Opposition to immunisation in polio-endemic countries, particularly Nigeria and Pakistan, is one of the final hurdles to eradicating the crippling disease, according to presentations at a conference this week (11-15 November) in Atlanta, United States.
Participants at the annual meeting of the American Society of Tropical Medicine and Hygiene (ASTMH) heard that, in Pakistan, fears surrounding the safety and religious implications of vaccines discourage parents from immunising their children. Meanwhile, in Nigeria similar issues have led to a surge in polio cases, against the backdrop of a global decline.
The number of polio cases worldwide has been reduced by more than 99 per cent since 1988, the year the Global Polio Eradication Initiative (GPEI), a multi-billion dollar programme involving 200 countries, was launched in Geneva.
One of the three naturally occurring types of poliovirus (type 2) has been already eradicated globally, with no reported cases since 1999.
But despite this success, the battle is not yet over.
The objectives of GPEI's 2010-2012 strategic plan, which called for an end to the transmission of wild (naturally occurring) polioviruses by the end of 2012, will not be reached, experts say.
"Reaching the goal of global interruption of poliovirus transmission by the end of 2012 is unlikely," said Stephen Cochi, special adviser to the director of the Center for Global Health at the Centers for Disease Control and Prevention in Atlanta, United States.
Pockets of disease transmission remain in Afghanistan, Chad, Nigeria and Pakistan.
"However, there has been remarkable progress and a steep decline in polio cases from 2010," Cochi said.
The number of cases dropped from 1,352 in 2010 to 650 in 2011, to the current record low of cases of 181 as of 7 November 2012. More than 97 per cent of the remaining cases were recorded in the three endemic countries, Afghanistan, Nigeria and Pakistan.
Speaking at the ASTMH meeting, Anita Zaidi, director of research at the Aga Khan University's paediatrics department in Karachi, Pakistan, reported that parents' refusal to have their children vaccinated is widespread in the Karachi area.
In a study published in the November issue of the Bulletin of the World Health Organization, Zaidi and her colleagues report that in Karachi, parental refusal accounted for 74 per cent of missed immunisations, with refusals being particularly common among low-income ethnic Pashtun and high income families of any ethnic background.
"Reasons commonly cited among Pashtuns for refusing vaccination included fear of sterility; lack of faith in the polio vaccine; scepticism about the vaccination programme; and fear that the vaccine might contain religiously forbidden ingredients," the study says.
"There is a clustering of persistently unimmunised children among Pashtun children in Karachi, creating a reservoir for continued polio transmission," said Zaidi. "There is a need for specially targeted non-campaign approaches for reaching these children, if eradication goals are to be met."
Similar problems are reported in Nigeria, where cases of polio increased from 42 in 2011 to 101 in 2012, with the continuous risk that people in neighbouring polio-free countries become infected again.
Experts cite parental fears over vaccination safety and a widespread belief that vaccinations spread HIV and cause sterility as key factors curtailing vaccination uptake.
Adamu Nuhu, head of the disease control division at the National Primary Healthcare Development Agency in Nigeria's capital, Abuja, said that unless Nigeria took action to eradicate polio, "all the resources committed over the years will have come to nothing".
"Nigeria and Pakistan are the greatest remaining challenges to a polio-free world," added Cochi.
Bulletin of the World Health Organization doi:10.2471/BLT.12.106260 (2012)