Varicella-Zoster Virus (Chickenpox):

Chickenpox (caused by the varicella-zoster virus) is one of the most contagious childhood diseases. Nearly every unvaccinated child becomes infected with it. The affected child or adult may develop hundreds of itchy, fluid-filled blisters that burst and form crusts.

The infection rarely causes complications in healthy children, but it is not always harmless. Five out of every 1,000 children are hospitalized and, in rare cases, it can be fatal. Before the vaccination became widespread, chickenpox resulted in about 11,000 hospitalizations and 100 deaths a year.

This is a close-up picture of chickenpox. Early chickenpox lesions consist of small red papules that quickly fill with a yellowish or straw colored fluid to form small blisters (vesicles), as seen in this photograph. Later, these vesicles will rupture, forming shallow erosions that crust over and then ultimately heal.

Chickenpox can be especially severe in adults and very serious in anyone with a compromised immune system. In addition, the varicella virus (which persists in the body after the childhood disease is gone) erupts as a painful and distressing condition called herpes zoster (shingles) in about 20% of adults with a history of chickenpox. Chickenpox itself usually occurs only once, although a few cases of mild second infections, marked by the telltale rash, have been reported in older children years after their first infection.

Vaccines for Chickenpox

A live-virus vaccine (Varivax) produces persistent immunity against chickenpox. Data show that the vaccine can prevent chickenpox or reduce the severity of the illness even if it is used within 3 days, and possibly up to 5 days, after exposure to the infection.

Recommendations for the Vaccine in Children. The vaccine against chickenpox is now recommended in the U.S. for all children between the ages of 12 months and adolescence who have not yet had chickenpox. Children are given 2 doses of the vaccine. The first dose is recommended at age 12-15 months. The second dose should be given at 4 - 6 years. However, doses can be as little as 28 days apart (note that the ideal minimum time for children under 13 years is 3 months). To date, more than 75% of children have been vaccinated.

Doctors recommend that the chickenpox vaccine be given at the same time as the measles-mumps-rubella (MMR) vaccine or that there is a delay of at least 1 month between the two vaccinations. (If the chickenpox vaccination is given within that 30-day period -- but not at the same time -- there is a higher risk for a breakthrough infection later on.)

A chickenpox vaccine is part of the routine immunization schedule. It is about 100% effective against moderate or severe illness, and 85 - 90% effective against mild chickenpox in children. Parents often express concern that the immunity from the vaccine might not last. The chickenpox vaccine, though, is the only routine vaccine that does not require a booster.

Recommendations for the Vaccine in Adults.

Some doctors suggest that every healthy adult without a known history of chickenpox, and who do not show immunity through testing be vaccinated. In general, however, the following adults should consider vaccinations:

• Older people without a history of chickenpox and who are at high risk of exposure or transmission (such as hospital or day care workers and parents of young children)
• People who live or work in environments in which viral transmission is likely
• Nonpregnant women of childbearing age
• Adolescents and adults living in households with children
• International travelers

As with other live-virus vaccines, the chickenpox vaccine is not recommended for the following people:

• Pregnant women (including the 3 months prior to pregnancy).
• People whose immune systems are compromised by disease or drugs (such as after organ transplantation). The vaccine is being studied, however, for its safety in some of these patients, particularly children with cancer or other high-risk conditions. An inactivated varicella vaccine may be safe and effective in patients undergoing bone marrow transplants, when given before and after the operation.
• Most patients who cannot be vaccinated but are exposed to chickenpox are given immune globulin antibodies against varicella virus. This helps prevent complications of the disease if they become infected.

Side Effects of the Chickenpox Vaccine

Discomfort at the Injection Site. About 20% of vaccine recipients have pain, swelling, or redness at the injection site.

Mild Rash and Risk of Transmission. The vaccine may produce a mild rash within about a month of the vaccination, which has been known to transmit chickenpox to others. Individuals who have recently been vaccinated should avoid close contact with anyone who might be susceptible to severe complications from chickenpox until the risk for a rash has passed.

Severe Side Effects. Between 1995 and 2001, 759 serious adverse effects were reported. Such events included seizures, pneumonia, anaphylactic reaction, encephalitis, Stevens-Johnson syndrome, neuropathy, herpes zoster, and blood abnormalities. Anecdotal reports have found a higher association of side effects when varicella vaccine is given at the same time as the measles, mumps, and rubella (MMR) vaccination. Because combined vaccinations are being developed, such effects should be closely studied.

Long-Term Protection and Booster Shots

There is intense debate over the long-term protection of the vaccine. However, any negative studies to date on long-term effectiveness simply raise the question of the need for booster or higher doses -- not the elimination of the vaccine altogether.

Long-Term Protection in Vaccinated Children. Most studies suggest that the vaccine is not wholly effective in up to 30% of vaccinated children. However, they also report if chickenpox occurs, more than 95% of the cases are mild. It is also usually less contagious. In such people, the infection appears to be caused by a wild virus, not a reactivation of the vaccine. The longer the interval since vaccination occurs, the higher the risk for a breakthrough infection.

Long-Term Protection in Vaccinated Adults. The protective effects for adults are even less clear.

Vaccine's Effect on Shingles. A primary concern is whether the vaccine protects against shingles later on, particularly in people who have breakthrough infections -- however mild. As more and more children get vaccinated, the actual protection of the vaccine and the implication of the breakthrough infection will become clearer.

[For more information, see In-Depth Report #82: Shingles and chickenpox (Varicella-zoster virus).]

In September 2005, the Food and Drug Administration approved a combination vaccine to protect against measles, mumps, rubella, and chickenpox. ProQuad, produced by Merck & Co., protects against all four infections with one injection. ProQuad is approved for use in children from 12 months to 12 years of age. However, due to manufacturing problems, the vaccine is in very limited supply at the time this report goes to print. In addition, the Food and Drug Administration (FDA) warns that children receiving ProQuad are at higher risk of developing febrile seizures, compared to children receiving the MMR and varicella vaccines separately.