June 26 (Bloomberg) -- The vaccine being developed to combat a pandemic of swine flu will require multiple shots to provide immunity from the new virus, and the added immunizations may overwhelm U.S. state agencies, health officials said.

Two injections will be required three weeks apart for swine flu, also known as H1N1, and a third will be needed for seasonal flu, health officials said at a meeting today at the U.S. Centers for Disease Control and Prevention, in Atlanta. Children younger than 9 years old will need four shots, the CDC said.

Because swine flu is a new virus, most people have no natural immunity. The first shot provides an initial exposure, and the second shot boosts antibody levels in the body, Schaffner said.

People older than age 50 are getting swine flu at far lower rates than younger people, evidence they may have some immunity from prior exposures to a similar virus, and will only need one shot, the CDC said.

Children under 9 have little immunity to any flu strain and need two shots for protection against seasonal flu, as well as two for swine flu, the CDC said. The agency is conducting tests to find the most effective dosing for different age groups.