Hazard warning

The World Health Organisation aims to have officially eradicated polio by 2005. But even if it succeeds, we still have reason to fear this potentially deadly disease, says Tom Clarke

Going by the figures, polio has had it. The regular outbreaks that killed or paralysed 300,000 children a year are a thing of the past. Polio has been driven from all but 10 countries worldwide: last year there were fewer than 600 cases. Last week public-health experts meeting in Copenhagen reviewed the evidence for certifying Europe, including the former Soviet Union, free of the disease. Now focusing on just a few problematic regions, the World Health Organisation wants to declare the war against the polio over by the end of 2005.

Poliomyelitis has probably been killing and paralysing infants for most of human history. Spread by water contaminated with human waste, the virus infects the gut from where it can spread into the spinal cord. This only occurs in about 1 per cent of cases, but in those it can lead to permanent paralysis and death.

In the 1950s two vaccines were developed. One made of killed poliovirus relies on the fact that the dead virus can prime the immune system to attack and destroy the real thing. This "inactivated" vaccine must be injected. The other vaccine is made of living, but weakened, polioviruses. While able to reproduce in the gut and provide strong immunity, the viruses are too feeble to invade nerves and cause disease.

Eradicating polio is not an unrealistic goal. In 1980, following a 12-year vaccination effort, the WHO declared the world free of smallpox. This was the first instance of a disease being vaccinated out of existence. In 1988 it set out to do the same for polio. When choosing its weapon, the WHO picked the oral polio vaccine. Simply dropped into the mouth, OPV can be given by untrained personnel. It boosts the immune response in the gut, meaning that vaccinated children destroy any virus passing through them, helping to break the cycle of transmission. Because it is a living virus, vaccinated children can also pass the vaccine on to other, perhaps unvaccinated children – boosting OPV's overall effect. OPV is also cheap.

Since 1988 the number of cases of polio has dropped steadily. But the challenges of the polio endgame are being thrown into sharp relief. Getting the vaccine tochildren who still need it is a major stumbling block. Of the 537 cases reported last year, nearly all were in areas of unrest. Fighting in polio hotspots such asPakistan and Afghanistan makes children, especially refugees on the move, difficult to reach. Poor sanitation spreads the virus.

Yet these problems pale by comparison to the problem of the vaccine itself. Experts knew all along that because the vaccine virus is living, it could, in theory, revert back to its wicked self. Polio is one of the fastest-mutating viruses known to science. The vaccine viruses are no different. Through a random process called "back-mutation" the virus can replace the lethal genes it lost while being turned into a vaccine. Polio experts concluded that such reversion would be safe provided every child in a population was vaccinated. Even if the virus did back-mutate, everyone would have immunity.

But in October 2000 on the Caribbean island of Hispaniola, two children were killed and 19 paralysed during a mysterious polio outbreak. The region had been declared polio free by the WHO. The virus originated from a single dose of OPV given to one child. Not only had the vaccine virus back-mutated, it had borrowed useful genes from other harmless viruses living in the gut of its victims, possibly adding to its strength. The outbreak had occurred, just as experts predicted, because in some parts of the island only 20 per cent of children had been properly vaccinated.

Countries with tight public-health budgets and other diseases to tackle have little incentive to fight a disease that officially no longer exists. The worry now is that as fear of polio wanes, so will commitment to eradication, and vaccine-associated outbreaks will become commonplace.Some experts are backing a spectacular vaccination finale. Children worldwide should be filled with vaccine, followed by a wave of vaccine withdrawal around the globe, phasing out the vaccine and its dangers once and for all. Others argue that OPV should be replaced with the injectable, killed virus vaccine or a totally new one.

If only if it were that simple. Eradicating polio, and then eradicating the vaccine that eradicated polio, is hard enough, but there is still the threat that the virus could be released back into the environment. Smallpox was eradicated over 20 years ago, but its potential as a biological weapon is still feared. Polio would make a poor weapon – it only kills 1 per cent of those it infects and spreads erratically – but in light of recent events, it is wise to fear the unthinkable.

But the greatest threat comes from a far more benign source. Countless laboratories around the world hold countless stool samples taken for a host of routine purposes. When polio is eventually removed from the wild, they will become a serious biohazard. Simply, polio will be around in one form or another for a long time to come, making the need for continued worldwide vaccination as important now as ever.

Spending on polio must continue long after the 2005 deadline. The belief that polio is vanquished has already caused the cashflow to slacken – the WHO is nearly $300m (£200m) short of its $1bn (£700m) budget for 2002-2005. The funds, and the will, must be maintained if the world is to be polio free.

Tom Clarke is a science writer for the journal 'Nature'