Surveillance — Finding Poliovirus in Its Last Hiding Places

Polio campaigns get the attention. Surveillance is what tells the programme whether they are working — and in the last mile of eradication, it is the more difficult of the two.

The problem surveillance solves

Roughly one poliovirus infection in two hundred causes paralysis. The other 199 produce mild illness or nothing at all, while the infected child still sheds virus for weeks. This means case counts measure visible harm, not the presence of the virus — and a country can have very few cases while poliovirus circulates widely.

Two systems exist to see what case counts cannot.

Acute flaccid paralysis surveillance

Every child under 15 in Pakistan who develops sudden onset of floppy paralysis is investigated — regardless of the suspected cause, because the only way to be sure polio is not responsible is to test.

Surveillance officers trace the case, collect two stool samples within a defined window, reconstruct the child's vaccination history, and search the surrounding community for other paralysis that was never reported. Samples go to the WHO-accredited Regional Reference Laboratory at the National Institute of Health in Islamabad.

Sample timing and condition matter enormously. A sample collected late or transported outside the cold chain can produce a false negative, which means a case the programme never sees. This is why the quality of AFP surveillance is measured separately from case counts.

Environmental surveillance

Sewage is sampled at a network of well over a hundred fixed sites nationwide, chosen to cover the catchments where poliovirus is most likely to circulate. Because a single site can represent the excreta of a very large population, this method detects transmission that no amount of paralysis reporting would reveal — usually weeks before a child is paralysed.

Environmental surveillance is why the programme knew poliovirus was still circulating during Pakistan's 15-month case-free period, and why it could see the risk building through 2018 before the 147 cases of 2019.

Genetic sequencing

Every isolate is sequenced. This establishes how closely a virus is related to strains found elsewhere — whether a case represents long-undetected local circulation or recent importation from another district or across the Afghan border. Those two findings require different responses.

What it means for certification

A country can only be certified polio-free after three consecutive years with no wild poliovirus detected in people or in the environment, with surveillance sensitive enough to be believed. Surveillance quality is therefore not a supporting function — it is the thing that makes the eventual claim of success credible.

Related: polio surveillance in Pakistan · environmental sampling results