American researcher Maurice Hillman and his team had long been researching the chicken pox vaccine. In 1981, Merk, an American pharmaceutical company, authorized the "Oka" strain of the varicella virus in the United States. Japanese virologist Michiaki Takahashi had removed the strain from a 13-year-old boy suffering from chicken pox, as stated by The History of Vaccines.
Merk further attenuated the strain, and it was used to develop the chicken pox vaccine. The invention of the chicken pox vaccine is accredited to Michiaki Takahashi. He started working on a chicken pox vaccine after his son suffered from the condition. By 1972, he was experimenting with the vaccine in clinical trials. After several years, Japan and other countries started using the vaccine. It was not until 1995 that the Food and Drug Administration approved the vaccine in the U.S. and added to the childhood immunization schedule, according to The New York Times.
The FDA delayed approving the vaccine because there were concerns that the immunity of the vaccine could not last for long and that there could be side effects, as stated by The New York Times. In 2006, the Centers for Disease Control approved a second dose of the vaccine. The first shot is given to kids when they are 12 to 15 months old and a second shot when they are 4 to 6 years old.