Routine vaccinations against the chicken pox have reduced the  incidence of the itchy potentially serious virus by as much as 90 percent  in the U.S. But the original one-shot protocol may not be as effective as the newer, two-dose regimen, new research suggests.

The vaccination program, launched in the U.S. in 1995, recommended that kids 12 months to 12 years old get immunized against chicken pox. Since then, the  number of cases has  dropped by 57-to-90 percent, according to government research published in the journal Pediatrics.

But the single shot, while 80-to-85 percent effective in preventing the disease, may not be enough to protect kids in schools and other "high-contact" settings. It's still too soon to tell if the double dosage, made routine in 2006, provides better protection, according to the study. (That regimen  calls for the first shot at around 12 to 15 months of age – and the second between ages 4 and 6.)

Will the findings encourage more parents to vaccinate their children? Chicken pox "parties" have been en vogue to varying degrees over the last decade among families who shun vaccines over fears of autism and prefer to have their kids develop immunity to the disease by exposing them to other children suffering from it.

Some other parents uneasy about the increase in vaccine dosing in childhood say one injection is good enough. While proponents of a vaccine link to autism often finger mercury preservative as a cause, others point to multiple shots at once.

One mom told the Chicago Tribune that while she abides by Illinois' vaccine requirements for children, she's wary of the expanding roster of shots, including the new vaccine against human papilloma virus. The one-shot dose against chicken pox is enough, Elizabeth Wright told the newspaper.

"If they don't have to have the vaccine," she said, "I'd rather they not have it."