TUESDAY, Oct. 8, 2013 (HealthDay News) -- Two doses of chickenpox vaccine are better than one, new research confirms.
After the introduction of the second dose of chickenpox vaccine, the rates of chickenpox infection dropped 76 percent and 67 percent at two U.S. sites tracked for the study on opposite sides of the country.
Rates of infection in adults and infants -- two groups who generally don't receive the vaccine -- also went down, suggesting that higher levels of immunity in the population are decreasing the amount of circulating chickenpox.
"The first dose of vaccine was highly protective for reducing hospitalizations, deaths and other severe complications, but it wasn't fully protective against mild disease. There were still mild breakthrough cases, and these people could transmit the disease to those who hadn't been vaccinated," said Dr. Rachel Civen, senior study author and a medical epidemiologist with the Los Angeles County Department of Public Health.
"In 2006, it was recommended that a second dose be given between the ages of 4 and 6. And, we've seen a continuous significant decline since then, and the drops are across all age groups. The transmission is less throughout the whole community," she said.
Results of the study were published online Oct. 7 and in the November print issue of Pediatrics.
Chickenpox, which is also known as varicella, is a highly contagious viral disease. Before the varicella vaccine was introduced in the United States in 1995, about 4 million people had the chickenpox each year, according to the U.S. Centers for Disease Control and Prevention.
Although many people tended to think of chickenpox as a relatively mild infection, it caused more than 10,000 hospitalizations each year and about 100 deaths annually, according to background information in the study.
After the vaccine was introduced in 1995, the incidence of chickenpox went down by 90 percent and deaths from the disease dropped by 88 percent, but there were still outbreaks occurring. That's why experts decided to add the second dose in 2006.
The current study was designed to see how well the second dose of vaccine kept chickenpox under control. It included two sites -- one in Antelope Valley, Calif., and the other in West Philadelphia.
Infections in these sites dropped dramatically between 2006 and 2010. The Antelope Valley site had 76 percent fewer cases of chickenpox in 2010 than they did in 2006. West Philadelphia saw a 67 percent decline in the same time period. Both sites had a 98 percent decline in the incidence of chickenpox from 1995 to 2010.
Of those who came down with chickenpox between 2006 and 2010 in the study sites, just 7.5 percent had been vaccinated with two doses. Slightly less than two-thirds had received one vaccination. Most of those who got the chickenpox even though they'd been vaccinated had a mild case with fewer than 50 lesions on their bodies. The study also found that hospitalization rates dropped even further.
During 2007 to 2010, just 12 chickenpox outbreaks occurred within the California site compared with 47 outbreaks during 2003 to 2006, and 236 outbreaks during 1995 to 1998, according to the study.
Civen said all of these declines can be attributed to the additional dose of vaccine. "This is solely about getting that second dose," she noted.
"The varicella vaccine is very effective and safe. It helps those who get the vaccine and others who can't get the vaccine, but are highly susceptible to infection, such as immunocompromised adults," she said.
An expert not involved with the study found its results convincing.
"I think the data is pretty clear that the second dose is having a dramatic effect," said Dr. Thomas Murray, a pediatric infectious disease specialist and associate professor of medical sciences at Quinnipiac University's Frank H. Netter MD School of Medicine in North Haven, Conn.
"The second dose in the varicella vaccine program is very effective in reducing varicella in the general population," he said.
The CDC recommends that children under 13 years get two doses of the chickenpox vaccine: one between the ages of 12 months and 15 months, and the second dose between the ages of 4 and 6 years.
People aged 13 and older who've never had chickenpox or the vaccine should get two doses, at least 28 days apart, the CDC says.
Learn more about the chickenpox vaccine from the U.S. Centers for Disease Control and Prevention.