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 4,100 euros ($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 can still avoid prison — their sentences were delayed to give them a chance to vaccinate their children. But if that deadline also passes without their children receiving the injections, the parents could be put behind bars.

Because of privacy laws, Belgian officials would not talk specifically about the case, such as why the parents refused the vaccine or how much longer they have to vaccinate their children.

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.

"Polio is a very serious disease and has caused great suffering in the past," said Dr. Victor Lusayu, head of Belgium's international vaccine centre. "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."

Some ethicists back the hardline 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 vaccinated against many diseases including polio, but most states allow children to opt out if their parents have religious or "philosophical" objections.

In the U.S. state of 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 nobody 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 — Afghanistan, India, Nigeria and Pakistan — 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.

Polio is a highly infectious disease spread through water that mainly strikes children under five. 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, children can die when their breathing muscles are immobilized.

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 like polio puts others at risk as well.

"Most of the time, polio outbreaks do spill into the general population," Cochi said.

Ethicists argue that people who refuse vaccinations are taking advantage of everyone else who has been vaccinated. Once the majority of a population is vaccinated, there are few susceptible people the disease can infect, thus lowering the odds of an outbreak.

People who refuse to be vaccinated are "free riders," Harris said. "They can only afford to refuse the vaccine because they are surrounded by people who have fulfilled their obligations to the community."

Health officials doubt that Belgium's strategy will be useful to countries 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.