The largest U.S. supplier of seasonal flu vaccines said it is running behind on shipping those vaccines — partly because of the crunch to produce millions of doses of the swine flu vaccine.

The pharmaceutical company Sanofi Pasteur said it has shipped more than half of the 50.5 million doses of seasonal flu vaccine order by U.S. health care providers. But the company has sent notices to customers indicating that additional doses may be delayed.

Company spokeswoman Donna Cary said it could be November before some customers get the rest of their orders.

The delay already has forced some doctor's offices to turn away parents seeking season flu vaccines for their infants and toddlers and caused some public health offices to cancel scheduled community vaccination clinics.

"We understand it does create an inconvenience for some people who wanted to hold their seasonal influenza campaigns earlier," Cary said. "We apologize for that, but we're doing every thing we can."

The federal Centers for Disease Control and Prevention said Sanofi Pasteur had alerted it a while ago that it may need to delay shipments, but the federal agency did not learn the details until Thursday, said spokesman Tom Skinner.

The delay isn't surprising nor cause for big concern, Skinner said, because about 70 million of the nation's expected 114 million doses already have been delivered and vaccinations started unusually early. October is the traditional time when seasonal flu vaccine clinics open.

"Vaccine, while it's coming out, may not be coming out to some of the providers when they thought they were going to get it," Skinner said. But, "it's coming, and people may have to be patient and persistent in inquiring about when they can get it."

Sanofi Pasteur, the Swiftwater, Pa.-based vaccines division of the French drug-maker Sanofi-Aventis SA, is producing about 45 percent of seasonal influenza vaccine, making it the largest of the country's five suppliers.

Right now, swine flu is the dominant virus. Even with the production delay, health officials expect plenty of the seasonal flu vaccine to be available by the time the regular flu typically becomes more widespread.

Cary attributed part of the backlog at Sanofi Pasteur to its simultaneous production of 75.5 million doses of H1N1 vaccine for the United States. Also contributing to the delay is the fact that this year's seasonal flu vaccine includes a strain that develops more slowly in laboratories, she said.

But "everybody who ordered vaccine from us will be getting it," Cary said.

At the Jefferson City Medical Group, which serves about 70,000 patients in mid-Missouri, the vaccine delay primarily is affecting children younger than three — considered to be a high-risk group for the flu.

The medical group has received just 150 of its 700 doses of preservative-free vaccines, which Missouri law stipulates must be used for children between six and 35 months old. Doctors used the last of those this week and have been told they may have to wait another month to get the rest of their supply, said Cindy Andrews, a nurse who is helping coordinate clinic's flu vaccines.

In northeast Kansas, at least three health care providers have postponed flu-shot clinics indefinitely because vaccine supplies from Sanofi Pasteur have been delayed.

The Cotton-O'Neil Clinic in Topeka, had planned a walk-in clinic next week. However, as of this week, 9,000 vaccine doses — or a third of the clinic's total supply for the season — hadn't arrived, spokeswoman Nancy Burkhardt said.

"We do not know why they haven't shipped it," Burkhardt said.

Lyon County, Kan., health officer Ann Mayo said an Oct. 6 drive-through vaccination clinic has been canceled because the health department has used the 1,660 doses it received in August and hasn't received an additional 1,500 doses that were due to arrive this week from Sanofi Pasteur.

The National Community Pharmacy Association, which advocates on behalf of 23,000 independent pharmacies nationwide, said it started hearing concerns about the availability of the seasonal flu vaccine in early September. Oklahoma, Kansas, Georgia were some of the hot spots, said association spokesman Kevin Schweers.

Walgreen Co., which has about 16,000 certified immunization providers in more than 7,000 pharmacies nationwide, said it ordered several times its normal supply of seasonal flu vaccines this year because of the increased public awareness caused by the swine flu. It used several different suppliers and expects the last of its doses to arrive in the next couple weeks, said spokesman Jim Cohn.

There is no shortage now. But Walgreens — which already has administered 2.5 million seasonal flu vaccines — doesn't expect its supply to last past the end of December, Cohn said.

"We're encouraging people who want to get a seasonal flu shot to do it now," he said.