Medical Health Encyclopedia

Breakthrough Infections and Waning Protection in Vaccinated Children. Studies report that more than 15% of vaccinated children still develop chickenpox (called breakthrough infections). Reasons for this include the following:

• Waning Immunity. Studies on children in day care centers report that nearly half of children who had been previously vaccinated develop chickenpox. In one study, children vaccinated within three years had a much lower risk than those whose vaccinations had occurred later. Another study reported a higher risk for breakthrough infections in children who were immunized before 15 months.
• Oral Steroids. Children on oral steroids are also at higher risk for a breakthrough infection. (Children taking inhaled steroids, such as for asthma, do not appear to have this risk.)

It should be noted that if vaccinated children develop chickenpox, the cases are nearly always mild and usually less contagious. In such children, the infection appears to be caused by a wild virus, not a reactivation of the vaccine.

This does not necessarily mean, however, that children who are vaccinated eventually lose total immunity. A breakthrough infection is often due to issues with the primary vaccine (improper storage, low potency) or the child's history (having asthma, being less than 14 months at the time of vaccination). Nevertheless, there is also some evidence that either having the vaccination or even having chickenpox itself is not as protective against a later infection as experts have thought.

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Long-Term Protection in Vaccinated Adults. The protective effect for adults is even less clear. An encouraging 2002 study of adults vaccinated between 1979 and 1999 reported that although 9% developed chickenpox months to years after their last vaccination, in all cases, infection was mild, with none of the serious complications of adult chickenpox. A 2003 study on booster shots in older adults suggests that revaccination with the live virus is safe and effective.