In response, the federal Advisory Committee on Immunization Practices has reversed its prior position of recommending ProQuad, which protects against measles, mumps, rubella (MMR) and chicken pox, over two separate vaccines for chicken pox and MMR.

“Safety, shortages, delivery issues - lots of reasons not to state such a strong preference,” advisory committee member Patsy Stinchfield said.

Previously, the committee had recommended ProQuad over the separate vaccines in order to reduce the number of shots that children would have to undergo.

ProQuad costs approximately the same as the chicken pox and MMR vaccines combined. Due to manufacturing problems, the vaccine has been in short supply since 2007.

Researchers in the new study looked at 43,000 children between the ages of 12 and 23 months who had been vaccinated with ProQuad and 315,000 who had received two separate MMR and chicken pox vaccines. They found that within seven to 10 days after vaccination, those given ProQuad suffered twice as many cases of fever followed by convulsions as those given the separate shots. This amounted to one extra convulsion per 2,000 children receiving ProQuad.

In absolute terms, nine of every 10,000 children receiving ProQuad suffered fever-related seizures, compared with four in 10,000 children in the dual vaccine group.

Children undergoing seizures did not appear to suffer any lasting effects, and there were no recorded deaths.

A similar study conducted by Merck also found a doubling of seizure risk within five to 12 days of vaccination with ProQuad, but said that the difference in seizure rates vanished in a 30-day time period. The absolute seizure rate from ProQuad was lower in the Merck-funded study, at five convulsions per 10,000 children vaccinated.