VARIVAX Refrigerated [Varicella Virus Vaccine Live (Oka/Merck)] is a lyophilised preparation of the Oka/Merck strain of live, attenuated varicella virus. The virus was initially obtained from a child with wild-type varicella, then introduced into human embryonic lung cell cultures, adapted to and propagated in embryonic guinea pig cell cultures and finally propagated in human diploid cell cultures (WI-38). Further passage of the virus for varicella vaccine was performed at Merck Research Laboratories in human diploid cell cultures (MRC-5) that were free of adventitious agents. VARIVAX Refrigerated, when reconstituted as directed, is a sterile preparation for subcutaneous administration. Each 0.5 mL dose contains: a minimum of 1350 PFU (plaque forming units) of Oka/Merck varicella virus when reconstituted and stored at room temperature for 150 minutes (2 ½ hours).