One spring morning  in 1902, E. Edwin Spencer, the city physician of Cambridge, Massachusetts, visited the home of Henning Jacobson, a local Lutheran preacher. Spencer had the vaccination for smallpox. Jacobson refused it for both himself and his family. The standoff between the two men would lead to what historian Michael Willrich called “the seminal case in modern American public health law.”

The Boston area was then experiencing what would be its last smallpox epidemic, which killed approximately 270 people over three years. At the time, smallpox—a highly contagious virus—was a leading cause of death around the world. Victims were diagnosed by telltale pustules, and during some outbreaks, as many as one in three infected people died.

The board of health in Cambridge, led by Spencer, had launched a citywide vaccination program that carried a financial ultimatum: Get vaccinated or face prosecution, including a $5 fine. The mandate applied to anyone who hadn’t received a vaccine since 1897. (The smallpox inoculation protected a person for three to five years.)

A similar mandate in neighboring Boston had mobilized a growing anti-vaccination movement. At least 19 people in Boston refused and were prosecuted in court. In February 1902, “anti-vaccinationists” had lined up at the Massachusetts State House to vividly describe how vaccines had harmed them—one citing the loss of feeling in one arm, and another saying that a five-year-old child died of lockjaw after receiving the vaccine.

In Cambridge, Spencer was an improbable enforcer. Before attending medical school in Worcester, he had studied alternative medicine at the Eclectic Medical Institute in Cincinnati. He preferred botanical solutions to chemical cures. After the refusal from Jacobson—an imposing figure with thick eyebrows and a goatee—he notified the preacher of his fine and left.

But in late June, death counts began to surge again, with cases only blocks from Jacobson’s home. City officials decreed that people who continued to refuse the vaccination would now be prosecuted, and on July 23, Jacobson became one of six refusers summoned to court.

As his case made its way through the courts, Jacobson preached to judge and jury with points borrowed from anti-vaccination societies. He said that his own childhood smallpox vaccination in Sweden caused “great and extreme suffering” and that vaccination led to “evil and dangerous effects.” In ringing oration, he likened vaccination to a “barbarous ceremonial of blood-poisoning.”

The U.S. Supreme Court, which decided the case in 1905, was not convinced. It ruled by a 7-2 margin that cities had the power and obligation to protect their citizens, through vaccination and other means, “as the safety of the general public may demand.” Jacobson paid his fines.

During the COVID-19 pandemic, Jacobson v. Massachusetts has been cited to defend some public health measures. So far, however, those measures have not included mandatory vaccination. According to bioethicist Arthur Caplan at the New York University Grossman School of Medicine, one reason is that current vaccines are being administered under emergency use authorization from the U.S. Food and Drug Administration, and officials are unlikely to require people to take a drug that is still under investigation. But over the next few months, he expects the fights over mandatory vaccination to begin.

“You undoubtedly will see mandates as soon as licensure occurs, and the government could act even before licensure,” Caplan says. “In the military, for example, if the Department of Defense says vaccination is necessary, the ability of a soldier to say, ‘I don’t want to do it’ is nothing.” A mandate for the armed forces would cover millions.

Immediately after the COVID-19 vaccines’ full licensure—“within a day, day and a half,” Caplan predicts—many more hospitals and nursing homes will most likely require vaccination among employees. These requirements will be similar to hospital mandates for workers to receive a flu vaccine. The future will probably also bring more vaccine mandates from airlines, cruise lines and other businesses.

“There will be lawsuits against any workforce mandates for health care workers, but we’ve already fought those out on the flu,” Caplan says, adding that the courts have upheld the requirements. Other lawsuits may follow, but Caplan doesn’t see those who press them having more luck than Henning Jacobson did. “You can’t have a worldwide plague that has destroyed the economy, killed millions and caused such anguish, and end up with sympathy for people who don’t want to vaccinate,” Caplan says.