Clearly this is raising a red flag - whether it is a small one or a big one remains to be seen

Roland Sutter, CDC

Although the presence of polio has been confirmed in fewer than a dozen patients, more are showing the classic symptoms of polio infection, including paralysis.

It has been known for many years that the living viruses used in the most popular oral polio vaccine can survive passing through the body.

In fact, the harsh acid conditions in the gut can mean that only the toughest viruses pass through and are carried in the faeces.

These can replicate successfully in watercourses and sewers, and over time, can regain a virulence that their weakened predecessors lacked.

They now appear to be able to not only infect humans and cause illness, but pass from human to human.

First acknowledged outbreak

There have been several examples of cases of polio illness thought to have been caused by vaccine viruses, particularly among those with suppressed immune systems.

However, the Hispaniola incident appears to be the first where there has been an oubreak, rather than individual isolated cases.

The island states are thought to be most at risk because vaccination rates were low, and as there has been no wild poliovirus found for some time, natural immunity was also low.

Dr Ciro de Qudros, director of the Division of Vaccines and Immunisation at the Pan-American Health Organisation (PAHO), said the outbreak was now under control.

The oral polio vaccine has been successful

But he added: "It is a powerful reminder that even polio-free areas need to maintain high coverage with polio vaccine until polio eradication has been achieved."

Others, however, are saying this is potential hurdle for the World Health Organisation's hopes of worldwide polio eradication by 2005.

Its experts had always hoped that while the occasional isolated case of polio caused by the oral poliovaccine viruses might occur, outbreaks were unlikely even in a community with low vaccination rates.

Funding further vaccination programmes using non-live viruses may prove too expensive for the WHO.

Roland Sutter, from the US Centers of Disease Control (CDC), told Nature magazine: "Clearly this is raising a red flag - whether it is a small one or a big one remains to be seen."