Vaccination against the H1N1 swine flu is off to a slow start in the United States, but states have ordered more than 2 million doses of mostly nasal spray for the first patients, a top health official said on Tuesday.

Every state has ordered a share of the pandemic vaccine, Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, told a news briefing.

This week, as of yesterday, about 2.4 million doses were available for ordering, Frieden told reporters in a telephone briefing. He said states had ordered 2.2 million of the doses -- a painstaking process because they must specify which vaccine they want and have a plan in place for delivering it.

The U.S. Food and Drug Administration must inspect each lot of vaccine as it is packaged. Each day as more vaccine is cleared, more vaccine becomes available for ordering, Frieden added.

I think what we are seeing now is the tap beginning to flow. We are seeing a substantial amount of vaccine beginning to get out.

The first batches available are AstraZeneca unit MedImmune's nasal spray vaccine, which is approved for people aged 2 to 49 without asthma or other lung conditions.

Many states are opting to vaccinate healthcare workers first, who have a high risk both of being infected and of passing infections along to vulnerable patients.

WAITING FOR SHOTS

But at least three Chicago-area hospitals that have received the nasal spray vaccine said they would not be using it to immunize hospital staff because it is made from a weakened version of a live virus, unlike shots, which are made using a killed virus, and there is a very small chance of infecting someone with a weak immune system.

Dr. John Segreti, an infectious diseases expert at Rush University Medical Center, said the hospital had received 2,000 doses of the nasal spray.

We're going to be distributing it to clinics that see children. We're not using this for our healthcare workers. We're waiting for the inactivated vaccine for our health workers, he said.

Officials at Northwestern Memorial Hospital and the University of Chicago Medical Center said they also will be waiting for the shots to immunize hospital workers.

The U.S. government has ordered 250 million doses and, given that many Americans skip flu shots every year, the CDC believes this will be enough to fill demand.

What we have decided to do is make vaccine available as soon as it comes off the production lines, Frieden said. That means it is becoming available in lots. It is a little bit of a messy process and we do expect it to be a little bit bumpy in the next few weeks.

And it will be a strain to keep up with the virus. As of today, influenza is widespread in most of the United States, Frieden said. We are seeing it continue to increase in some areas.

Nonetheless, he noted, when H1N1 goes through a community, it infects about 5 percent of the people, leaving 95 percent vulnerable to a fresh round.

You don't know what the rest of this long flu season is going to hold. We haven't had a flu season like this in at least 50 years, Frieden said.

Pennsylvania Medicine CDC