What was important about this report is that it drew conclusions from an enormous sample size of children between 12 and 23 months of age collected from seven health organizations across the United States. That large of a sample size gives researchers greater confidence in its reliability.
The data were collected between January 2000 and October 2008 from Group Health Cooperative in Washington State, Kaiser Permanente systems in Colorado, Northern California and Oregon, Harvard Pilgrim Health Care in Massachusetts, HealthPartners in Minnesota and Marshfield Clinic in Wisconsin.
The project discovered that seizure incidence peaked during days seven through 10 after vaccination, "with the most prominent peak following MMRV." However, there was no peak in seizures for children vaccinated against just varicella. Other complications, such as the severity of the seizures or death rates, were not evaluated in this portion of the project although that may be included in a later analysis.
The MMRV vaccine was licensed in the U.S. in September 2005. It is made by ProQuad, Merck & Co. Inc. "At the time of its licensure, use of MMRV vaccine was preferred for both the first and second doses over separate injections of equivalent component vaccines (MMR vaccine and varicella vaccine), which was consistent with the CDC's Advisory Committee on Immunization Practices' 2006 general recommendations on use of combination vaccines," the MMWR said on May 7.