Varicella vaccine

Varicella vaccine, also known as chickenpox vaccine, is a vaccine that protects against chickenpox.^[9] One dose of vaccine prevents 95% of moderate disease and 100% of severe disease.^[10] Two doses of vaccine are more effective than one.^[10] If given to those who are not immune within five days of exposure to chickenpox it prevents most cases of the disease.^[10] Vaccinating a large portion of the population also protects those who are not vaccinated.^[10] It is given by injection just under the skin.^[10] Another vaccine, known as zoster vaccine, is used to prevent diseases caused by the same virus – the varicella zoster virus.^[11]

The World Health Organization (WHO) recommends routine vaccination only if a country can keep more than 80% of people vaccinated.^[10] 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.^[10] Either one or two doses of the vaccine are recommended.^[10] In the United States two doses are recommended starting at twelve to fifteen months of age.^[9] As of 2017, twenty-three countries recommend all non-medically exempt children receive the vaccine, nine recommend it only for high-risk groups, three additional countries recommend use in only parts of the country, while other countries make no recommendation.^[12] Not all countries provide the vaccine due to its cost.^[13] In the United Kingdom, Varilrix, a live viral vaccine^[14] is approved from the age of 12 months, but only recommended for certain at risk groups.

Minor side effects may include pain at the site of injection, fever, and rash.^[9] Severe side effects are rare and occur mostly in those with poor immune function.^[10] Its use in people with HIV/AIDS should be done with care.^[10] It is not recommended during pregnancy; however, the few times it has been given during pregnancy no problems resulted.^[9][10] The vaccine is available either by itself or along with the MMR vaccine, in a version known as the MMRV vaccine.^[10] It is made from weakened virus.^[9]

A live attenuated varicella vaccine, the Oka strain, was developed by Michiaki Takahashi and his colleagues in Japan in the early 1970s.^[15] American vaccinologist Maurice Hilleman's team developed a chickenpox vaccine in the United States in 1981, based on the "Oka strain" of the varicella virus.^[16][17][18] The chickenpox vaccine first became commercially available in 1984.^[10] It was first licensed for use in the US by Merck, under the brand name Varivax, in 1995. It is on the WHO Model List of Essential Medicines.^[19][20]