WASHINGTON (AP) -- An acute shortage of vaccines for many common childhood diseases should subside by year's end, but drug makers, doctors and lawmakers said Wednesday that without government intervention more shortfalls could occur.

Immunization in the United States has been enormously successful, reducing preventable diseases by more than 95 percent, according to the American Academy of Pediatrics. Yet the vaccine supply is fragile, depending on only four major vaccine manufacturers using live viruses and bacteria that can grow unpredictably.

Supplies have been running low in vaccines against eight of 11 preventable childhood diseases: diphtheria, tetanus, pertussis, pneumococcal infection, measles, mumps, rubella, and varicella, or chickenpox.

The shortages are "unique and unprecedented," said Walter Orenstein, director of the National Immunization Program at the Centers for Disease Control and Prevention.

But, he added, "I would like to point out some good news - current information from manufacturers indicates that many of these shortages will be over before the end of the summer, and most will be resolved by the end of this year."

The causes of the problem will persist, several witnesses said during a hearing of the Senate Governmental Affairs Committee. The industry and physician groups want expanded government stockpiles, along with boosting the Vaccine Injury Compensation Program, which protects companies from being sued over rare but serious side effects caused by vaccines.

Sen. Bill Frist, R-Tenn., has proposed legislation with those goals in mind, and much of his plan is being pushed by physicians and industry groups.

The result of the shortages has been a delay in immunizations for many children as well as changes in immunization requirements.

Several reasons for supply problems were cited. Some drug companies have dropped out of the vaccine market, and some manufacturers temporarily halted production to upgrade facilities or fix manufacturing problems.

Also, recommendations from public health officials can prompt an increase in demand. That happened when a wider range of people was urged in 2000 and 2001 to get flu shots, or when officials recommended removing mercury-containing preservatives from vaccines.

"It is not a matter of opening up a tap and pouring out vaccine, no matter who controls the tap," said Wayne Pisano, executive vice president of Aventis Pasteur North America. "Production lead times are long, and the quality control process is the strictest possible. As a result, supply and demand will be misaligned when policy changes increase demand before supply is available."

Copyright 2002 The Associated Press. All rights reserved.