Before the
chickenpox
vaccine’s debut in 1995, it was fairly usual for parents to try to expose their kids to a chickenpox-infected neighbor or friend—comfortable in the knowledge that once their kids recovered from the generally mild condition, they would develop an immunity to it. Now that, instead, most children receive the chickenpox vaccination, it’s time to ask, how effective is the chickenpox vaccine?
Chickenpox is a highly contagious childhood disease caused by the varicella-zoster virus. The virus is spread easily through the air via coughs and sneezes, or through contact with an infected person. A person with chickenpox is contagious 1-2 days before, and up to five days after, the rash appears.
The most recognizable symptom of chickenpox is the fluid-filled blisters that appear as a rash on the skin. These itchy blisters range from 250 to 500 in number, and appear all over the body. Fever, itching, headache, loss of appetite, and fatigue may also be part of the chickenpox package.
Most cases of chickenpox resolve over time, yet complications can occur. Some 12,000 people are hospitalized each year for chickenpox. The most common complication is skin infections caused by frequent scratching. Also possible, but less frequent, are pneumonia and brain infections. Certain groups are at higher risk of complications from chickenpox. People with weakened immune systems, babies younger than one year, and teens and adults are all at increased risk for complications.
Active cases of chickenpox have faded since the vaccine has become available. Before the vaccine, there were about four million cases of chickenpox each year in the United States. Most of these cases occurred in children 6-10 years old during the winter and early spring.
Since 1995, more than 40 million doses of chickenpox vaccine have been distributed. The American Academy of Pediatrics recommends that children between 12 and 18 months receive a single dose of chickenpox vaccine. Persons older than 13 years should receive two doses of the vaccine, 4-8 weeks apart.
Most children who are vaccinated do not get chickenpox. However, a small number of vaccinated children still get the disease, often called “breakthrough cases”. Since these children have been vaccinated, they get a milder case of the disease and recover faster.
Vaccination side effects are usually minimal. The most common side effect is a soreness or swelling at the injection site. Fever or a mild rash is less common.
Since the chickenpox vaccine is relatively new, clinical studies have followed its course to determine how effective it is. In a study by Barbara Kuter, PhD and colleagues, 2,216 children who received the vaccine were followed for 10 years. Half the children received one dose of vaccine, while the other half received two doses. During this time span, 71 children of the one-dose group, and 25 of the two-dose group developed chickenpox. The researchers estimated the vaccine to be 94.4% effective after one dose, and 98.3% effective after two doses. They conclude their study by saying that a two-dose regimen is worthy of additional consideration.
In another study. Marietta Vazquez, MD and colleagues examined whether time or age affected the vaccine’s effectiveness. From 1997 to 2003, twenty different group physician practices in Connecticut were monitored for cases of chickenpox in vaccinated children. A total of 339 vaccinated children were diagnosed with chickenpox during this time. The researchers found that the overall effectiveness of the vaccine was 87%, but there was a substantial difference in effectiveness over time. The vaccine was 97% effective in the first year after vaccination, versus just 84% effective 2-8 years after vaccination. They also found the vaccine to be more effective in the first year when administered to children 15 months or older. They conclude that the chickenpox vaccine is effective, although its effectiveness decreases significantly after one year. They also noted that breakthrough cases were mild.
In 2005, the prestigious group ACIP recommended that children immunized as infants not routinely receive a varicella booster in later childhood. However the ACIP also advised that all persons born after 1965 be screened for immunity to varicella and receive immunization if found to be susceptible.
In the world of vaccines, the chickenpox vaccine is still a relative newcomer. While most experts agree that overall the vaccine is effective, there are still unanswered questions. Will a booster be necessary at some point? Does the vaccine prevent
shingles
from occurring later in life? While the evidence today is showing the vaccine to be effective, these questions and others can only be answered by continued monitoring over time.
