The U.S. began recommending a single dose of the chickenpox (varicella) vaccine for children aged 12 months and older in 1995. A second dose was added to the immunization schedule in 2006.
Before the vaccine was recommended, infants were four times more likely to die from chickenpox compared to children aged 1 to 14.
Not anymore.
There has been close to a 90% drop in the rate of chickenpox among infants from 1995 to 2008, even though they are not eligible for the vaccine.
"Vaccinating children aged 12 months and older protects infants who are too young to be vaccinated," says study researcher Adriana S. Lopez, MHS. She is an epidemiologist at the CDC in Atlanta.
Lopez and colleagues analyzed chickenpox rates and severity among infants aged 0-5 months and 6-11 months from 1995 to 2008. Their goal was to find out some of the indirect benefits of routine chickenpox vaccination in the U.S.
The study is published in Pediatrics.
The researchers found the viral infection was actually milder among infants aged 0-5 months, compared with those aged 6-11 months.
This is likely because younger infants are still benefiting from maternal antibodies to chickenpox. These protective antibodies do wane over time.
Protecting Infants From Chickenpox
There have been major strides against chickenpox, but we can't afford to become complacent, Lopez says. "Even though the number of cases has gone down, there are still cases out there," she says. "It is important to have high vaccine coverage with the recommended two doses so that the risk is low for infants and others who can't get vaccinated."
"This is very good news," says Gail Demmler-Harrison, MD. She is a professor of pediatrics at Baylor College of medicine in Houston. Most people think of vaccines as benefiting only the person who receives the shot, she says.
But "this study shows that there are lesser-appreciated secondary benefits to the chickenpox vaccine, namely protecting vulnerable young infants," Demmler-Harrison tells WebMD.