CDC received notice from Merck & Co., Inc., that it has lower amounts of varicella-zoster virus (VZV) than expected from recently manufactured bulk vaccine. Bulk vaccine production is an intermediate step in the manufacture of VZV-containing vaccines. Varicella bulk is stored frozen until it is needed in the final preparation phase of each vaccine. Production of VZV bulk has been suspended temporarily while the manufacturer identifies the cause of the low virus yield. Merck is the only U.S. supplier of VZV-containing vaccine, including varicella vaccine (Varivax®); combined measles, mumps, rubella, and varicella (MMR-V) vaccine (ProQuad®); and zoster vaccine (Zostavax®). This lower virus yield does not affect the quality of any of Merck's VZV-containing vaccines currently on the market, any lots of vaccine manufactured and ready for release to the market, or any VZV-containing vaccines presently being manufactured.