Drive to stamp out polio looks hopeful
New vaccine programme could rid the world of polio soon.
Polio has been making a comeback in India, but researchers say that a new vaccination campaign has a very good chance of finally eradicating the disease, by the end of the decade.
Polio is a highly infectious virus that can invade the nervous system, leading to paralysis and in some cases death. Vaccines have helped eliminate the virus from most of the world, but it still remains endemic in some countries.
This year particularly has seen a jump in the number of cases in India, with a total of 490 recorded, up from 66 in 2005.
Nicholas Grassly at Imperial College London, together with colleagues at the World Health Organisation, modelled the factors contributing to the outbreak in India and found that, perhaps unsurprisingly, poor sanitation and high population density are to blame. This not only means that polio spreads more easily, but also reduces the efficiency of protective jabs vaccines need time to infect the gut in order to confer immunity, and so children with diarrhoea often lack protection.
The good news is that Grassly's team predicts that a targeted campaign of high-density areas using a more specific vaccine should boost the efficiency of the treatment enough to allow polio to be eliminated from India. They report their work in Science1.
India's programmes use two types of live vaccine to counter the polio virus, which comes in three strains. Trivalent vaccines contain weak versions of all three strains, whereas monovalent forms contain only one specific type.
Using trivalent vaccines can lead to interference between the three types and reduce protection leaving only 10% of children protected after one dose, according to Grassly. Monovalent vaccines are three times as effective. Because type 1 is by far the most common strain in India (type 2 has been completely eliminated and type 3 is sporadic) using only this type of vaccine should ensure that a greater number of children are covered.
India began to introduce the monovalent vaccine in 2005. But officials faced difficulties in getting enough people vaccinated, thanks to myriad social factors including a lack of medical staff. "Unfortunately, at the same time as this new monovalent vaccine has been rolled out there was a significant drop in coverage among some communities, particularly in Uttar Pradesh," explains Grassly. But he maintains that the monovalent vaccine itself is doing its job. "It simply looks like it's a matter of coverage," he says.
In January of this year, the campaign was only able to vaccinate 89% of households in India, compared with 93% in 2005.
If the coverage can be boosted, they are optimistic that polio cases will decline with the new vaccine. "There's a golden opportunity for elimination of polio from India using the monovalent vaccine right now," Grassly says. "I think it's the best chance yet."
Sona Bari, a spokesperson for the World Health Organisation's Polio Eradication Initiative in Geneva, doesn't think this estimate is too far off the mark. But she cautions that the scientists can't account for some of the factors affecting the eradication programmes. "The model can't take into account political or social factors". Even so, she says, "it's not wildly optimistic."
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- Grassly N., et al. Science, 314. 1150 - 1153 (2006).