Following an unusual outbreak of poliovirus affecting mostly adults, Namibia, a country in southwestern Africa, inoculated its entire population in the last week of June. The country is now preparing for follow-up campaigns.

The outbreak has spurred major concerns across the world because the disease has resurfaced 10 years after Namibia was declared polio free; the last polio outbreaks were reported in 1993 and 1994. Besides, in the vast majority of cases, the virus poliomyelitis attacks children under the age of five, not adults. But this time, as per media reports, at least 75 per cent of the suspected and confirmed cases are above 20 years of age. By the time the Namibia government initiated immunisation, 14 of the affected (all adults) had succumbed to paralytic polio. Earlier, Albania, a southeastern European country that was declared polio-free, had a similar outbreak in 1996 and ran a polio-campaign for all its residents.

According to epidemiologists, the virus traced in Namibia is a strain found in neighbouring Angola last year.

Two clusters of cases are found around the capital city of Windhoek and in the country's northern region, besides isolated cases in several other parts, say health officials of Namibia. The outbreak in Namibia may thus be difficult to contain. Health experts do not discount the possibility of the virus spreading to neighbouring Botswana and South Africa, besides causing a resurgence of the disease in Angola.

"Such recurrence of poliovirus in countries that have already been declared polio-free is a setback to the global polio eradication initiative and implies that the whole initiative has a conceptually flawed strategy,' says Onkar Mittal, a consultant on health policy and governance in India. It is likely that those who fell ill did not receive immunisation as children, for the country began its routine immunisation for polio in 1990, say Namibian health officials. But according to Mittal, the issue is more serious as the "epidemiology of polio is changing'.

Health experts are now more concerned about the potential for more outbreaks due to both wild and vaccine-derived virus as a consequence of effects on the natural history of disease. "Zero polio status is suggestive but not conclusive of the interruption of transmission of wild poliovirus, which may circulate silently,' Mittal says.

Experts now call for a change in strategy of the global polio eradication initiative. Otherwise, they caution, the world should get ready for more polio outbreaks.