45.1% in Balochistan to 94.9% in Punjab. No district in Balochistan, Khyber Pakhtunkhwa or Sindh reached 80% — against 31 of 36 districts in Punjab.

Campaigns

Since type 2 wild polio was declared eradicated (2015) and trivalent OPV was withdrawn worldwide (2016), most campaigns have used bivalent OPV (types 1 and 3); type 2 vaccines were used against the 2019–2021 cVDPV2 outbreaks.

• 2022: two national immunization days — each targeting about 44 million children under 5 — and six subnational rounds, plus outbreak responses in south Khyber Pakhtunkhwa in March, April and June. In June and August, children there also got fractional-dose IPV (one-fifth of a dose, given into the skin).
• 2023: a national round in January and subnational rounds in February, March, May and June.

Children still missed

South Khyber Pakhtunkhwa's seven districts have about 1.1 million children under 5, of whom about 50,000 are regularly missed — including 19,500 in the Mehsud tribal area of South Waziristan, where militants have intimidated health workers and stopped vaccination since August 2022. Some communities boycott campaigns over demands unrelated to polio, such as electricity.

Campaign quality checks mark each vaccinated child's fingernail with ink. With a 90% pass mark, only 56%–80% of union councils in south Khyber Pakhtunkhwa passed in rounds from August 2022 to February 2023. Nationally, about 505,750 children were missed in the January 2023 national round, 22,466 (4%) of them because of refusals. And fake finger-marking — inking children who weren't vaccinated — means some checks overstate coverage.

What's needed

Pakistan has made substantial progress, but interrupting transmission by the end of 2023 faces big obstacles. The report calls for:

• meticulous microplanning of campaigns and tracking of children missed again and again, including mobile populations crossing the Afghan border;
• synchronizing vaccination with Afghanistan where possible;
• combining polio vaccination with other health services to win the trust of hesitant communities;
• protecting the safety and morale of frontline workers, who are sometimes targeted in attacks along with their security escorts;
• responding urgently to any new detection.

With a 2021–2022 outbreak in southeastern Africa traced to Pakistan apparently winding down, GPEI's focus stays on Pakistan and Afghanistan and on containing vaccine-derived outbreaks. Ending polio in Pakistan means reaching, vaccinating and protecting every child.

Limits

Refusals usually account for under 5% of missed children; operational problems cause most of the rest. Coverage estimates may be distorted by caregivers' recall and by fake finger-marking.

Sources

Based on Mbaeyi C, Baig S, Safdar RM, et al., "Progress Toward Poliomyelitis Eradication — Pakistan, January 2022–June 2023," MMWR volume 72, number 33, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report describes the two remaining clusters as both indigenous while identifying one of them as circulating in eastern Afghanistan, so only their number is given here.