Well, the disease is also more dangerous than chicken pox.
Side effects and adverse reactions from the smallpox vaccine range from fever to tissue necrosis and extensive lesions to encephalitis. A death rate of one per 1 million vaccinations is noted by the Centers for Disease Control and Prevention.
| Varicella (chickenpox) vaccine |
Printable version (vaccine information sheet) (.pdf) 
Mild Problems
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Soreness or swelling where the shot was given (about 1 out of 5 children and up to 1 out of 3 adolescents and adults)
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Fever (1 person out of 10, or less)
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Mild rash, up to a month after vaccination (1 person out of 20, or less). It is possible for these people to infect other members of their household, but this is extremely rare.
Moderate Problems
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Seizure (jerking or staring) caused by fever (less than 1 person out of 1,000).
Severe Problems
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Pneumonia (very rare)
Other serious problems, including severe brain reactions and low blood count, have been reported after chickenpox vaccination. These happen so rarely experts cannot tell whether they are caused by the vaccine or not. If they are, it is extremely rare.