African countries on Thursday vowed to wipe out all forms of polio on the continent after efforts to crush the virus were stymied by the coronavirus pandemic.

A year ago, wild forms of poliovirus were declared eradicated in Africa after four years elapsed without a case.

However, further sporadic outbreaks have occurred because of vaccine-derived —a weakened form of the that is used in vaccines and which is excreted by people for a time after they receive it.

Such cases "occur in communities where not enough children have received the ," the World Health Organization's Africa region said after a two-day virtual conference on the disease.

"Cases increased last year in part because of disruptions to polio vaccination campaigns caused by COVID-19," it said.

"Since 2018, 23 countries in the region have experienced outbreaks and more than half of the global 1,071 (vaccine-derived) cases were recorded in Africa."

At a press conference, regional WHO chief Matshidiso Moeti said that polio vaccination efforts in Africa had been hit by postponements or suspensions after the COVID-19 crisis got underway early last year.

The conference agreed to speed up responses to polio outbreaks, including the fast deployment of "surge staff" to support countries as soon as cases are detected.

There will also be greater efforts to "build trust with communities" to encourage uptake of vaccines, and wider use of a new oral immunisation called nOPV2 that is more effective at ending vaccine-derived outbreaks.

Polio is a transmissible disease caused by the highly contagious poliomyelitis virus, which spreads through contact with faeces or sneezes and coughs.

The virus infects the , causing paralysis of the parts of the body.

Once a global peril, the disease has been rolled back since the advent of a vaccine in the 1950s by the US physician, Jonas Salk.

The WHO said that despite the disruption, nearly 100 million African children had been vaccinated against polio since July 2020.