This year’s flu season has started slowly in the United States, where there are concerns about a shortage of vaccine caused by production problems at a major manufacturer, the head of the Centers for Disease Control and Prevention said on Monday.

None of the 50 states or Washington, D.C., reported widespread influenza activity between Nov. 21 and 27, the latest period for which full data was available, according to CDC Director Dr. Julie Gerberding. Most saw only sporadic outbreaks of the potentially deadly virus.

“The flu is getting off to a slower start this year than last year,” Gerberding said during a presentation at an American Medical Association meeting in Atlanta.

But she added that it was still too early to say whether the United States was poised for a relatively easy flu season. “We can’t be confident that we’re not going to see more flu by the end of the season,” Gerberding said.

Influenza kills about 36,000 Americans and hospitalizes 200,000 each year. Exact figures on flu-related deaths are difficult to come by because U.S. doctors are not required to report such deaths.

The U.S. flu season typically runs from October through March. Last year’s epidemic hit the nation early, but then moderated in the winter.

That early start in 2003, coupled with recent production problems at Chiron Corp. which was supposed to supply about 48 million of the estimated 100 million doses needed in the United States for this flu season, prompted fears that the nation would be overwhelmed by influenza.

British authorities in October suspended Chiron’s license to make vaccines at its Liverpool plant. The company, based in Emeryville, California, announced that it would not be able to deliver any of the promised vaccine.

U.S. health officials rounded up several million more doses but still estimate that only 61 million are available.

The Atlanta-based CDC, which is spearheading efforts to allocate vaccine to the most needy, has recommended that only high-risk people - seniors, people with chronic diseases, babies, and pregnant women - get the vaccine.

Those who care for anyone in those groups should also be vaccinated. All others are urged to take alternate steps, such as frequent washing of hands and taking anti-flu treatments, to avoid getting sick.

About 90 percent of annual U.S. flu deaths occur among those 65 and older.