The World Health Organization is kicking into high gear to distribute millions of polio vaccines in Africa after Nigeria had its first cases in two years.

Nigeria announced earlier this week that two children were paralyzed due to the debilitating virus, which the WHO hopes to eradicate by 2020. The new cases have sparked a fervent response from WHO, which wants to vaccinate children not only in Nigeria but in surrounding African countries.

The WHO said Friday that it would conduct five rounds of immunization targeting several million children starting next week. The immunizations would target five African countries: Nigeria, Cameroon, Chad, Niger and Central African Republic.

"The first round will take place next week," said Dr. Michel Zaffran, director of polio eradication for the WHO. "Then there will be several rounds of vaccination that will take place at two or three intervals that will target an increasing round of children and increasing circles of populations."

Zaffran said during a conversation on Facebook Live that the new cases come after great progress in Nigeria at eradicating polio.

"It is a particularly disturbing piece of news," he said.

The WHO said earlier that as recently as 2012 Nigeria accounted for more than half of all polio cases worldwide.

"The two cases in Nigeria particularly highlight the need to prioritize immunization of children in hard-to-reach areas such as the Lake Chad region, which spans several countries and is often affected by conflict and large population movements," the WHO said.

The world has been working to eradicate polio, and Zaffran said the goal is still to wipe out the disease by 2020.

Only 21 wild polio cases have been reported in 2016, compared with 34 cases at the same point last year, according to WHO.

The two other countries reporting active polio cases are Afghanistan and Pakistan.