The World Health Organization says Niger has been struck by a new outbreak of polio, following the suspension of immunisation activities during the COVID-19 pandemic.

The UN health agency reported that two children were infected by the highly infectious, water-borne disease and that one was paralysed.

The outbreak was sparked by a mutated virus that originated in the vaccine and was not connected to a previous polio epidemic Niger stopped last year, WHO said, in a statement.

"The poliovirus will inevitably continue to circulate and may paralyse more children as no high-quality immunisation campaigns can be conducted in a timely manner," said Pascal Mkanda, WHO's coordinator of polio eradication in Africa.

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Children paralysed

In rare cases, the live virus in oral polio vaccine can evolve into a form capable of igniting new outbreaks among non-immunised children; stopping the epidemic requires more targeted vaccination.

Earlier this month, WHO and partners announced they were forced to halt all polio vaccination activities until at least June 1, acknowledging the decision would inevitably result in more children being paralysed.

The Africa Centres for Disease Control and Prevention says there have been 33,500 cases and 1,469 deaths as of Tuesday, but experts suspect the real numbers are far higher due to lack of testing and poor surveillance.

Eradicating polio requires more than 90 per cent of children being immunised, typically in mass campaigns involving millions of health workers that would break social distancing guidelines needed to stop the spread of the new coronavirus.

Across Africa, 14 other countries are struggling to contain their polio epidemics, which have also been caused by a rare mutation of the virus in the oral vaccine. Health officials had initially aimed to wipe out polio by 2000, but that deadline has been pushed back and missed repeatedly.

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