CHICAGO (AP) – The maker of flu shots for children 3 and under said Monday that most of this year’s U.S. supply will be delayed for a least a month because it is taking longer than expected to produce the vaccine – a development that worries some doctors.

Sanofi Pasteur, the sole manufacturer of FluZone, the injected flu vaccine for children, said some health care providers will not receive their full allotment until late November or early December.

But the vaccine will still be delivered within the time frame recommended by the federal Centers for Disease Control and Prevention, and well before the usual height of the flu season, which typically peaks between late December and March, the company said.

Spokeswoman Patricia Tomsky said about one-third of the 50 million doses of FluZone have been distributed. She said the children’s vaccine is taking longer than expected to prepare in the laboratory.

The CDC said that as of Oct. 6, a total 32 million doses of flu vaccine for children and adults had been distributed, and that 75 million doses are expected to be delivered by the end of October.

“We’re still saying there’s going to be plenty of vaccine available for October and November,” CDC spokesman Curtis Allen said.

Dr. Richard Lander of the American Academy of Pediatrics said the delay is significant because children should be vaccinated as early as possible.

“The longer the flu vaccine is in the body, the greater the chance the body can build up antibodies against the flu,” said Lander, a New Jersey pediatrician and chairman of the AAP’s section on administration and practice management.

Nevertheless, the AAP says that children “will still benefit greatly from receiving the vaccine into December, January and beyond.”