The United States federal government originally promised 120 million doses of influenza A (H1N1) vaccine by now. Only 13 million have come through.

As nervous Americans clamour for the vaccine, production is running several weeks behind schedule, and 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.

Delay in developing tests

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

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

"We're in this race against the virus, and only Mother Nature knows how many cases are going to occur over the next six to 10 weeks," said Michael Osterholm, a vaccine expert at the University of Minnesota.