<P> The World Health Organization (WHO) recommends routine vaccination only if a country can keep more than 80% of people vaccinated . If only 20% to 80% of people are vaccinated it is possible that more people will get the disease at an older age and outcomes overall may worsen . Either one or two doses of the vaccine is recommended . In the United States two doses are recommended starting at twelve to fifteen months of age . As of 2012 most European countries either recommend it for all children or just those at high risk, but not all countries provide the vaccine due to its cost . </P> <P> Minor side effects may include pain at the site of injection, fever, and rash . Severe side effects are rare and occur mostly in those with poor immune function . Its use in people with HIV / AIDS should be done with care . It is not recommended during pregnancy; however, the few times it has been given during pregnancy no problems resulted . The vaccine is available either by itself or along with the MMR vaccine, in a version known as the MMRV vaccine . It is made from weakened virus . </P> <P> The chickenpox vaccine first became commercially available in 1984 . It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system . In the United States it costs between 100 and 200 USD . </P> <P> Varicella vaccine is 70% to 90% effective for preventing varicella and more than 95% effective for preventing severe varicella . Follow - up evaluations have taken place in the United States of children immunized that revealed protection for at least 11 years . Also, studies were conducted in Japan which indicated protection for at least 20 years . </P>

When was the chickenpox vaccine introduced in the united states