Vaccine switch can loosen polio's tenacious grip in India
Nov 21, 2006 - 5:09:12 PM
, Reviewed by: Priya Saxena
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But the vaccine's weakened strains can interfere with each other inside the body, producing immunity to one strain but not another.
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By Arun Kumar, Indo-Asian News Service,
[RxPG] Washington, Nov 21 (IANS) 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 latest issue of the journal Science, published by American Association for the Advancement of Science.
Nicholas Grassly of Imperial College in London and his colleagues analysed 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 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 polio virus 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 parents and 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.
'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 programme 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 programmes,' he said.
Grassly's team focused on Uttar Pradesh and Bihar, 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 immunised against the dominant strain of poliovirus by the end of 2005. In contrast 85 percent of children were successfully immunised 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 faeces 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 diarrhoea, which can prevent the vaccine from working effectively. So, it is in exactly those areas where polio transmission is favoured 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.
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