The COVID-19 pandemic has dealt an unprecedented blow to the world's battle against polio, the head of the global organisation to combat the disease said Thursday after it suspended vaccination campaigns for the first time in three decades.

With the coronavirus marching swiftly across the world and nations imposing strict travel restrictions to slow its spread, the Global Polio Eradication Initiative (GPEI) has announced that its health workers cannot continue their immunisation drives -- and warned this risks a resurgence of the poliovirus.

"We're devastated by the fact that we have to stop the activities for a disease that we were working so hard to eradicate," said the World Health Organisation's Michel Zaffran, who heads GPEI.

There are only two nations remaining where the wild version of the poliovirus continues to spread -- Pakistan and Afghanistan -- but a strain that has mutated from the vaccine itself has also caused outbreaks in several nations in Africa.

Immunisation campaigns protect against both wild and vaccine-derived outbreaks of the virus, which spreads in areas of poor sanitation and contaminated water and can cause irreversible paralysis. Children under five are particularly vulnerable.

- Polio could advance again -
Zaffran told AFP that beyond travel restrictions put in place by governments, the new coronavirus itself was considered too great a risk for health workers and the community to continue vaccination drives.

"Many of these activities have been suspended because they bring people together, they increase the mass gathering effect and also the delivery of the vaccine uses a dropper which could actually get contaminated, either by the recipients, or by the health worker," he said.

An announcement last week by GPEI said it would halt immunisations until at least June, but Zaffran said it was impossible to predict when they will resume, with decisions likely made on a country-by-country basis.

In the meantime, a devastating disease that the world had come achingly close to snuffing out will be free to spread.

Zaffran said the group was "extremely concerned" that the poliovirus could now start to advance again within Afghanistan and Pakistan and warned that in Africa it could cross borders into countries currently unaffected.

- 'Terrible pandemic' -
When the GPEI was set up in 1988, polio paralysed more than 1000 children worldwide every day in dozens of endemic countries.

In the last decade, GPEI says more than 10 billion doses of oral polio vaccine have been given worldwide, estimating that this prevented some 6.5 million children from being paralysed by the virus.

While the vaccine-derived outbreaks have presented a grave setback, causing hundreds of cases, GPEI had put its hopes in a new vaccine that is awaiting WHO pre-licence approval for emergency use later this year.

Despite halting some services, GPEI will continue to try to monitor outbreaks and thousands of people in its network will be redeployed to help with the COVID-19 response in the countries where they work.

"Because we've got this infrastructure of very competent epidemiologists and surveillance officers and laboratories and logistics networks and so on on the ground, we believe that we have a role to play in supporting the response to this terrible pandemic," he said.

- 'Substantial amount of time' -
Zaffran said the weeks and months of suspension will be used to ensure that manufacturers keep sufficient stocks of the existing vaccine to deploy quickly once it is possible to resume immunisations.

But he cautioned that the suspension in some countries could last a "substantial amount of time", leaving remote communities potentially in need of an array of vaccines and medical help.

"Keeping in mind that we do want to eradicate polio, but we cannot go to remote communities and only offer polio drops if those communities are also affected by the lack of health services over the next several months," he said.