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Science study explains polio's tenacious grip in India

Suggests vaccine switch may enable eradication

New research helps explain polio's persistence in India despite massive immunization efforts and offers hope for the campaign to stamp out the virus once and for all. The study, whose authors include some of the experts heading the global polio eradication effort, appears in the 17 November issue of the journal Science, published by AAAS, the nonprofit science society.

Polio is a highly infectious disease that primarily affects children under three years of age. Paralysis develops in a small minority of polio cases but is permanent and can be fatal.

Nicholas Grassly of Imperial College London and his colleagues analyzed reports of children with paralysis in India, collected since 1997, and used a computer model to determine which conditions most strongly influence the persistence of polio in the country.

In the impoverished states of Uttar Pradesh and Bihar, where India's recent polio epidemics originate, poor sanitation and high population density are the key obstacles to eradicating the virus, the researchers report. These conditions pack a one-two punch: they make it easier for the poliovirus to spread, and they decrease the efficacy of the vaccine.

In place of the standard, "trivalent" form of the vaccine, which contains weakened versions of each of the three types of poliovirus, using the strain-specific "monovalent" vaccine should compensate for these obstacles and boost the vaccine's efficacy to the point that rapid polio eradication in India should be possible, the researchers report. India began introducing the monovalent vaccine in 2005.

"The question from the parents and the government officials in these regions is 'why, with so much vaccination, is polio still around?' Our paper shows that we're still seeing polio in India because the conditions there prevent the standard trivalent vaccine from working optimally, but this new vaccine should be able to finish the job," said Grassly.

"This study makes an important contribution to the discussion about reaching the polio endgame," said Barbara Jasny, Deputy Editor for Commentary at Science. "Time will tell whether the ambitious goal of polio eradication is possible, but these findings should be part of the discussions about the best way forward."

Since the late 1950s, vaccination has made this disease a distant memory in most of the world, but the virus has hung on in India and three other countries – Nigeria, Pakistan and Afghanistan – leaving children immobile or with weakened limbs.

In 1988, when the governments of the world committed to globally eradicating polio, the disease was still paralyzing more than 1,000 children every day, according to the Global Polio Eradication Initiative. This coalition is spearheaded by the World Health Organization (WHO), UNICEF, Rotary International and the U. S. Centers for Disease Control and Prevention. About 1,500 polio cases have been reported for 2006 worldwide, mostly occurring in the four countries where the disease is still endemic.

Intensive immunization campaigns take place regularly in India, and children there have received many more doses of vaccine than children in the other endemic countries. But endemic polio transmission continues, and this year has even seen a dramatic rise in cases, leaving families, public health workers, and government officials in India frustrated, according to Grassly.

Since the year 2000 -- the target date for global eradication -- has come and gone, some experts have suggested that eliminating polio altogether is an unrealistic goal. A recent Policy Forum in Science, for example, proposed that it would be better to focus on controlling the disease instead (I. Arita et al, Science, 12 May 2006).

Grassly and his coauthors say the world shouldn't give up.

"Our study has a positive message. It identifies the problem with the standard trivalent vaccine in these particular states but suggests that new monovalent vaccine will be very effective, provided a very high proportion of children, from all population subgroups, are immunized during each campaign," Grassly said.

"The eradication program has come so far, and it's getting very close. Finishing the job is very important. It's been a $4 billion investment, but it has a finite goal. That's the aim of eradication -- to save children from paralysis and permit countries to place resources they would have to spend for polio prevention and treatment into other disease control programs," he said.

Grassly's team focused on the states of Uttar Pradesh and Bihar in northeastern India, which the country's vaccination campaign has also targeted. These states were some of the smallpox virus' last refuges as well.

Basing their estimates on the number of times children in these states were vaccinated with the standard trivalent vaccine, the authors calculated that only 74 percent of children under five years old in these states were successfully immunized against the dominant strain of poliovirus by the end of 2005. In contrast, 85 percent of children were successfully immunized in the rest of India with this same vaccine.

The reduced efficacy of the trivalent vaccine in Uttar Pradesh and Bihar appears to be the result of unsanitary and overcrowded living conditions, according to the researchers. The poliovirus typically passes through the feces of infected people and spreads when people eat or drink contaminated food or water. These conditions also mean a child is more likely to have other infections and diarrhea, which can prevent the vaccine from working effectively. So, it is in exactly those areas where polio transmission is favored that the efficacy of the vaccine is compromised.

The type 1 strain of the polio virus is now the dominant strain in India, eliminating the need for the standard trivalent vaccine, the authors report. The trivalent vaccine is typically used when more than one strain of the virus is at large in the population. But, the vaccine's weakened strains can interfere with each other inside the body, producing immunity to one strain but not another.

Carefully planned vaccine strategies using the monovalent vaccine could increase the immunization efficacy, enabling polio's eradication in India, Grassly and his colleagues conclude.

"Putting some numbers on the efficacy of the vaccine gives a clear basis for expanding and continuing with the monovalent vaccine strategy," Grassly said.

The obstacles to eradication vary among the endemic countries. In some areas of Afghanistan, for example, where violent conflicts are occurring, health workers must wait for negotiated cease-fires, called "days of tranquility," to reach children for vaccination.

Natasha Pinol | EurekAlert!
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