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ATLANTA—A serious shortage of the combined DTP vaccine, for diphtheria, tetanus and pertussis, may soon lead pediatricians to delay the scheduled vaccinations of millions of American infants, the Centers for Disease Control and Prevention said.

Spot shortages of the combined vaccine, also known as DTaP, have already occurred in some areas. The shortages result from efforts by manufacturers to remove mercury, which was used as a preservative, from vaccines. Aventis SA is having trouble ramping up production of its mercury-free DTP vaccine, and GlaxoSmithKline PLC hasn't been able to make up the difference, so supplies are running Short. American Home Products Corp., which stopped production last year of its mercury-containing DTP vaccine, said that the cessation had nothing to do with mercury.

Officials from the CDC estimate that nearly 1.4 million vials of the vaccine have been back-ordered. That is about a month's supply of the CDC's public vaccine program, which provides vaccinations to roughly 60% of U.S. children. "It could be touch and go for the next several months," said Walter Orenstein, director of the National Immunization Program.

Critics want all mercury-containing vaccines withdrawn. "Our organization is asking, and this will be the third time, that the [Food and Drug Administration] recalls them all," said Lyn Redwood, president of Safe Minds, a group of parents of children with mental problems they believe were caused by vaccinations. But more than 9% of DTP vaccines on doctors' shelves have mercury in them. Recalling them would worsen an already serious shortage, officials say.

In an announcement reflecting the concern about mercury, GlaxoSmithKline said Wednesday that it would exchange any vaccines containing mercury that still sit on doctors' shelves for mercury-flee vaccines. While GlaxoSmithKline's DTP vaccine never contained mercury, its childhood vaccine against hepatitis B, called Engerix-B, contained mercury until March 2000. "This is not a recall, it's a voluntary exchange program," said Carmel Hogan, a GlaxoSmitixKline spokeswoman.

Concerns over the use of mercury in vaccines began in June 1999, when the FDA issued a report showing that children receiving the entire recommended schedule of vaccinations might ingest more mercury during a six-month period than is considered safe by the U.S. Environmental Protection Agency. The CDC's Advisory Committee on Immunization Practices responded by urging manufacturers to remove mercury from their vaccines "as rapidly as possible."

But some parents of children who developed mysterious aliments soon after being immunized—including autism and other neurological disorders—protested that no child should be vaccinated with potentially dangerous medicines. When ingested in large quantities, mercury causes a variety of problems including neurological deficits. Experts at the CDC, however, noted that there was no proof that mercury in vaccines had harmed anyone, while the evidence is overwhelming that vaccinations save lives.

The CDC asked the nonpartisan Institute of Medicine to analyze the relevant science. The institute released a report on Oct. 1 that, while acknowledging that there is no evidence that mercury-containing vaccines caused harm, recommended that such vaccines be removed. A week later, a group of lawyers sued Vaccine manufacturers, claiming mercury in their products had injured children. As a result, the CDC vaccines committee recommended yesterday that mercury-containing vaccines no longer be used in children after March 31, 2002.

Six mercury-containing vaccines are still being used by physicians but mostly to treat adults. One is RecombivaxHB and is made by Merck & Co. to fight hepatitis B. Tom Vernon, a Merck vice-president, said the Whitehouse Station, N.J., concern stopped distributing RecombivaxHB last month.

He said the company is "having serious conversations" about whether to withdraw supplies of the vaccine still in doctors' offices and warehouses.

Aventis officials said they hoped to ramp up production of its mercury-free DTP vaccine in the next several months. The Franco-German company has all but stopped selling vaccines to the CDC, which buys the drugs cheaply and in bulk, in favor of supplying private doctors, who generally pay more.

Philip Hosbach, an Aventis spokesman, said the pharmaceutical firm wasn't making enough of the vaccine to supply the CDC.