Most vaccines must be injected, but there are exceptions, such as the human oral polio vaccine.

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The oral polio vaccine is the type widely used all over the world today, as it is cheaper.

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The oral polio vaccine can - in very rare cases - trigger polio.

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Working under 150, 000 supervisors they administered Oral Polio Vaccine (OPV) to children in numbers equal to the entire population of Pakistan (almost 172 million under-five children).

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Without exception, of the vaccines given to their children over the first 15 months of life, the one they felt most comfortable with was the oral polio vaccine.

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And last autumn the government was forced to withdraw an oral polio vaccine after it became clear it had been developed from serum which could potentially have been infected with BSE.

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But the injected killed virus version is what's given in the United States today because of the rare instances of people developing polio from the oral vaccine.

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The earliest polio shot, for example, was superseded by an oral vaccine, which was easier to administer and lasted longer.

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But with naturally occurring polio cases now so low there is a minority which claims the oral live vaccine is causing significant harm.

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