A young girl walks into a National Health Service walk-in center Tuesday in west London, which posted a sign on the door requesting that people with flu symptoms stay away. Countries worldwide are debating the best advice to give on avoiding swine flu.

Geneva ? Some Muslim countries are advising pregnant women not to attend the hajj pilgrimage. China is quarantining any visitor suspected of having a fever, while priests in New Zealand have been banned from placing Communion wafers on worshippers’ tongues.

It’s all part of a global effort to slow the spread of swine flu until a vaccine is ready, although experts are divided on whether the measures will work.

Students across Europe may have their summer vacations extended after the World Health Organization said Tuesday that closing schools was one option countries could consider.

Deaths from the H1N1 virus have doubled in the past three weeks, to over 700 from about 330 at the start of July, the agency said.

“We expect to see more cases and deaths in the future,” WHO spokeswoman Aphaluck Bhatiasevi told The Associated Press in Geneva.

The agency gave no breakdown, but as of last week, the United States had reported 263 deaths, Canada had 45 and Britain had 29. According to WHO’s last update on July 6, there were 119 deaths in Mexico.

Yet even the latest figures may seriously underestimate the true toll because not all swine flu cases are being picked up due to testing limitations.

The race is now on to develop a vaccine that is effective against the pandemic strain before the flu season begins this fall in the northern hemisphere. Estimates for when a vaccine will be available range from September to December.

In the meantime, the U.N. health agency is working with its national counterparts around the world to examine what countries can do.

“School closures is one of the mitigation measures that could be considered by countries,” Bhatiasevi told reporters.

Experts have argued that school closures may be among the most effective measures, but warn there may be a considerable economic downside, too.

Religious leaders have been drawn into the debate after authorities in Jordan and health officials at a conference in Saudi Arabia recommended that people thought to be most at risk, including pregnant women and those with chronic diseases, skip the hajj pilgrimage this year.

Arab health ministers are holding an emergency meeting today in Cairo to come up with a unified plan to confront the pandemic.

In New Zealand, the Roman Catholic Church imposed a ban on priests placing Communion wafers on the tongues of worshippers and on the sharing of Communion wine. It also asked parishioners to avoid bodily contact at services, including shaking hands.

In Chile, where 40 people have died from swine flu, authorities canceled a popular religious festival that normally draws tens of thousands of worshippers to the northern town of La Tirana, prompting protests from the faithful.

“The key question is whether citizens will accept the measures governments impose,” said Christian Drosten, head of the Institute for Virology at the University of Bonn in Germany.

“You need to get the population on board, otherwise your efforts won’t work,” he said. “Once people take the disease seriously, you’ll begin to see the kind of social distancing that limits infection.”

“But it’s all a question of culture,” Drosten added. “What works in Europe may not work in other countries, and vice versa.”

In Switzerland, supermarket chains are considering requiring customers to disinfect their hands and put on a face mask as they enter the store.

“We can put these measures in place as quickly we get food into the stores,” said Urs Peter Naef, a spokesman for the Migros chain, Switzerland’s biggest.

China’s practice of forcibly quarantining visitors has caused bewilderment elsewhere, particularly when hundreds of American, British and other foreign students have been sealed off in hotels for days on just the suspicion of infection.

Chinese officials in masks or hazmat suits board planes, pointing temperature guns at passengers’ foreheads. If a passenger is diagnosed with swine flu, anyone seated within three rows is often tracked down. Those quarantined get to leave if they are healthy seven days from the date they landed.

In Britain, health officials’ advice that women put off planning to have children due to the global outbreak was met with ridicule since the swine flu pandemic may last years.