Health officials are blaming the slow production of swine flu vaccine in part on an antiquated process that relies on millions of chicken eggs.

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

The 50-year-old technology involves injecting the virus into eggs and allowing it to feed on the nutrients in the egg white. It is a slow process, and the pressure on manufacturers to produce two vaccines -- for both swine flu and ordinary flu -- has made it even slower.

In the meantime, many states have had to postpone mass vaccinations. Clinics around the country that managed to obtain doses of the vaccine have been swamped.

Since April, swine flu has killed more than 800 people in the U.S.

Federal officials counsel patience, saying that eventually there should be enough vaccine.

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