Swiftwater, Pa. ? The federal government originally promised 120 million doses of swine flu vaccine by now. Only 13 million have come through.

As Americans clamor for the vaccine, production is running several weeks behind schedule. Health officials blame the pressure on pharmaceutical companies to crank it out along with the ordinary flu vaccine, and a slow and antiquated process that relies on millions of chicken eggs.

There have been other bottlenecks, too: Factories that put the liquid into syringes have become backed up. And the government ran into a delay in developing the tests to assess each batch before it is cleared for use.

What effect the delays will have on the outbreak is unclear, in part because scientists cannot say how dangerous the virus is, how easily it spreads, or whether it will mutate into a more lethal form.

Since April, swine flu has killed more than 800 people in the U.S., including 86 children, 39 of them in the past month and a half, according to the Centers for Disease Control and Prevention. More than half of all people hospitalized since September were 24 and under.

Many states have had to postpone mass vaccinations. Clinics around the country that managed to obtain doses have been swamped.

The delays have led to renewed demands for a quicker, more reliable way of producing vaccines than the chicken-egg method, which is 50-year-old technology and involves injecting the virus into eggs and allowing it to feed on the nutrients in the egg white.