The world is “tantalisingly close” to eradicating polio – with no confirmed cases of wild polio anywhere so far this year. But experts warn that vaccination efforts – and funding – must not falter if the world is to rid itself of a human infectious disease for the second time in history, after smallpox.

There have been no reported cases of wild polio infection in people for the last 19 weeks. Figures from the World Health Organization reveal that the last confirmed cases were on the borders of Pakistan and Afghanistan in October and September 2023 respectively; these are the last nations on Earth where polio is endemic.

“To have gone 19 straight weeks … is a long period to go without a single case, that’s why there is some hope [of eradication],” Gordon McInally, president of Rotary International, a founding partner in the Global Polio Eradication Initiative (GPEI), told the Observer. “All of us who are involved in this, every week we get an email giving us the updated figures … and every week when I click open that email my heart rate goes up until I see the number in the hope that it will be zero and not one, or worse. But we take it week by week.”

But those involved in eradication efforts are taking nothing for granted. The programme has come under fire before for its “almost-there narrative”, as described in a report last September by the Independent Monitoring Board, led by Liam Donaldson, a former chief medical officer for England.

Still, said McInally, if they can get through another 33 weeks (one full year after the last case), they will be “celebrating cautiously”, and if the world stays two years disease-free they can officially declare the global eradication of polio.

While the absence of confirmed cases is “really encouraging”, Aidan O’Leary, director for polio eradication at the WHO, said: “It’s important we don’t call the figures great.” The campaign needs to be aggressive in closing any immunisation gaps, he added.

The latest WHO figures do reveal 34 samples of wild poliovirus detected from environmental and other sources in the first three months of this year, including from surveillance samples of sewage (where shed virus may circulate). “We are identifying environmental isolates which does indicate there’s some transmission,” said O’Leary, adding this needs to be zero too.

O’Leary, McInally and other stakeholders met Andrew Mitchell, the minister for development, and his team on Wednesday to update them on polio, and ask the UK government to continue with its funding, which ends this year. They are asking the Foreign, Commonwealth and Development Office for £100m for the next two years. GPEI’s goal is to eradicate polio by 2026.

The UK government has been the second biggest contributing government after the US towards eradicating polio, said McInally. He said funding is crucial because “we’re at that challenging point in time where because of the geography, the nomadic nature of many of the people we’re trying to reach, it’s not easy to reach everybody to get it finished, and there is a realisation that unless we finish it completely we run the risk of it bubbling up again.”

If they fail to eradicate polio in Afghanistan and Pakistan, WHO modelling suggests there could be a “global resurgence” resulting in some 200,000 new cases of polio each year within 10 years. Until this is done, McInally said, polio is only a “plane-ride away”. Imported polioviruses caused alarm in 2022 when an unvaccinated adult in New York was paralysed by the disease, and poliovirus was detected in sewage in London.

But McInally is hopeful, in part because of the programme’s success in India – 27 March will mark the country’s 10-year anniversary since it became officially polio-free. There are some parallelswith rural India which give him hope. “Many people said ‘you will never get rid of polio from India’…and it was done.”

Immunisations have been stepped up in Afghanistan and Pakistan; with the programme extending its target age for immunising children from under five to under 10, said O’Leary, and synchronising on both sides of the border.

Another crucial challenge is “vaccine-derived” poliovirus transmission. This derives from the oral polio vaccine, still used in some regions, which harnesses live but weakened poliovirus.

This would not cause issues of itself, but if immunisation rates are low in a population, the vaccine strain can circulate and genetically change over time, and in rare cases cause paralysis like wild polio.

“Any case is a worry,” said McInally. “But once we can eliminate wild virus then the vaccine-derived cases will clear up relatively quickly.”

O’Leary notes that there were 1,000 cases of polio a day in 125 countries in 1988 when the GPEI started. He likens the polio programme to running a marathon – with a few hundred metres to go. Nonetheless he remained cautious: “Eradication is a zero-sum game. We have to be very clear-eyed – when we look back and everything is zero, then we can say it’s great.”