May 6 (Bloomberg) — The World Health Organization has determined that at least two influenza shots will be needed to protect against both the seasonal form of the disease and the new swine flu.

The United Nation’s health agency will ask drugmakers to start producing a vaccine for swine strain once they finish making shots for seasonal flu, (Snip) WHO is waiting to hear from Sanofi-Aventis SA, GlaxoSmithKline Plc and other vaccine manufacturers when they are ready to switch to making inoculations for the new pig- derived virus, Kieny said. The virus, formally known as A/H1N1, has spread to 20 countries since Geneva-based WHO first identified cases in Mexico and the U.S. on April 24.

“It is important to have seasonal vaccine available,” Kieny said. The health agency will ask flu-vaccine makers to produce immunizations against the swine variant even before it is certain H1N1 will re-emerge in the Northern Hemisphere’s coming winter, she said. The agency wants to make sure inoculations for seasonal flu are readily available before the companies switch to making swine flu shots.

“Come November, if this isn’t a killer virus, and we don’t have seasonal flu vaccine, we’ll be crucified,” Kieny said. She hopes a recommendation can be made “within days” to start manufacturing the swine flu shot, she said. A recommendation to stop making vaccine for the seasonal strain will depend on supplies, and may be made a few weeks later, Kieny said.

Sanofi-Aventis, the world’s largest maker of seasonal flu vaccine, said it plans to produce seasonal vaccine until August. “If we stop too fast, to produce the vaccine against the new strain, if it is decided to produce the new strain, we will have a lower amount of seasonal,” (Snip) The three main seasonal flu strains — H3N2, another form of H1N1, and type B — cause 250,000 to 500,000 deaths a year globally, according to the WHO.