London ? An ugly scramble is brewing over the swine flu vaccine — and when it becomes available, Britain, the United States and other nations could find that the contracts they signed with pharmaceutical companies are easily broken.

Experts warn that during a global epidemic, which the world is in now, governments may be under tremendous pressure to protect their own citizens first before allowing companies to ship doses of vaccine out of the country.

That does not bode well for many nations, including the United States, which makes only 20 percent of the regular flu vaccines it uses, or Britain, where all of its flu vaccines are produced abroad.

“This isn’t rocket science,” said Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota. “If there is severe disease, countries will want to hang onto the vaccine for their own citizens.”

Experts say politicians would not be able to withstand the pressure.

“The consequences of shipping vaccine to another country when your own people don’t have it would be devastating,” added David Fedson, a retired vaccine industry executive.

About 70 percent of the world’s existing flu vaccines are made in Europe, and only a handful of countries are self-sufficient in vaccines. The U.S. has limited flu vaccine facilities, and because factories can’t be built overnight, there is no quick fix to boost vaccine supplies.

Earlier this week, the U.S. Department of Health and Human Services announced it was spending $884 million to buy extra supplies of two key ingredients for a swine flu vaccine. The U.S. has contracts to get swine flu vaccines from Sanofi Pasteur, MedImmune, GlaxoSmithKline and Novartis. Sanofi Pasteur and MedImmune both have vaccine plants in the U.S., while GlaxoSmithKline and Novartis have plants in Europe.

Even if the U.S. held onto all the swine flu vaccine produced domestically, it would still not be enough for all Americans. About 80 million Americans are vaccinated against the seasonal flu every year.

In 2004, when problems with the U.S.’ flu vaccine supply at a British factory hit, there were less than 54 million shots available. Flu vaccines were saved for those in high-risk groups like the elderly and pregnant women, and officials asked other people to simply forgo their usual flu shot.

If there are limited swine flu shots during a pandemic that turns more serious, experts are not sure people will be as willing to skip getting a vaccine.

Last week, the World Health Organization reported nearly 95,000 cases of swine flu, including 429 deaths worldwide. If swine flu turns deadlier in the winter, the main flu season in the Northern Hemisphere, countries will likely be clamoring for any available vaccines.

“Pandemic vaccine will be a valuable and scarce resource, like oil or food during a famine,” said David Fidler, a professor of law at Indiana University who has consulted for WHO. “We’ve seen how countries behave in those situations, and it’s not encouraging.”