Last week, 40 million doses of the government's H1N1 vaccine expired, and by year's end another 30 million doses will go bad as well. That will be 70 million doses, or about 43 percent of the total reserved for the American public, incinerated at a cost of nearly half a billion dollars.

In April 2009 the H1N1 virus (better known as the swine flu, much to the dismay of pig farmers and pork producers) leapt onto the world stage after being diagnosed in Mexico. Mexico City and other large cities were soon (temporarily) shut down as infections spread across the globe. The Centers for Disease Control and Prevent (CDC) and the World Health Organization (WHO) soon declared the flu a serious threat that could infect millions. Governments ordered huge quantities of vaccine to protect their citizens, and some people panicked.

Yet the swine flu epidemic never materialized, and in fact killed only about 12,000 people worldwide — about one-third the number killed just in the United States each year by seasonal flu. What happened?

For one thing, far more vaccine than usual was produced. The U.S. government, under pressure from CDC and WHO scientists, ordered nearly 200 million doses, about twice the usual amount created to prevent the seasonal flu. Secondly, the H1N1 vaccine manufacturing process is time-consuming (the virus is incubated in chicken eggs) and cannot be rushed. The bulk of the vaccines were not shipped until late 2009, by which time most of the infections had already occurred.

So was it all a waste, a false alarm?

Hindsight is always 20/20, and public health officials must strike a delicate balance between over- and under-estimating the threat. While some critics believe that doctors overreacted, others point out that the danger was very real.

Flu is deadlier than most people realize, and epidemiologists feared a repeat of the 1918 Spanish flu pandemic that infected half a billion people around the world. The flu virus is constantly mutating, making it difficult to predict who will catch it and how severe the infection will be. Until better science is developed, the best defense is vigilance.