A top U.S. health official has urged healthy people to defer their influenza shots this season, after the supply of vaccinations was abruptly cut in half.

The head of the Centers for Disease Control and Prevention, Dr. Julie Gerberding, appealed for the voluntary rationing Wednesday. She also called for a long-term solution to avoid future supply shortages.

U.S. health officials say the flu vaccines should be reserved for babies and toddlers, people older than 65, pregnant women, health care workers and people with chronic health problems.

The United States will get roughly one half of its expected supply of influenza vaccines this year because British regulators have suspended the license of a Liverpool factory that produces a major brand.