Don't count on a vaccine to protect against bird flu during the first six months of a pandemic.

Health and Human Services Secretary Mike Leavitt said Monday it would take at least that long to produce a vaccine because the virus is changing and there's no way to know which strain might become capable of human-to-human transmissions.

As a result, the government will have to maintain stockpiles of vaccine against each of the main H5N1 strains circulating the globe.

Once the particular strain is identified, it will take time to ensure that a vaccine is safe and to mass-produce it, he said.

"If we have a person-to-person, transmissible virus and we enter a pandemic condition, we will be operating without a vaccine for the first six months," Leavitt said. "We will be dependent upon traditional public health measures to contain and limit it."

Leavitt updated reporters Monday on what the federal government is doing to monitor and prepare for bird flu. A report he released noted that the virus has spread from 16 countries to about 37 in a span of about four months.

Leavitt said the federal government is operating under the assumption that an infected bird will be found in the United States by the fall. He stressed that people should not be overly concerned once a sick bird is found.

"As long as it is a bird disease, it is not a crisis," he said.

Scientists are concerned that the virus could mutate and become contagious among people. Currently, people have only been infected after close contact with sick birds. So far, 175 people have been infected and 96 of them have died.

HHS officials have held 23 bird flu summits throughout the country to emphasize the need for state and local communities to take the threat seriously, and to begin preparations for a pandemic.

Leavitt said his top bird flu concern was the ability of the government to get medicine quickly into the hands of people who need it.

"Doing anything millions of times is hard. Doing it fast makes it even more difficult," he said.

He also said he was worried about how the country would handle a surge in demand for health care.

"It will need to be the focus of individual community planning, because every community's surge will be handled differently," he said.