Health and Human Services Secretary Kathleen Sebelius said on Tuesday U.S. health officials are working with vaccine makers to speed up production of the seasonal flu vaccine and get ready to make a H1N1 flu shot.

The World Health Organization and the U.S. Centers for Disease Control and Prevention agree it is too soon to decide whether to go ahead with manufacture, but because making a flu vaccine is a long process, work needs to start now.

Vaccine makers grow the virus in fertilized chicken eggs in a process that can take four to six months.

Here are some factors health officials will be considering as they decide whether to make a swine flu vaccine:

— One Vaccine or Two?

Vaccine experts say manufacturing of the seasonal flu vaccine for the northern hemisphere is already under way and the best approach will be to develop a separate vaccine for the H1N1 virus. "If we make a vaccine, it will be two," said Dr. William Schaffner of Vanderbilt Medical School in Nashville, Tennessee and the Infectious Diseases Society of America.

Manufacturing of next season's flu vaccine is "pretty much finished," and manufacturers have not yet been given the green light to make an H1N1 vaccine, Schaffner said.

If they do, he said "manufacturers could redirect their entire manufacturing capacity to the new vaccine, which would, we hope, become ready for use sometime in the fall. It would become available in phases. We won't have it all available on September 1."

— Would There Be Enough Eggs?

One fear about making a new vaccine is that traditionally, vaccine production capacity is limited by the number of available eggs. But this may not be as much of a problem as once thought.

"The manufacturers have greatly increased their ability to scale up egg production," said Dr. John Treanor of the University of Rochester Medical Center, who tests flu vaccines in people.

Vaccine maker Sanofi-Aventis says capacity should not be a problem. "We have enough eggs on a year-round schedule that we have for our regular production and, because of the pandemic threat, flocks that are in use year-round," said Rick Smith, a vice president at the company's Sanofi Pasteur unit.

"We would probably be reaching the end of our routine seasonal production anyway, and then those eggs would be available for use in a swine flu vaccine."

Smith said it is too early to speculate how many doses the company could make.

— If They Make a Vaccine, Would They Use It?

The decision to make a vaccine and to use it are two very different steps. Officials at the CDC and the WHO will be closely monitoring how this new virus affects people in the southern hemisphere, where flu season is just beginning.

"If this virus doesn't cause any mischief in the southern hemisphere, or if it only produces mild disease, they might well not deploy the vaccine," Schaffner said.

He said it is possible that the new vaccine would need to be given in two doses to be effective, and distributing that on top of a seasonal flu shot would be a huge burden on the public health system. It might not be worth that effort if the virus only causes mild disease.

"Yes, we had to pay for it, but the juice is not worth the squeeze," Schaffner said.

— How Many People Could Be Vaccinated?

"There is much greater vaccine capacity than there was few years ago, but there is not enough capacity to make vaccine for the entire world's population for influenza," WHO's Keiji Fukuda told a news briefing on Tuesday. One question is whether adjuvants - compounds that stimulate the immune system and help make vaccines more effective - will be added to make the vaccines stretch farther.