The number of Americans with influenza has jumped since January, suggesting the worst of the 2004-2005 flu season may be ahead, federal health officials said on Thursday.

The U.S. flu season, which typically runs from October through March, began slowly despite fears a vaccine shortage would leave millions of unvaccinated Americans vulnerable.

It picked up speed recently, with 33 states reporting widespread flu activity for the week ending February 19, 2005, according to data released by the Centers for Disease Control and Prevention.

A total of 8.5 percent of deaths reported by 122 cities across the nation were attributed to pneumonia and influenza for that week, the highest level in the current flu season and above the weekly epidemic threshold, the CDC said.

Nine children have died from flu in the past two months.

“This may be as bad as it gets, but it could continue to go up,” said Lynnette Brammer, an epidemiologist in the CDC’s influenza branch.

Influenza, which is marked by respiratory inflammation, fever, muscular pain and intestinal tract irritation, is believed to kill about 36,000 people and hospitalize 114,000 in the United States every year.

Exact figures on flu-related deaths are difficult to get because U.S. doctors are not required to report such deaths.

The rise in flu cases came amid efforts by the federal government to ensure the most needy were vaccinated.

Millions of people were advised last fall to forgo a flu shot due to an anticipated shortage of vaccine following production problems at vaccine manufacturer Chiron Corp., which was supposed to supply about half the needed vaccine.

British regulators on Wednesday cleared Chiron’s flu vaccine plant in England to resume production in a move that should ensure supplies for the next flu season.

The Atlanta-based CDC, which is spearheading the bid to allocate vaccine, has recommended 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 anti-flu treatments.

Although health experts say it is better to get vaccinated early in the flu season, Brammer noted people could still get protection from a shot at this time of the year.