The chickenpox vaccine has not been a slam-dunk success. With some people, it hasn't scored any points at all.
The varicella vaccine was introduced in the United States in 1995 as a single shot given to children ages 12 to 18 months, and by many accounts, the program has been effective. A generation of children has now been vaccinated against chickenpox, and cases of the disease have dropped by 85% since 1995. Deaths from severe cases of the disease fell from 124 in 1994 to 26 in 2001.
But the itchy, infectious disease is still cropping up.
A recent study in the New England Journal of Medicine found that a single shot does not produce a sufficient immune response in as many as 20% of people who receive it. As a result, older children, teens and young adults are developing the disease even though many have already been vaccinated -- and they historically become much sicker from chickenpox than young children.
Now public health officials are trying to figure out how to control a disease that was recently thought to be well-in-hand.
A federal government advisory committee last summer recommended adding a booster shot to the varicella vaccine regimen, and the Centers for Disease Control and Prevention is expected to publish a final recommendation this summer supporting the two-shot regimen.
"Varicella immunity has unique factors associated with it," says Dr. Roger Spingarn, a Newton Centre, Mass., pediatrician who has followed the development and use of the vaccine. "If the immunity wears off, a population of adults, now susceptible to infection, may be created. Disease in adulthood, and especially during pregnancy, is a serious concern."
Public health officials say a two-shot regimen will create a larger pool of fully immunized people and should reduce so-called breakthrough disease.
But health officials admit they don't yet know if a second dose will provide permanent immunity. It's possible that repeated chickenpox booster shots may be needed to protect adults from getting the disease.
"The main public health concern here is that teens and young adults will get chickenpox and need to be hospitalized or will have severe illness that requires intensive care. That is the group everyone is concerned about," says Dr. Matthew Davis, an assistant professor of pediatrics at the University of Michigan who studies immunization issues.
The value of vaccinating children against chickenpox was widely debated before the vaccine, called Varivax, was approved in 1995. Chickenpox usually strikes toddlers and preschoolers and is rarely complicated, prompting some doctors at that time to wonder if vaccination was worth the cost and trouble.
But the disease causes missed school time -- and time away from work for parents -- so the shot has proved popular, with many school districts now requiring it for admission to kindergarten
Tammy Steensland had her daughter Ava vaccinated at age 1, as recommended by the family's doctor. Then, at age 5, Ava developed a fever and a few scattered, fluid-filled blisters.
"I was surprised when the doctor said it was the chickenpox," says the Mission Viejo mother of three. "I said, 'Wait a minute -- she was vaccinated.' It made me wonder what was the point of the vaccine."
Older children affected
Outbreaks are now occurring regularly in "highly vaccinated" school communities, according to the study published in March in the New England Journal of Medicine.
And the disease pattern is changing. Before 1995, 73% of cases of varicella occurred in children age 6 or younger; now the peak incidence of the disease among vaccinated children is ages 6 to 9, and among unvaccinated children, ages 9 to 12. The study also found that the more time that elapsed since getting the shot, the more likely children were to have moderate to severe cases of the disease.
Though many breakthrough cases in vaccinated children are reported to be mild, the children are still infectious and can pass the disease to unvaccinated older people who could become very sick. Unvaccinated adults who get the disease later in life have a 20-times greater chance of dying compared with a child and have a much higher chance of needing hospitalization from complications.
"The new data on waning immunity puts the recommendation of a second dose in a different light," says Davis. "It may be very important to help protect individuals for longer."
No one is really certain, however, why so many kids are not fully protected from chickenpox after receiving one shot. According to the New England Journal of Medicine study, which was conducted by the CDC and the Los Angeles County Department of Health Services, rates of breakthrough disease increased steadily with the time elapsed since vaccination. That implies that the vaccine works initially but loses its punch over time.
But another study, presented last fall at the annual meeting of the Infectious Diseases Society of America, suggests that the vaccine simply fails in some children from the start.