The United States could find itself short of swine flu vaccine if the virus becomes much more lethal and countries start to scramble for more of the vaccine, experts warn.

They noted that the United States makes only 20 percent of the flu vaccines it uses. The situation is even worse in Britain, which imports all its flu vaccines. Only a few countries are self-sufficient in vaccines.

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

Leaders of countries with adequate supplies of swine flu vaccine won’t be willing to share with other nations, experts predict.

“Pandemic vaccine will be a valuable and scarce resource, like oil or food during a famine,” David Fidler, an Indiana University law professor who has consulted for the World Health Organization, told the AP. “We’ve seen how countries behave in those situations, and it’s not encouraging.”