Vaccine Safety

Varicella (Chickenpox)

Varivax : The Food and Drug Administration (FDA) licensed this vaccine for use in 1995. It is a single-antigen vaccine that protects only against chickenpox. Many studies were done before the vaccine was licensed that showed Varivax is safe. Also, millions of people in the United States have gotten the vaccine safely. During 1995 through 2011, over 120 million doses of Varivax were distributed in the U.S. Learn about a combination vaccine called ProQuad that protects against measles, mumps, rubella, and varicella (chickenpox).

As with all licensed vaccines, CDC and FDA continue to closely monitor the safety of the chickenpox vaccine. Any problems that are detected will be reported to health officials, healthcare providers, and the public, and actions will be taken to ensure the public’s health and safety.

How CDC Monitors Varicella Vaccine

Vaccines continue to be monitored for safety after they are licensed. This is because pre-licensure trials are often too small to detect rare events and special populations may not be adequately represented. Since licensure, CDC and FDA have been closely checking the safety of chickenpox vaccine through 3 monitoring systems. These systems can monitor adverse events (health problems) already known to be caused by vaccines, as well as detect rare adverse events that were not identified during pre-licensure clinical trials. The 3 systems are:

The Vaccine Adverse Event Reporting System (VAERS)–an early warning public health system that helps CDC and FDA detect possible side effects or adverse events following vaccination.
The Vaccine Safety Datalink (VSD) –a collaboration between CDC and several health care organizations which monitors and evaluates adverse events following vaccination.
The Clinical Immunization Safety Assessment (CISA) Network– a collaboration between CDC and medical research centers in the U.S. which conduct research on adverse events that might be caused by vaccines.

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A Closer Look at the Safety Data

VAERS

In 2008, a CDC-FDA report analyzed VAERS data on patients who reported health events after getting chickenpox vaccine during May 1995 through December 2005. Merck distributed 50 million doses during this time. VAERS received 25,000 reports of health events that occurred after vaccination with the chickenpox vaccine (approximately 53 reports per 100,000 doses distributed).
The most commonly reported health problems after vaccination were:
- rash,
- fever, and
- pain, redness, and soreness at the injection site
Serious health problems after vaccination are extremely rare. Only a few have been confirmed by lab testing as due to vaccine-strain VZV, including:
- pneumonia,
- hepatitis,
- severe rash, and
- shingles with meningitis.
Some children who had these serious health problems after vaccination had weakened immune systems before they were vaccinated, but they had not been diagnosed by a doctor at the time of vaccination.
Other serious health events after vaccination have been reported, such as thrombocytopenia (low platelet count), acute cerebellar ataxia (brain injury that leads to balance problems), and acute hemiparesis (paralysis on part the body). It is not known if these were caused by the vaccine. Lab testing was either not done or did not confirm if the health effects were caused by vaccine-strain virus.

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Comparison of side effects from one and two doses of Varivax

In pre-licensure studies, 2 doses of chickenpox vaccine was about as safe as 1 dose. In children 12 months through 12 years old—
- 1 out of 5 children had local reactions (i.e., soreness, swelling, and redness) within 3 days of getting the first dose compared with 1 out of 4 children after the second dose
- 7 out of 100 children had fever after the first dose compared with 4 out of 100 children after the second dose
- 3 out of 100 children had chickenpox-like rash after first dose compared with 1 out of 100 children after second dose

Other safety information

Chickenpox vaccines contain weakened live VZV, which may cause latent (dormant) infection. The vaccine-strain VZV can reactivate later in life and cause shingles. However, the risk of getting shingles from vaccine-strain VZV after chickenpox vaccination is much lower than getting shingles after natural infection with wild-type VZV. For more information about how natural infection with wild-type VZV causes shingles, see Shingles Overview. Lab testing is needed to determine if a person got shingles from vaccine-strain VZV or from wild-type VZV.
People who get chickenpox vaccines can spread the vaccine-strain VZV to others. But, this is very rare.
Pregnant women should not get chickenpox vaccine. The Merck/CDC Pregnancy Registry for VZV-containing Vaccines monitors side effects in pregnant women who got the vaccine but should not have. There have been no cases of congenital chickenpox syndrome noted in 15 years of monitoring the vaccine. But, the theoretical risk to the fetus, although small, cannot be absolutely ruled out. For more information, see the Guidelines for Vaccinating Pregnant Women.

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Further Vaccine Safety Studies

CDC and FDA monitor health events following the first and second doses of chickenpox vaccine.
CDC and FDA vaccine safety experts review serious reports to VAERS.
Merck reviews reported exposures to vaccine-strain virus from Varivax®, ProQuad® or ZOSTAVAX® from inadvertent vaccination during pregnancy.
The CDC National VZV Laboratory tests specimens from people who suspect they have a health event caused by chickenpox vaccine. This helps determine if their health problem may be caused by vaccine-strain VZV or wild-type VZV. Laboratory testing is an essential part of evaluating health problems from live viral vaccines, like chickenpox vaccine.
CDC continues to consult other vaccine safety experts, such as the Clinical Immunization Safety Assessment (CISA) Network, when needed on complex vaccine safety questions.