U.S. President Joe Biden announced sweeping COVID-19 vaccine mandates, requiring most federal employees to get the shots and pushing large employers to have their workers inoculated or tested weekly.

Vaccine mandates by states have been around for more than a century in the United States. In the 1850s, Massachusetts became the first state to mandate smallpox vaccination for school children, according to a Centers for Disease Control and Prevention publication. In 1905, the Supreme Court ruled in Jacobson v. Massachusetts that states could enforce vaccination mandates.

By the early 1900s, nearly half of the states had vaccination requirements for children entering school and by the early 1980s, all 50 states had vaccination laws covering students first entering school, according to the CDC publication.

U.S. vaccine mandates in the past have mostly been administered by state and local governments in relation to public venues, schools and healthcare facilities. The military also requires certain vaccines. Biden’s measures, which Republicans have vowed to fight, reignite a long-standing battle over individual rights and the powers of the executive branch.

The spread of the Delta variant has raised concerns as children head back to school.

In July, a federal judge refused to block Indiana University’s COVID-19 vaccine mandate and ruled that students must comply with the inoculation requirement. On Sept. 9, Los Angeles County school officials ordered COVID-19 vaccinations for all students aged 12 and over, the largest U.S. school district to take that dramatic step.

Vaccine mandates, however, are not absolute. All states and the District of Columbia have a medical exemption, which allows children with medical conditions that prevent them from receiving vaccines to be exempt.

Many states also allow exemptions based on philosophical or religious reasons. According to the CDC, about 2.5% of U.S. kindergartners used an exemption last year.