A genetic investigation is ongoing to determine the precise origin of the outbreak, says WHO. Experts fear that the number of cases will rise in the immediate future.

Teams of WHO and Ministry of Health epidemiologists and pediatricians remain on the ground to investigate and control the outbreak, and to intensify the planning for appropriate supplementary immunization activities.

Low Immunization Rates Among Yemen's Children

Four cases of polio were confirmed on April 20 in just one governorate in the southwestern part of Yemen on the Red Sea coast. The latest 18 cases occurred across five governorates throughout Yemen, including in two districts in the country's capital Sanaa. This suggests the virus has spread across the country.

Ongoing field investigations have identified additional suspected polio cases across the affected governorates in Yemen. Low immunization rates among Yemen's children may facilitate the spread of the virus.

Experts are planning to use the recently developed monovalent oral polio vaccine type 1(mOPV1) in the outbreak response. This new vaccine enables a precisely tailored immunological response to the type 1 poliovirus that is causing the outbreak.

Compared to the commonly used trivalent OPV, which offers protection against all three types of wild poliovirus, mOPV1 provides a greater immunity to type 1 wild poliovirus with fewer doses, according to WHO.

Use of mOPV1 is expected during a nationwide immunization campaign in the second half of May. Yemen already had conducted a mass campaign on April 11-14, as the country was considered to be at high-risk of polio re-infection from nearby Sudan where an outbreak of polio continues.

Global Funding Gap of $50 Million

Experience in polio eradication demonstrates that outbreaks can be contained quickly with high-quality immunization campaigns that reach every child under five years old.

Dedicated donor support and strong partnerships with the private sector, as well as swift development of the mOPV1 vaccine, enabled the previous campaign in Yemen. However, a global funding gap of US$50 million must urgently be filled by July, says WHO, to finance continued intensification of immunization campaigns in the second half of the year.

Global eradication efforts have reduced the number of polio cases from 350,000 annually in 1988 to 1,267 cases in 2004.

Six countries remain polio-endemic, with a further six where polio transmission is re-established. Concern is high that the ongoing outbreak of polio in Africa might lead to re-infection of more countries in the polio-free Horn of Africa and the Middle East.

Yemen is the most recent of fifteen polio-free countries that have reported cases of polio since the epidemic began in late 2003, notes WHO.