The disease

What is chickenpox? Chickenpox is an infection caused by the varicella virus, and it is highly contagious. It can spread in one of three ways: by coughing or sneezing, physical contact with broken blisters, or by virus particles from the blisters that are sprayed in the air. The rash of chickenpox begins as red bumps that turn into blisters that cover the entire body. As many as 300-500 blisters can occur during a single infection. Chickenpox is usually a relatively benign infection. However, chickenpox infections can have severe complications. About 1 of every 1,000 children infected with varicella will develop severe pneumonia (infection of the lungs) or encephalitis (infection of the brain). In addition, about 1 of every 50 women infected with varicella during their pregnancy will deliver children with birth defects. These birth defects include developmental delay and shortened or atrophied limbs. Finally, a bacterium called Group A streptococcus, commonly known as "flesh-eating" bacteria, can enter through the skin during a varicella infection and cause severe, and sometimes fatal, disease. How contagious is chickenpox? Chickenpox is very contagious. If 100 people are sitting in a room together for several hours talking and one of them has chickenpox and the other 99 have never been infected with chickenpox or vaccinated with the chickenpox vaccine, about 85 of the remaining 99 will get chickenpox.

The vaccine

How is the varicella vaccine made? Like the MMR vaccine, the varicella vaccine is a live, "weakened" form of natural varicella virus. The varicella vaccine virus is "weakened" by a process called "cell-culture adaptation." (see How Are Vaccines Made?). "Cell-culture adaptation" modifies natural varicella virus so that it behaves very differently once it is injected into the body. Natural varicella virus normally grows in cells that line the skin or the back of the throat. Cells are the building blocks of all the different parts of the body, like skin, heart, muscles and lungs. Natural varicella virus reproduces itself thousands of times, occasionally causing severe disease, and is then passed on to the next person unchanged. But the process of "cell-culture adaptation" changes all of that. Natural varicella virus was first taken from a young child infected with varicella in Japan. The family name of the child was Oka, and the strain of vaccine virus is now called the "Oka" strain. The virus was then "grown" in human embryo fibroblast cells. Fibroblast cells are the cells needed to hold skin and other connective tissue together. The vaccine virus was also grown in guinea pig fibroblast cells. By growing varicella virus in fibroblast cells it became less and less able to grow in human cells that lined the back of the throat or skin. This happened because the genes that told varicella virus how to reproduce itself were changed. Now the virus reproduced itself very poorly in human cells. When this vaccine virus (a now modified form of the natural virus) was put back into other children, it grew very poorly. Whereas varicella virus typically reproduces itself thousands of times during natural infection, the varicella vaccine virus reproduced itself probably fewer than 20 times. That is why natural infection with varicella virus causes illness, but varicella vaccine virus doesn't. However, because the varicella vaccine virus reproduces itself a little bit, it induces immunity against varicella that is, like the immunity to rubella, measles or mumps virus, likely to be life-long. (See how vaccines work.) Who should get the chickenpox vaccine? The varicella vaccine is given to children between 12 and 15 months and again between 4 and 6 years of age. Children, adolescents and young adults who have received only one dose should get a second dose. For previously unimmunized adolescents (13 to 18 years old) or adults, the vaccine is given as a series of two shots, separated by four to eight weeks. Does the varicella vaccine work? Since the introduction of the single-dose varicella vaccine into the United States in 1995, and the second-dose vaccine in 2006, the number of people infected with chickenpox has decreased from about 4 million per year to about 12,000 per year. Although this represents a 99 percent decrease in the incidence of infection, chickenpox infections still occur fairly frequently in the United States. What are the side effects of the varicella vaccine? Side effects of the varicella vaccine include tenderness in the local area of the shot, and occasionally a low-grade fever. A rash occurs in about 4 of 100 children who get the vaccine, generally around the area of the shot. A rash can also occur on parts of the body other than the area of the shot. There are usually fewer than 30 blisters that are a consequence of varicella vaccine, and children with these blisters are not contagious to other people. Because the chickenpox vaccine contains gelatin, people who are severely allergic to gelatin cannot get the vaccine. Gelatin is also contained in foods, like Jell-O, so a child's allergy may already be known before it is time to get the vaccine.

Other questions you might have

My daughter had chickenpox at 10 months old, but now that she is 1 year old, her doctor said she should still get the chickenpox vaccine. Is this really necessary? The Centers for Disease Control and Prevention (CDC) recommends vaccination of all children at 12 months of age who do not have evidence of immunity. Evidence can include: A diagnosis of chickenpox or shingles made by a healthcare provider

Laboratory confirmation of disease

Laboratory evidence of immunity (blood test) Because many rashes are misclassified, the CDC states that “if there is any doubt that the illness was actually varicella [chickenpox], the child should be vaccinated.” While you could get your daughter tested for evidence of antibodies to chickenpox, the test will require a blood draw and if it is negative, she will still need to get the vaccine. Further, a dose of vaccine even if she has had chickenpox will not hurt her; it will make her immunity stronger. If we immunize children against chickenpox, will they be more likely to develop shingles later in life? Shingles is a rash with extremely painful blisters that occur along a nerve, usually on the face, chest or abdomen. Shingles usually affects people 45 years old or older and occurs when the varicella virus reawakens (or reactivates). Although children usually completely recover from a chickenpox infection, the virus never really goes away. It lives silently in the nervous system and, when we get older, it can occasionally reawaken, causing shingles. Like the natural virus, the vaccine virus can also live silently in the nervous system. However, it has been shown that the varicella vaccine virus is much less likely to reawaken and, that when it reawakens, is much less likely to cause severe shingles than natural varicella virus. This makes sense because the varicella vaccine virus is much, much weaker than the natural virus. The important question you need to ask yourself: Would you rather have the more destructive, "wild type" virus reawaken later in life, or the weakened virus contained in the vaccine? Also, since the introduction of the varicella vaccine in 1996, the incidence of shingles in the United States has decreased. If we immunize children with the varicella vaccine, won't they be more likely to get chickenpox as adults? Chickenpox is much more likely to cause severe disease in adults than in children. Adults are 10 times more likely to be hospitalized when they have chickenpox than are children. Therefore, one thing that you would never want the vaccine to do is shift the disease from childhood to adulthood. However, for a number of reasons, this is unlikely to happen: Several studies have shown that immunity to chickenpox lasts at least 20 years and is probably life-long.

The varicella vaccine is made in a manner similar to the rubella vaccine. We immunize little girls with the rubella vaccine to protect them from catching rubella when they become pregnant as adults — an event that occurs 20, 30, or even 40 years later. And it works. The incidence of birth defects from rubella has decreased from as high as 20,000 cases per year to fewer than five cases per year.

Measles vaccine, also made in a similar way, has been successfully used for more than 40 years without seeing a similar shift in age of disease. Therefore, fading immunity, and a consequent shift of chickenpox infections from childhood to adulthood, is extremely unlikely to occur. Should teenagers and adults get the varicella vaccine? Any teenager or adult who has not had chickenpox or the chickenpox vaccine should receive the vaccine. Adults are 10 times more likely than children to be hospitalized with the severe consequences of chickenpox. These consequences include pneumonia and encephalitis (inflammation of the brain). I got the chickenpox vaccine as an adult, but I got a fever and a rash, so should I get the second dose of the vaccine? About 4 of every 100 adults who get the vaccine will develop a rash, usually with less than 10 blisters. You should still get your second dose of vaccine in one to two months as recommended. Your reaction to the first dose indicates that your body has made an immune response to the vaccine, so you are less likely to have the same thing happen after the second dose. However, the second dose will strengthen your immunity, so that if you are exposed to the chickenpox virus, you will be better protected.

Relative risks and benefits

Do the benefits of the varicella vaccine outweigh its risks? Before the varicella vaccine was recommended for use in all children in 1995, about 4 million children were infected every year. In addition, chickenpox caused about 10,000 hospitalizations and 100 deaths a year — most of these hospitalizations and deaths occurred in previously healthy young children. So, although chickenpox causes much less severe disease, permanent disabilities and death than other diseases such as polio, measles or rubella, it still is an important cause of severe disease in children. On the other hand, the varicella vaccine does not have serious side effects. Therefore, the benefits of the varicella vaccine clearly outweigh its risks. Disease risks Rash (300 -500 blisters)

Pneumonia or encephalitis (1 of 1,000)

Birth defects (1 of 50 infants born to women infected during pregnancy)

Bacterial co-infections

Disease can be fatal Vaccine risks Pain and tenderness at the injection site

Low-grade fever

Rash around injection site (4 of 100 recipients) or rash more distant to site of injection (less frequently)

Reference