One step closer to eradicating polio : New vaccine strategies will help make the world completely free of polio, says new research

A simple change to the way people are vaccinated against polio could help achieve global eradication of the virus, according to research published today in the journal Science.

The new study, by researchers from Imperial College London and international partners, explains the persistence of the disease in northern India, one of the few remaining places in the world where endemic polio has not yet been eradicated.The researchers found that poor sanitation and overcrowded living conditions in Uttar Pradesh and Bihar pose a dual challenge to the eradication effort. They encourage poliovirus to spread because the virus passes through the faeces of infected people and spreads when people eat or drink contaminated food or water. They also encourage other infections and diarrhoea that interfere with the efficacy of the oral polio vaccine.The researchers found that the oral poliovirus vaccine has a significantly lower efficacy in Uttar Pradesh and Bihar, where polio transmission persists, compared with the rest of India, where transmission of the virus is no longer supported. This means children who have received multiple doses of vaccine may still become infected with the virus.The researchers argue that the simple measure of using a 'monovalent' form of the polio vaccine alongside the standard 'trivalent' form in these areas could sufficiently increase the effectiveness of vaccination programmes to wipe out the poliovirus where it persists.The trivalent vaccine currently in use contains weakened strains of all three types of poliovirus, unlike monovalent vaccines, which are strain-specific. The trivalent vaccine is typically used when more than one strain of the poliovirus is at large in the population. The problem with trivalent vaccines is that the three strains can interfere with each other inside the body, producing immunity to one strain but not another.The researchers argue that as the type 1 strain of the virus is now the dominant one in India, it would be more effective to focus on the monovalent form of the vaccine.Dr Nick Grassly, lead author of the paper, from the Department of Infectious Disease Epidemiology at Imperial College London, said: "The global polio eradication programme has achieved a great deal. As expected, the last remaining pockets of transmission are the biggest challenge. These pockets of transmission act as sources for all the outbreaks we see around the world today. Our research shows that in northern India the efficacy of the trivalent vaccine is compromised. Given that this same vaccine has achieved remarkable results in other parts of the world, this suggested that there was biological interference with the vaccine. The new monovalent vaccine has potential to significantly boost immunity to the dominant poliovirus in these areas. A focus on high coverage with this new vaccine has the potential to eliminate polio from India, and bring the world one step closer to eradication."Polio is a highly infectious disease that primarily affects children under three years of age. Paralysis develops in a small minority of cases, which is permanent and can be fatal. The World Health Organisation's Global Polio Eradication Initiative, which started in 1988, has seen endemic polio eradicated everywhere except in parts of India, Pakistan, Afghanistan and Nigeria. When this initiative began it was estimated that polio was paralysing 1,000 children every day, whereas only 1,500 cases have been reported so far in 2006.(Source: Science : Imperial College London : December 2006.)