Yemen was first certified polio-free, after more than a three-year safety period, in February 2005, but the country was reinfected around the same time after a pilgrim returning from Saudi Arabia caught the polio virus from a fellow pilgrim from Nigeria. The reinfection resulted in an outbreak of 473 cases, according to WHO. Since 2006 no case of polio has been reported.
"We can say now that Yemen has been free of polio for the last three years. It took more than three years, during which many anti-polio campaigns were conducted, to eradicate the disease," Mohammed Osama Mere, a technical adviser to WHO's Expanded Programme on Immunisation, told IRIN on 16 May.
Mere said WHO went through a validation process - involving the deployment of consultants to monitor anti-polio drives over the past three years - before declaring Yemen polio-free.
"We also have the surveillance system, which the Ministry of Health established in cooperation with us. This system was sensitive and mature enough to detect any polio virus and any suspected cases of polio," he said.
Samples of suspected polio cases were also sent weekly by the Health Ministry to the WHO office in Sanaa, which in turn sent them to the WHO regional laboratory in Cairo, the official said.
WHO also closely monitored routine polio immunisations. Mere said coverage was 87 percent, which was significant.
According to Mere, the last measure taken before granting Yemen polio-free status was the laboratory contamination check. The Health Ministry recruited a large team to visit every single medical laboratory in the country and issued questionnaires about stool specimens (for detecting polio cases).
"Again we as WHO sent independent teams to verify the information we got from the Ministry of Health and reviewed all the questionnaires. We then brought in an international consultant to evaluate the situation on the ground," Mere said.
Threat from Sudan
Despite the WHO declaration, national anti-polio campaigns would continue - to prevent any possible introduction of the polio virus from neighbouring African countries, according to Essa Mohammed, head of the Health Ministry's National Programme for Vaccination.
He said anti-polio campaigns would be conducted from time to time "as supplementary measures". Mohammed told IRIN a new anti-polio drive - targeting 4,181,729 children under five - began on 17 May throughout Yemen.
He said one of the aims of the current campaign was to ensure protection against the virus which, WHO says, is currently found in Sudan.
Some 38,487 health workers and volunteers are involved in the door-to-door campaign, which is funded by WHO, the UN Children's Fund (UNICEF) and the government, according to him.
According to WHO, there are still four countries in the world that remain polio-endemic - Afghanistan, India, Nigeria and Pakistan.