And in the U.S., there are no licensed flu vaccines that use adjuvants.

WHO's vaccine experts recommend that countries decided that certain groups should get the vaccine first - like pregnant women, people with chronic respiratory problems or obesity, children, and possibly young to middle-aged adults, who have been disproportionately affected by the virus.

The decision to start vaccinating people against swine flu - which so far remains a mild virus in most people - will ultimately be a gamble, since there will be limited data on any vaccine. Until millions of people start receiving the shots, experts will not know about rare and potentially dangerous side effects.

The public health community may still be scarred by the U.S.' disastrous 1976 swine flu vaccination campaign, which was abruptly stopped after hundreds of people reported developing Guillain-Barre syndrome, a paralyzing disorder, after getting the flu vaccine.

Several drugmakers are currently considering using adjuvants, ingredients used to stretch a vaccine's active ingredient, which could allow for many more vaccine doses. But little or no data exists on the safety of vaccines with adjuvants in populations including children and pregnant women. And in the U.S., there are no licensed flu vaccines that use adjuvants.