ATLANTA (AP) -- Children suffered higher rates of fever-related convulsions when they got a Merck & Co. combination vaccine instead of two separate shots, according to a new study presented Wednesday.

The results prompted a federal advisory panel on vaccines to water down their preference for the combo vaccine ProQuad, which protects against measles, mumps and rubella as well as chickenpox.

In the study of children ages 12 months through 23 months, the rate of seizures was twice as high in toddlers who got ProQuad, compared with those who got one shot for chickenpox and one for the three other diseases.

The risk translates to about one extra case of convulsion for every 2,000 doses of ProQuad given said Dr. Nicola Klein, who lead the federally funded study. She presented the data at a meeting of the Advisory Committee on Immunization Practices.

The study focused on children who develop fevers and then go into convulsions -- an occurrence that frightens parents but usually has no lingering consequences. There were no deaths in the new study.

ProQuad was licensed in 2005. It's been in extremely short supply since last year, when Merck suspended production because of manufacturing problems. The company expects to resume ProQuad production next year.

The panel had previously taken a position that they preferred doctors give children as few needlesticks as possible, and that ProQuad is preferable to giving separate shots.

It voted Wednesday to amend that, to say they're no longer voicing a preference for ProQuad over the separate shots.

"Safety, shortages, delivery issues -- lots of reasons not to state such a strong preference," said member panel Patsy Stinchfield, an infectious disease expert at Children's Hospitals and Clinics of Minnesota.

Merck officials said their own research, though preliminary, also showed a doubling of the risk in children within five to 12 days of vaccination. However, the occurrence was low -- about 5 cases in 10,000, Merck officials said.

They said there was five times more chickenpox antigen, the key ingredient, in the ProQuad shot than in the stand-alone chickenpox shot. But they said it's not clear that would explain the difference in seizure rates.

For some reason, the difference disappears when comparing rates for 30 days, Merck officials added.

Klein's research checked seizure rates only at seven to 10 days after vaccination, and looked at about 43,000 kids who got ProQuad and 315,000 who got the two other shots together. It found fever-related seizures occurred at a rate of 9 per 10,000 children vaccinated with ProQuad, compared with 4 per 10,000 for those who got separate shots.

Klein is co-director of Kaiser Permanente Vaccine Study Center in Oakland, Calif., one of seven sites in the study. Her work was funded by the U.S. Centers for Disease Control and Prevention.

ProQuad costs $124 per dose, about the same as the two other shots combined.

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