Health officials in Pakistan say they have failed to immunise over 160,000 children against polio due to rumours the vaccine causes sexual impotence.
Parents in parts of northern Pakistan told the BBC news website they feared an "American conspiracy" to cut the fertility of the next generation.
Pakistan is one of four countries the World Health Organisation (WHO) says is a source of polio.
The WHO has led a $196m-a-year campaign to control the disease in Pakistan.
At least 39 cases of polio were reported in 2006, 15 of them in the North West Frontier Province (NWFP). The NWFP and the tribal areas account for 20% of those targeted for immunisation.
Worldwide 1,902 cases of polio were reported during the year, a recent WHO report said.
A WHO meeting in Geneva last October heard that children paralysed by polio around the world were infected by viruses originating from Pakistan, Afghanistan, India and Nigeria.
Radio rumours
The main opposition to the drive in Pakistan came from local clerics who run illegal FM radio channels in many NWFP districts and the tribal areas, say officials.
Amirullah Khan, a resident of NWFP's Swat district, quoted Maulana Fazlullah of a local FM channel as telling his listeners the vaccination drive was "a conspiracy of the Jews and Christians to stunt the population growth of Muslims".
Maulana Fazlullah confirmed this to BBC, saying if the international organisations were keen on improving the health of the Muslims, they should help the hepatitis-C patients in the area.
Last year samples of the vaccine were laboratory-tested after a petition in Peshawar High Court alleged they contained oestrogen.
The hormone was not found, said Dr Waheed Khan, a health official.
But the FM channels have won many supporters in the more conservative areas of NWFP and the tribal region, officials have said.
Vaccine teams beaten up
A WHO report for 2006 said 66 localities in these areas were not covered by the immunisation staff due to logistical problems, and coverage in 320 localities was "poor" because of the anti-vaccine propaganda.
In some areas immunisation teams were beaten up by local people, officials said.
Elsewhere parents just refused to get their children immunised.
The WHO says global polio eradication efforts have reduced the annual incidence of polio worldwide from 350,000 cases in 1988 to 1,902 in 2006.
But the NWFP health officials believe anti-vaccine propaganda may jeopardise WHO's aim of making the world polio-free.
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