vaccine or varicella zoster vaccine (VZV) is an injection that protects
children from contracting chickenpox (varicella), one of the most common
VZV consists of living but attenuated (weakened) varicella zoster, the
virus that causes chickenpox. The weakened virus induces a child's
immune system to develop antibodies against the varicella virus without
causing the disease. Thus it prevents children from contracting
chickenpox. Prior to the introduction of VZV, approximately 4 million
Americans contracted chickenpox each year, and 95 percent of children
contracted the disease before the age of 18. The vaccine first became
available in the United States in 1995 and is produced by Merck &
Company under the trade name Varivax.
A sample of the varicella zoster virus was isolated from the blood of a
three-year-old Japanese boy in 1972. A Japanese researcher, Michiaki
Takahashi, attenuated the virus by growing it in various animal and human
cell cultures. He then tested it on children and found that it was
effective in preventing chickenpox. This "Oka" varicella
strain, named after the original infected child, was licensed by Merck in
1981 and used to develop Varivax. The vaccine was clinically tested for
safety and effectiveness.
Producing sufficient quantities of the vaccine to immunize all children
against chickenpox has proven to be a major obstacle. Weakened viruses for
vaccines are grown in cell cultures. However unlike other weakened
viruses, varicella zoster remains in the cell rather than being secreted
from the cell and collected from the culture medium. Thus the infected
cells must be collected and broken open by ultrasound. The released virus
is extremely sensitive to heat caused by ultrasound, and Merck scientists
had to determine the precise conditions for opening the cells and
releasing the virus unharmed. The company built a new production facility
for Varivax that uses robots to strictly control the ultrasound procedure.
In addition to the live attenuated varicella virus, Varivax contains:
VZV is considered to be safe and 70 to 90 percent effective. Vaccinated
children who do contract chickenpox usually have milder symptoms. The
vaccine also prevents chickenpox in children exposed to the virus three to
five days prior to
The Centers for Disease Control and Prevention (CDC), the American Academy
of Pediatrics, and the American Academy of Family Physicians all recommend
that healthy children be vaccinated against chickenpox. In 2001 child-care
facilities and public schools began phasing in a varicella vaccination
requirement for enrollment. By 2002, some 81 percent of American children
had been vaccinated with VZV, and the CDC determined that the number of
chickenpox cases had declined substantially. The CDC expects that
widespread childhood vaccination against chickenpox will further reduce
the incidence of the virus in the general population. This, in turn, will
reduce the incidence of chickenpox among those who cannot receive VZV,
including children who are most at risk for serious complications from the
As of 2004 it was unclear whether VZV provided life-long immunity to
chickenpox. The U.S. Food and Drug Administration (FDA) required Merck to
follow several thousand children for 15 years, to determine the long-term
effects of the vaccine and whether additional booster shots of VZV would
be necessary. It is possible that vaccinated children obtain booster
immune effects through repeated contact with the virus from infected
Early evidence suggested that the rate of breakthrough chickenpox
infections (infections in previously vaccinated children) was about 2
percent annually and that the likelihood of such infections did not
increase with time after vaccination. Breakthrough infections in
vaccinated children usually are very mild. They last only a few days and
there are fewer than 50 lesions on the child's body and little or
. It is not clear whether breakthrough chickenpox infections are less
contagious than infections in unvaccinated children.
Some physicians remain reluctant to vaccinate against an usually mild
childhood disease such as chickenpox. Some also are concerned that
vaccinated children may contract chickenpox as adults when it can be a
much more serious disease with a 20 percent higher risk of death.
Although children who have had chickenpox are immune to the disease and
cannot contract it a second time, the varicella zoster virus can remain
inactive in the human body. These dormant viruses are concentrated in
nerve cells near the spinal cord and may reactivate in adults, causing the
disease herpes zoster or shingles. The reactivated virus further infects
nerve cells, causing severe
, burning, or
. Shingles usually occurs in people over the age of 50 and may be
associated with a weakening immune system.
It is not known whether the weakened virus used for VZV can remain dormant
in the body, eventually causing shingles in the same way that the
naturally occurring varicella virus can. In 1998 the CDC found 2.6 cases
of post-vaccination herpes zoster for every 100,000 distributed doses of
VZV. In contrast there were 68 cases of herpes zoster in healthy children
under age 20, following natural infection with varicella. However, as of
2004, it is too early to determine whether vaccinated children are more or
less likely to develop shingles in adulthood as compared with adults who
were naturally infected with chickenpox as children.
A 2002 study indicated that exposure to varicella is much higher in adults
living with children and that such exposure substantially boosts immunity
against shingles. The authors of the study predicted that mass vaccination
against varicella will create an epidemic of herpes zoster, affecting as
many as 50 percent of those who were between the ages of ten and 44 at the
time that the vaccine was introduced.
Chickenpox is highly contagious and easily transmitted among children
through personal contact, coughing, or sneezing. The disease is
characterized by red spots on the face, chest, back, and other body parts.
These spots fill with fluid, rupture, and crust over. Symptoms of
chickenpox may not appear for as long as two to three weeks following
infection. The virus is contagious
from one or two days before the first rash appears until the blisters
have formed complete scabs and no new rash has appeared for 24 hours. This
may take from five days to two weeks. Thus the varicella virus can spread
very rapidly within families and among groups of children in school and
In most instances chickenpox is not a serious disease, although the itchy
lesions and fever and other mild flu-like symptoms may cause a week or two
of discomfort. However the disease can have serious complications.
Scratching the pox can cause bacterial infection that can lead to
permanent scars. In rare cases chickenpox can lead to the following:
In the United States more children die of chickenpox than of any other
disease that can be prevented by a vaccine. Prior to the introduction of
VZV, there were about 100 deaths and 12,000 hospitalizations annually as a
result of chickenpox infections. Approximately 40 percent of the deaths
and 60 percent of the hospitalizations occurred in children under age ten.
Teenagers and adults, as well as children with leukemia or other cancers
or with impaired immune systems, are at particular risk for severe
chickenpox and its complications. Babies whose mothers contracted
chickenpox during pregnancy are at risk for multiple birth defects. Babies
whose mothers contract chickenpox shortly before or after giving birth are
at risk of developing a severe form of the disease. As many as 5 percent
of these babies die. Most high-risk children and non-immune adults
contract chickenpox from unvaccinated children.
Children with chickenpox miss an average of five to six days of school and
their parents miss an average of three to four days of work while caring
for them. The CDC estimates that, including direct medical costs and
indirect societal costs, $5.40 is saved for every $1.00 spent on childhood
It is recommended that babies receive a single-dose injection of Varivax
between the ages of 12 and 18 months, usually at the same time that they
receive their first
(MMR) vaccine. Children and adolescents who have not already had
chickenpox can be vaccinated at any time. However, adolescents aged 13 or
older, as well as adults, require two doses of Varivax, four to eight
weeks apart, to obtain the same level of immunity as children under 13.
The reason for this is not known.
VZV usually is covered by health insurance. In the United States the
Vaccines for Children program covers the cost of chickenpox vaccination
for children without health insurance and for specific other groups of
children, including Native Americans.
In rare instances it is possible to contract the weakened vaccine strain
of varicella from a recently vaccinated child.
Children on long-term steroids for any reason, including
, should consult their physician about the timing of the vaccination.
Children should not receive VZV if the following applies:
In addition, infants under one year and pregnant teenagers should not
receive VZV. Females should not become pregnant within one month of
Children at high risk for severe chickenpox or its complications,
including newborns and premature infants exposed to chickenpox after
birth, often are given varicella-zoster immune globulin (VZIG). VZIG is
made from the blood serum of people with high antibody levels against the
varicella virus. It must be administered within 96 hours of exposure to
chickenpox, and it results in a passive immunity against the disease for
about three months.
Additional CDC precautions for administrating VZV pertain to the following
groups of children:
Medical conditions that preclude vaccination against chickenpox include
and any other moderate or serious illness. Moreover, children with
weakened immune systems should not receive a live virus vaccine such as
VZV. This restriction applies to children who have the following
Children with leukemia in remission or HIV-infected children with normal
immune function may be eligible for VZV. However, chickenpox can cause
serious complications in HIV-infected children with compromised immune
systems. Therefore, the National Institute of Allergy and Infectious
Diseases (NIAID) and the National Institute of Child Health and Human
Development (NICHD) are as of 2004 sponsoring a clinical study of the
safety and effectiveness of Varivax in HIV-infected children. In the
initial phase of the study, HIV-infected children who were without
symptoms tolerated Varivax well. Since shingles is very common in
HIV-infected children, the NIAID and NICHD also launched a clinical study
to determine whether Varivax can prevent shingles in HIV-infected children
who have had chickenpox.
Reactions to VZV are usually mild and may include:
Moderate or severe reactions to VSV have been reported very rarely. These
reactions include: high fever or seizures one to six weeks after
vaccination in fewer than one out of 1,000 children; pneumonia; and
, an allergic reaction that may include weakness, wheezing, breathing
, a fast heart rate,
, or behavior changes, within a few minutes to a few hours after the
injection. Other reactions, such as a low blood count or brain
involvement, including encephalitis, occur so rarely that they may not be
associated with VZV.
Following the distribution of the first 10 million doses of VZV, it was
determined that severe reactions occurred with a frequency of
approximately one in 50,000. This is far lower than the risks associated
with chickenpox. There is no evidence that healthy children who have had
chickenpox or who received VZV previously are at a greater for adverse
effects from an additional dose of Varivax.
The National Vaccine Injury Compensation Program helps pay for medical
expenses resulting from vaccine reactions. In case of a serious reaction
to VZV, parents should do the following:
VZV is not known to interact with any foods or drugs. However,
for treating herpes viruses, including acyclovir or valacyclovir, should
not be administered within 24 hours of Varivax, because these drugs can
reduce the effectiveness of the vaccine.
—A special protein made by the body's immune system as a
defense against foreign material (bacteria, viruses, etc.) that enters
the body. It is uniquely designed to attack and neutralize the specific
antigen that triggered the immune response.
—An additional dose of a vaccine to maintain immunity to the
—A disease that is contracted despite a successful vaccination
Herpes zoster virus
—Acute inflammatory virus that attacks the nerve cells on the
root of each spinal nerve with skin eruptions along a sensory nerve
ending. It causes chickenpox and shingles. Also called varicella zoster
—Ability to resist the effects of agents, such as bacteria and
viruses, that cause disease.
—The virus that causes chickenpox (varicella).
Varicella-zoster immune globulin
—A substance that can reduce the severity of chickenpox symptoms.
Atkinson, William, and Charles (Skip) Wolfe, eds.
Epidemiology and Prevention of Vaccine-Preventable Diseases
, 7th ed. Atlanta, GA: National Immunization Program, Centers for Disease
Control and Prevention, 2003.
Brisson, M., et al. "Exposure to Varicella Boosts Immunity to
Herpes-Zoster: Implications for Mass Vaccination Against
20 (June 7, 2002): 2500–07.
National Immunization Program.
NIP Public Inquiries, Mailstop E-05, 1600 Clifton Rd. NE, Atlanta, GA
30333. Web site: http://www.cdc.gov/nip.
National Vaccine Information Center.
421-E Church St., Vienna, VA 22180. Web site:
"Guide to Contraindications to Vaccinations."
National Immunization Program
, May 18, 2004. Available online at
December 20, 2004).
"Varicella Vaccine (Chickenpox)."
National Immunization Program
, December 20, 2001. Available online at
(accessed December 20, 2004).