Americans are still debating whether to roll up their sleeves for a swine flu shot, but companies have already figured it out: vaccines are good for business.

Drug companies have sold $1.5 billion worth of swine flu shots, in addition to the $1 billion for seasonal flu they booked earlier this year. These inoculations are part of a much wider and rapidly growing $20 billion global vaccine market.

“The vaccine market is booming,” says Bruce Carlson, spokesperson at market research firm Kalorama, which publishes an annual survey of the vaccine industry. “It’s an enormous growth area for pharmaceuticals at a time when other areas are not doing so well,” he says, noting that the pipeline for more traditional blockbuster drugs such as Lipitor and Nexium has thinned.

As always with pandemic flus, taxpayers are footing the $1.5 billion check for the 250 million swine flu vaccines that the government has ordered so far and will be distributing free to doctors, pharmacies and schools. In addition, Congress has set aside more than $10 billion this year to research flu viruses, monitor H1N1’s progress and educate the public about prevention.

Drugmakers pocket most of the revenues from flu sales, with Sanofi-Pasteur, Glaxo Smith Kline and Novartis cornering most of the market.