The almost instant shortage of flu vaccine caused by a bad influenza season points to a deeper, more chronic problem: The malfunctioning of the small but vital marketplace for preventive vaccines in America.

The influenza-vaccine shortage, reported Friday by the only two suppliers of flu shots, is more than a one-time mismatch of supply and demand. This is the eighth major shortage of preventive vaccines in the United States since the beginning of 2000.

Shortages of vaccines for diphtheria, tetanus, chickenpox and measles have occurred since then. Flu vaccines have been in short supply for three of the past four years. Now, doctors and clinics around the country say they are running low on supply as Americans rush to get some protection from the most severe outbreak in years, one that already has killed several youngsters in Colorado and elsewhere.

Vaccine shortages "aren't a one-time act of nature," says Frank Sloan, an economist at Duke University, who served as chairman of an Institute of Medicine study that explored vaccine issues in a report issued last August.

The report by the institute, an arm of the National Academy of Sciences, noted that there has been a steady erosion in the number of vaccine producers over the past three decades. In the 1970s, there were 25 vaccine makers; today — because of slim profit margins and legislative and liability issues — there are just five. With such a small number of producers, shortages can develop quickly as a result of manufacturing problems or a bad guess on the expected demand.

Many vaccines, such as those for tetanus and chickenpox, have only a single supplier in the U.S. market. The flu vaccine has only two: Aventis Pasteur, a unit of the large French drug company Aventis SA, and Chiron Corp. A newcomer, MedImmune Inc., has recently launched a nasal spray vaccine, FluMist. Merck & Co. and Pfizer Inc. are among the drug makers that no longer make flu vaccines.

Stockpiling of various vaccines is one solution to reducing the shortages, at least for some vaccines. The institute's report noted that "of the 10 vaccines that the [Centers for Disease Control and Prevention] has targeted for stockpiling, only three were stockpiled in 2002." Among those were vaccines for measles, mumps and rubella, as well as a small stockpile of polio vaccine.

The report noted that stockpiling is expensive and that the CDC has been conservative about developing stockpiles to minimize the financial risk. However, the flu vaccine can't be stockpiled every season, so manufacturers have to start every year from scratch, public-health officials say.