Many 'defenceless' against polio

People in Namibia are being immunised against polio

Increasing numbers of people around the world are vulnerable to polio because they have not been previously exposed to the disease, health experts warn.

The World Health Organization says adults who missed being vaccinated as children and who have not been exposed to the wild virus are most at risk.

There have been 19 confirmed and 150 suspected polio cases in Namibia, which had been polio-free since 1995.

New Scientist magazine reports the virus has arrived via Angola.

There are bigger immunisation gaps than at any time in the last 25 years Dr Bruce Aylward, World Health Organization

The trail began in Uttar Pradesh in India.

Because people in Namibia had not previously been exposed to polio, they had no defence against the disease, which came from an area where it was still present.

Polio, a highly infectious disease which affects the nervous system and can result in paralysis, is transmitted through contaminated food and drinking water, contact with faeces from an infected person or contaminated swimming pool water.

It has been eradicated in much of the world but is still endemic in some countries.

Those affected in the Namibian outbreak have been aged between 15 and 45.

In contrast, most polio outbreaks predominantly affect children under five.

The disease is more likely to cause paralysis in adults.

Last month, a campaign was launched to immunise the whole population of Namibia against polio.

Susceptible people

People can develop an immunity to the disease by being infected naturally, through vaccination or by catching the live but weakened version of the virus which is in the oral polio vaccine from newly immunised children.

However, natural infections are disappearing and vaccination rates are dropping in many countries as the risk fades.

In addition, 35 industrialised countries - including the UK - have switched from using the oral vaccine to a safer version that uses a killed virus, which will not infect others.

The UK only switched to the safer vaccine last year, partly because of its links to places such as Nigeria and India where polio is still endemic.

Bruce Aylward, head of the World Health Organization's polio eradication drive, said: "This just shows that we have to eradicate polio everywhere because, while endemic areas persist, the virus will find susceptible people."

He added: "There are bigger immunisation gaps than at any time in the last 25 years.

"If the virus exists, it will blow up in an area."

Dr Aylward said that the two ways of preventing the spread of polio were for all countries to aim to vaccinate everyone and for good surveillance to be in place so any outbreaks are picked up early.