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Timing Affects Effectiveness of Chicken Pox Shot

CDC: Don't give it right after measles, mumps, rubella vaccine

FRIDAY, Nov. 30, 2001 (HealthDayNews) -- Children who get immunized against chicken pox risk developing the disease anyway if the shot comes too soon after bulk vaccination for other infections, according to a new report from the Centers for Disease Control and Prevention.

When children receive the varicella vaccine less than 30 days after getting a shot for measles, mumps and rubella -- the so-called MMR injection -- their likelihood of contracting chicken pox is significantly increased, researchers say.

However, officials note, their overall odds of developing the disease are still extremely low -- but far higher than if they don't get vaccinated at all. An unvaccinated person's risk of catching chicken pox by the time he is 30 years old is essentially 100 percent, experts say.

The study "supports the recommendations that we have put in place," says Aisha Jumaan, an epidemiologist with the CDC's National Immunization Program and a co-author of the report. "This was not a surprise, but it was good to have the recommendations verified."

Chicken pox is caused by the varicella-zoster virus, which in adults also leads to shingles. The varicella vaccine guidelines are based on the experience with a far deadlier microbe: In the 1960s, doctors recognized that smallpox shots were less effective when delivered within a month of an measles injection, and vice versa.

This happens because the body's immune system is suppressed while it works to generate defenses against the proteins in the inoculation. Although administering both vaccines at the same time weakens neither, waiting for periods of less than a month doesn't give the immune system enough time to rebound, Jumaan says.

In the latest study, Jumaan and her colleagues compared chicken pox infection "breakthrough" rates in more than 114,000 children in California and Oregon between 1995, when the varicella vaccine was introduced, and 1999. Almost 70 percent of the children received the shot when they got their MMR vaccine, 29 percent got it at least 30 days later than the MMR, as health officials suggest, and the rest received the shot less than 30 days after the first inoculation.

Those who didn't wait the requisite month were 2.5 times more likely to be infected with chicken pox in the next several years than those in the other two groups, the researchers say. The risk of chicken pox was also somewhat higher in children who didn't wait 30 days for the shot after getting immunized against hepatitis B or who received the oral polio vaccine.

All pediatricians should be aware of the immunization guidelines, but Jumaan says some parents switch health care providers while their children are receiving their early childhood shots, which could possibly create confusion about what vaccines were administered and when. Fortunately, she says, the number of children who get the varicella vaccine too quickly is small, less than 1 percent in the new study.

Health officials believe that children who get their chicken pox shot less than a month before their measles-mumps-rubella shot may also be at increased risk of the latter diseases.

Jumaan, however, says there's no evidence yet to support that hypothesis. "We think the mechanism would be the same, although we haven't seen that happen in actual life," she says.

The chicken pox vaccine, which is recommended for children over 1 year old who haven't already contracted the infection, has met with mixed reviews. Roughly 69 percent of all American kids received the injection in 1999, which is a lower percentage than for all other major childhood inoculations, according to the CDC.

A Washington state study last year of almost 600 parents found that most believed the shot was worthwhile even if its only benefit was shielding children from a rare but serious complication from the disease. But most parents didn't approve of the vaccine simply to save them lost work time.

Before the varicella vaccine was introduced, 4 million Americans -- mostly children -- contracted chicken pox annually, the CDC says. The disease caused between 4,000 and 9,000 hospitalizations and led to approximately 100 deaths each year. Most of the fatalities occurred in adults, for whom the disease is far more serious.

What To Do

To find out more about chicken pox, try the Centers for Disease Control and Prevention.

For more on the varicella vaccine, check out the University of Colorado or the Immunization Action Coalition.

SOURCES: Interview with Aisha Jumaan, Ph.D., epidemiologist, National Immunization Program, Centers for Disease Control and Prevention, Atlanta; Nov. 30, 2001, Morbidity and Mortality Weekly Report
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