As doctors struggle to eradicate polio worldwide, one of their biggest problems is persuading parents to vaccinate their children. In Belgium, authorities are resorting to an extreme measure: prison sentences. Two sets of parents in Belgium were recently handed five-month prison terms for failing to vaccinate their children against polio. Each parent was also fined $8,000. "It's a pretty extraordinary case," said Dr. Ross Upshur, director of the Joint Centre for Bioethics at the University of Toronto. "The Belgians have a right to take some action against the parents, given the seriousness of polio, but the question is, is a prison sentence disproportionate?" The parents' sentences were delayed to give them a chance to vaccinate their children. But if that deadline also passes without the children receiving the injections, the parents could be put behind bars. Because of privacy laws, Belgian officials would not talk specifically about the cases, such as why the parents refused the vaccine or how much longer they have to get their children vaccinated. The polio vaccine is the only one required by Belgian law. Exceptions are granted only if parents can prove their children might have a bad physical reaction to the vaccine. There are no exceptions for people who object to vaccinations on religious grounds. "Polio is a very serious disease and has caused great suffering in the past," said Dr. Victor Lusayu, head of Belgium's international vaccine center. "The discovery of the vaccine has eliminated polio from Europe and it is simply the law in Belgium that you have to be vaccinated. ... At the end of the day, the law must be respected." The highly infectious disease is spread through water and mainly strikes children under 5. Initial symptoms include fever, headaches, vomiting, stiffness in the neck and fatigue. The polio virus invades the body's nervous system and can lead to irreversible paralysis within hours. In extreme cases, patients can die when their breathing muscles are immobilized. Some ethicists back the hard-line Belgian stance. "Nobody has the right to unfettered liberty, and people do not have a right to endanger their kids," said John Harris, a professor of bioethics at the University of Manchester. "The parents in this case do not have any rights they can appeal to. They have obligations they are not fulfilling." Aside from Belgium, only France makes polio vaccinations mandatory by law. In the United States, children must be immunized against many diseases including polio, but most states allow children to opt out if their parents have religious or "philosophical" objections. In Maryland, prosecutors and school officials in one county threatened truancy charges against parents who failed to vaccinate their children. The measure sharply reduced the number of unvaccinated children although no one has been charged. The only other case of mandatory polio vaccines is during the Muslim yearly Hajj pilgrimage in Saudi Arabia. Pilgrims from polio-endemic countries must prove they have been vaccinated. Saudi officials even give them an extra dose upon arrival at the airport. Since the polio virus can live in the human body for weeks, it jumps borders easily. That makes health officials even in developed countries nervous, since the threat of an outbreak remains as long as the virus is circulating anywhere. Incidence has dropped by 99 percent since the World Health Organization and partners began their eradication effort in 1988. But the virus is still entrenched in Afghanistan, India, Nigeria and Pakistan, and occasionally pops up elsewhere. For developed countries, imported polio cases could cause chaos in the health system, warned Dr. Steve Cochi, an immunization expert at the United States' Centers for Disease Control and Prevention. He said that unlike other medical problems, in which rejecting treatment only affects the individual, refusing a vaccine for a transmissible disease such as polio puts others at risk as well. Still, health officials doubt that Belgium's strategy will be useful to countries that are still battling polio. "It is up to individual countries to decide their own policies, but we do not feel that imprisonment would help," said Dr. David Heymann, WHO's top polio official.