"What we're facing now is a major epidemic in Yemen," said David Heymann, chief of the World Health Organization's polio eradication campaign. The latest cases bring to 22 the number of confirmed instances of polio in the country.
Heymann noted that the disease had spread across the country from the initial four cases that were recorded in the Red Sea port of Al-Hudaydah last week. The country was previously thought to be to be free of the disease.
"Ongoing field investigations have identified additional suspected polio cases across the affected governorates in Yemen," WHO said. "Low immunization rates among Yemen's children may facilitate the spread of the virus."
Heymann said all the infected so far were children.
"It will never be possible to tell how this virus came into Yemen," Heymann told reporters. "What's important is that the virus is there and that we have to stop it."
A nationwide immunization campaign is planned for the second half of May, Heymann said. WHO conducted one such vaccination drive last month, after it declared Yemen a high-risk country for polio reintroduction.
A third vaccination campaign might also be necessary in June, he added.
"We are confident that this vaccine will help finish polio" in Yemen, Heymann said.
Yemen is the most recent of 15 previously polio-free countries that have reported new cases since 2003 after a vaccine boycott in Nigeria was blamed for causing an outbreak that spread the disease to other countries.