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.
Doctors from the Columbia University College of Physicians and Surgeons took blood tests from children before they received the vaccine and again about one month after. In at least 15% of the children, there was no evidence of immunity -- meaning the vaccine didn't take.
"If it's a matter of primary vaccine failure, a second dose should take care of that," says Dr. Anne Gershon, a professor of pediatrics at the Columbia University College of Physicians and Surgeons who worked on the study. "It's very important that we sort out exactly what is going on. Waning immunity is more of a concern than is primary vaccine failure."
Waning immunity to the vaccine poses a potential problem to adults, experts say. In the pre-vaccine days when chickenpox spread throughout communities, even teenagers and adults who already had the disease received a natural boost to their immune systems just by being around someone with chickenpox.
But as more children are immunized, the natural virus isn't in widespread circulation anymore. That means that anyone who had the vaccine or the disease but whose immunity wanes will not get a natural boost and may require booster shots.
"We're in a situation where it's unlikely we'll get chickenpox through natural spread of the disease because it's so much less common now," says Davis. "People who don't get the vaccine or whose immunity wanes will be less likely to get reboosted in adolescence and adulthood."
Shingles in the mix
Another looming question is how the vaccine may influence rates of herpes zoster -- or shingles.
Once a person contracts the varicella virus, it never completely leaves the body, lying dormant in nerves. Sometimes, however, it is reactivated, for reasons that are not clearly understood. When this happens, the result is a painful rash -- shingles -- along the path of one or more nerves.
The rash usually strikes people age 50 and older, and a shingles vaccine is now recommended for people 60 and older to boost their immunity to this reactivated virus.
Although many doctors think the vaccine given in childhood will protect against shingles much later in life, public health experts aren't sure.
And adults who had chickenpox may end up more prone to shingles at a younger age because they won't get that natural immunity boost from disease that is circulating in the population, suggests Dr. James Cherry, a professor of pediatrics at Mattel Children's Hospital at UCLA and an expert on childhood vaccination.
"Our immunity is stimulated by being exposed to the chickenpox," he says. "When that stimulation goes away, our protection is going to decrease, so we'll see more cases of shingles. My guess is that we're going to be giving doses of the [varicella] vaccine to 30- and 40-year-olds to prevent shingles. The better we do [in eradicating chickenpox], the more we're going to see shingles."
(BEGIN TEXT OF INFOBOX)
Giving it a shot
Infectious disease experts generally recommend the chickenpox vaccine for these age groups:
* Babies: First dose should be given at 12 to 18 months, followed by the second dose at 4 to 6 years. The vaccine can be given alone or in a measles-mumps-rubella-varicella combination shot.
* Children, adolescents and adults: Those who have had one dose should get a second dose.
* Adults: Those who aren't sure if they have had chickenpox can have a blood test to see whether they have had the disease. The two-shot vaccine should then be administered if needed. (The first dose is followed by a second dose four to eight weeks later.)
The vaccine can also be administered even without the blood test. (It won't hurt those who have already had the disease.)
Adults who haven't been vaccinated and know they've never had chickenpox should have the vaccine.
For more information on chickenpox and the vaccine:
* From the National Library of Medicine: www.nlm.nih.gov/medlineplus/chickenpox.html
* From the Centers for Disease Control and Prevention: www.cdc.gov/nip/publications/VIS/vis-varicella.pdf
* From an abstract of a March 2007 New England Journal of Medicine study: content.nejm.org/cgi/content/abstract/356/11/1121