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 might 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 everything we can."

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