Kids are less likely to get chicken pox when they receive two doses of the chicken pox vaccine, according to a new study.
When kids are vaccinated twice, infants who are too young to get the shots are better protected as well, researchers found.
In 1995, the U.S. Centers for Disease Control and Prevention (CDC) recommended children between 12 and 18 months old get one dose of a new vaccine against the varicella virus, which causes chicken pox.
"By the year 2000, there was about a 90-percent drop in cases of chicken pox among kids," Dr. Rachel Civen, who worked on the new study, said.
The vaccine prevented up to 95 percent of severe chicken pox cases but wasn't 100-percent effective, said Civen, an infectious disease specialist at the Los Angeles County Department of Health.
So in 2006, the CDC recommended kids get a second dose of the vaccine between four and six years of age. It also said that kids, teens and young adults who had only had one shot should get a catch-up vaccine.
Civen and her coauthors set out to see whether the new two-dose vaccine schedule made a difference. They examined reports of chicken pox cases between 1995 and 2010 in Antelope Valley, California and West Philadelphia, Pennsylvania.
In 1995, there were 103 cases of chicken pox for every 10,000 children and adults in Antelope Valley and 41 per 10,000 people living in West Philadelphia, they found.
By 2006, that had declined to 11 cases for every 10,000 people in Antelope Valley and four per 10,000 in West Philadelphia.
After the second CDC recommendation, rates continued to fall, to three cases per 10,000 people in Antelope Valley and one case per 10,000 West Philadelphians in 2010.
Between 2006 and 2010, 62 percent of people who got chicken pox in both areas had been vaccinated with one dose of the varicella vaccine and eight percent with two doses.
Because pediatricians are already giving kids two doses of the vaccine, nothing much will change now; the new study just affirms that two doses are better than one, Civen said.
"Varicella is a rare vaccine because it is attenuated, or a live virus," she said. That means people with compromised immune systems and infants can't get the shot because it could make them sick, unlike other vaccines.
But according to the new study, chicken pox became less common among every age group, including infants, after 2006. That's not surprising, Civen said.
With a more effective vaccine schedule, "There is less disease being transmitted in the community, fewer little kids getting sick and transmitting it to the baby in the house," she said.
"You see this with influenza also," she said.
But even two doses didn't completely prevent the virus, according to results published in Pediatrics.
Cases that occurred after two vaccine doses were often very mild, Civen said, with a few scattered itchy red spots on the stomach, chest and face and a very mild fever, or no fever at all.
Some of the cases were so mild that doctors had trouble actually verifying they were chicken pox, she said.
Kids with mild chicken pox may not feel as sick, but they can still spread the virus at school or to younger siblings.
Although vaccinating with two doses seems to be very effective for kids and is now the standard for all pediatricians, the CDC is still encouraging more teens and young adults who may have missed a second shot to catch up now, Civen said.