When it comes to eradicating polio, the world is facing a dilemma. Developing countries can no longer be sure that using relatively cheap live polio vaccines will extinguish the virus, yet the replacement killed polio vaccines used in richer developed countries remain too expensive. However, two countries, Indonesia and Mexico are striking out with a new strategy.
The dilemma has been brought into focus by news that a strain of polio the world has already eradicated has re-emerged in northern Nigeria.
In 2003, cases of wild polio virus skyrocketed in Nigeria after its religious leaders denounced vaccination. 鈥淰accination only really recovered last year,鈥� says Bruce Aylward, head of the World Health Organization鈥檚 polio eradication campaign, and the number of people with polio has fallen with it. This year Nigeria has had 191 cases of polio as of 25 September, compared to 836 by the same date in 2006.
Advertisement
However, in the past two years 93 children were paralysed by type 2 polio, the last week. This strain stopped circulating in the wild in 1999, but is still used in some oral polio vaccines (OPVs).
Its re-emergence is not surprising, says Paul Fine of the London School of Hygiene and Tropical Medicine. Children given oral vaccine would have excreted live type 2 vaccine virus in their faeces, and this could lead to infection in children not yet vaccinated. An analysis of the gene sequences of the virus shows that this has happened on seven separate occasions. 鈥淭hat means thousands more were infected,鈥� says Fine, as many children would have been infected without displaying symptoms.
Aylward says the best way to contain type 2 polio in Nigeria is to continue vaccinating. Oral vaccines work well against type 2 polio, and the live virus cannot spread if everyone is immune. For example, the vaccine-derived polio virus also reached Niger, but it only caused three cases there and eventually petered out.
鈥淭he best way to contain polio in Nigeria is to continue vaccinating, as it cannot spread if everyone is immune鈥�
But the fact that a vaccine that has successfully eradicated polio could also be responsible for its resurgence poses a dilemma 鈥� one which may become a growing issue in countries where polio has been eradicated for so long that many people are un-vaccinated, and so are not immune to the disease.
No one really knows what the chances are that the virus in the oral vaccine will turn virulent in this situation. Some people harbour the virus for an unusually long time, and the WHO recently reported that such people are more widespread than previously thought, turning up in low-income countries as well as the industrialised world. 鈥淧eople have debated for years what happens when you stop OPV,鈥� says Fine.
We could soon find out. Last month, Jogjakarta province in Indonesia stopped administering oral polio vaccine, and Mexico plans to follow suit soon. Instead, they will give a killed polio vaccine already used in industrialised countries, which they hope will keep people immune while the live oral polio virus disappears from circulation.
However, the killed vaccine costs ten times as much as live oral vaccine, and many developing countries may not want to spend that much on keeping an eradicated disease at bay. But if they do not, they will remain vulnerable to an outbreak.
To break the impasse, scientists met last month in Bethesda, Maryland, to discuss new, cheaper ways of making killed vaccine, and drugs that can clear the virus from chronic carriers. Several candidates are already being tested.
