New Jersey ? 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.

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.

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 4 and 6 years old, or – when available – they can receive the ProQuad vaccine twice.