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It may be hard for Soraya Souza to hear her 14-month-old son Christopher scream while getting his vaccinations, but she says that his getting preventable illnesses would could cause a lot more pain and heartache down the road.

"I think it's important to vaccinate my son in order to prevent diseases. I just want to keep him healthy," says Souza.

Christopher received his measles, mumps and rubella shot in one leg and the varicella (chicken pox) vaccine in another.

That two-shot approach could become the permanent gold standard for vaccinating children across the board, thanks to a new study from Kaiser Permanente funded by the Centers for Disease Control. The study verifies earlier findings that giving all four shots at once as a combination vaccine, known as the "MMRV," doubles the risk of fever-induced seizures, or febrile seizures, in one- to two-year-old children.

"With vaccines it is very common to get fevers, and some of the kids who get fevers get seizures. So it's a known complication of vaccinations that kids have an increased risk of febrile seizures," says Dr. Barry Kosofsky of NewYork-Presbyterian/Komansky Center for Children's Health. "What this study was designed to find out is whether the incidence of febrile seizures was more in one group for vaccinated children than another."

Febrile seizures do not result in epilepsy and are also temporary and rare. While researchers found double the risk with the combined MMRV vaccine, that boils down to one seizure for every 2,300 doses given.

Giving the chicken pox vaccine separately from the measles, mumps and rubella shots cuts the risk down to about one in every 5,000 doses, which could change how vaccines are administered to infants.

"I suspect the American Academy of Pediatrics will now, based on this second group of data that is even stronger and now more compelling than the first, make the recommendation that the vaccinations be split and that children get three in one leg and the other in one leg," says Kosofsky.

School-age children receive MMRV booster shots between ages four and six. No increased risk for seizure with the combination vaccine has been reported for that age group.

Doctors say that is a good thing for children who may not be so keen on needles.

"It means four- to six-year-olds, who are very aware of what's going around them, can have one less vaccine" says Dr. Maura Frank of NewYork-Presbyterian/Komansky Center for Children's Health.

It makes vaccinations easily for young children who might not forget about the fear of multiple shots as fast as most babies do.