updated 5/10/2007 6:49:57 PM ET 2007-05-10T22:49:57

Young children needing immunization against chickenpox and three other diseases likely will have to get an extra shot, due to manufacturing problems that have halted production of a four-disease combo vaccine made by Merck & Co.

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Merck said Thursday that its ProQuad vaccine, which protects against measles, mumps and rubella as well as chickenpox, won’t be available from about July until at least year’s end.

“It’s too early to say at this point whether new (ProQuad) supplies will be available in 2008,” said Mary Elizabeth Blake, spokeswoman for Merck’s vaccines division.

However, the drugmaker expects to have plenty of two separate vaccines that cover the same diseases: Varivax, for chickenpox, and M-M-R II, for measles, mumps and rubella.

The federal government recommends children get each of those shots twice, once at age 12 months to 15 months and again between ages four and six years old, or — when available — they can receive the ProQuad vaccine twice.

Last year, the government recommended the second chickenpox shot because of outbreaks among schoolchildren, apparently due to waning potency of the vaccine.

Whitehouse Station, N.J.-based Merck is the only U.S. source for vaccines against the varicella zoster virus, which causes chickenpox in children and shingles in adults.

Blake said Merck decided to shift available supply of the key vaccine ingredient against chickenpox — live but weakened varicella virus — from ProQuad to Varivax and its shingles vaccine, Zostavax.

The drugmaker recently informed the U.S. Centers for Disease Control and Prevention of its decision.

“It makes sense to cut back on ProQuad first” because that requires more virus, said Dr. Lance Rodewald, director of the CDC’s immunization services division.

He said the agency had been assured there would be an adequate supply of both Varivax, with an expected demand of more than 8 million doses, and Merck’s Zostavax vaccine against shingles.

Zostavax was just approved last May and is recommended for all adults aged 60 and older. Shingles, also called herpes zoster, is a painful skin rash that strikes roughly one in four adults who previously had chickenpox.

The government does not have a firm projection of demand for Zostavax, Rodewald said.

According to the CDC, Merck charges private doctors about $45 for M-M-R II and $75 for Varivax, slightly less than the $125 price for ProQuad. All or most of the cost of getting the shots is generally covered by private insurance and the federal Vaccines for Children program for the poor.

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