In 1902, the city of Cambridge, Massachusetts, faced a smallpox outbreak. In response, the local health board ordered the city’s residents over the age of 21 to be vaccinated against this disease. Violators faced a $5 fine.

After a local pastor was fined for violating this vaccine mandate, he appealed his case all the way to the Supreme Court. The Court told him to pound sand in Jacobson v. Massachusetts (1905).

“The liberty secured by the Constitution of the United States to every person within its jurisdiction does not import an absolute right in each person to be, at all times and in all circumstances, wholly freed from restraint,” Justice John Marshall Harlan wrote for the Court. He added that “there are manifold restraints to which every person is necessarily subject for the common good.”

Under Jacobson, state and local governments — though not necessarily the federal government — may mandate vaccines for nearly all of their residents.

That decision has obvious relevance today. We now have multiple vaccines against Covid-19 that are both safe and shockingly effective, and they are available for free for all Americans. Yet the pandemic continues to rage in the United States because a large minority of Americans have yet to get a shot. While some people may face legitimate obstacles, others are just obstinate. Policymakers and other leaders, in other words, may need to take a page from Cambridge’s early 20th-century health board.

Some already are. Many of the first mandates are from employers: The state of New York, for example, recently announced that all of its employees will have to either get vaccinated or submit to weekly coronavirus testing, and President Joe Biden plans to impose similar requirements on federal employees.

Many private employers also require vaccines — Google, for example, will insist that its employees be vaccinated in order to enter the company’s offices. More than 600 colleges and universities require at least some of their students, faculty, and staff to be vaccinated.

These sorts of mandates will undoubtedly trigger lawsuits from vaccine resisters. In some cases, individuals with religious objections to vaccines or people with disabilities that preclude them from being vaccinated will have strong legal claims — much like schoolchildren who can already seek exemptions from schools’ vaccination requirements if they have religious objections.

But, assuming that the courts follow existing law — and assuming that Republican state governments do not enact new laws prohibiting employers from disciplining workers who refuse to be vaccinated — most challenges to employer-imposed vaccination requirements should fail.

Under Jacobson, moreover, states should be free to order everyone within their borders to be vaccinated against Covid-19, although it’s far from clear whether the federal government could do the same.

Of course, there is no guarantee that the Roberts Court, which is eager to impose limits on public health officials and not especially bothered about overruling precedents, will follow Jacobson if a state does enact a vaccine mandate. But there is good reason to believe that it will. Even Justice Neil Gorsuch, one of the most conservative members of the current Court, recently described Jacobson as a “modest” decision that “didn’t seek to depart from normal legal rules during a pandemic.”

The bottom line, in other words, is that, under existing law, numerous institutions within the United States may require their employees — and, in some cases, their citizens — to be vaccinated against Covid-19.

Your boss probably can require you to get vaccinated

Employment relationships in the United States are typically “at-will,” meaning that an employee can be fired at any time and for any reason, even if that reason is completely arbitrary. If you have an at-will relationship with your employer, your boss can fire you because they don’t like your haircut. Or because they don’t like what you had for breakfast last Tuesday.

Or, for that matter, because you refuse to get a Covid-19 vaccine.

The general rule, in other words, is that your employer can fire you for any reason unless some outside legal force — a federal or state law, or maybe an individual or collective bargaining contract between you and your employer — intervenes to give you additional job security. And there is no federal law prohibiting employers from requiring nearly all of their employees to get vaccinated.

That said, some federal laws may allow a small number of workers to seek an exemption from their employer’s decision to mandate vaccination.

Neither of these laws prevents an employer from “requiring all employees physically entering the workplace to be vaccinated for Covid-19,” according to the EEOC’s guidance on Covid-19 in the workplace. But employees may be entitled to a “reasonable accommodation” if their religion or disability precludes them from getting vaccinated, so long as this accommodation does not “pose an undue hardship on the operation of the employer’s business.”

Some examples of reasonable accommodations that might be offered to certain employees include requiring these workers to “wear a face mask, work at a social distance from coworkers or non-employees, work a modified shift, get periodic tests for COVID-19, be given the opportunity to telework, or finally, accept a reassignment.” But not every employee will be entitled to each of these accommodations, even if they are protected by a law like the ADA or the Civil Rights Act.

The specific accommodation will depend on an individual employee’s job duties — someone who does work that can only be done at a particular job site, for example, may not be allowed to telework. And employers are not required to employ people who cannot do their job even with reasonable accommodations.

It should be noted that some states may have existing laws that place additional restrictions on employers. And there’s always a risk that Republican state lawmakers will pass new laws prohibiting employers from requiring their employees to be vaccinated. But the law should permit most employers to require nearly all of their workers to get vaccinated.

State and local governments can require their residents to get vaccinated

So long as Jacobson remains good law, state and local governments may require their residents to get vaccinated. Indeed, states currently require their residents to get a wide range of vaccines by mandating that children be vaccinated before entering school or certain forms of child care. The only reason why a Covid-19 vaccine mandate would need to apply to adults is that the virus recently emerged, so most Americans were well past school age when they needed a vaccine.

That said, the Supreme Court will likely permit some individuals to seek exemptions from a Covid-19 vaccine mandate. Ever since Justice Amy Coney Barrett gave conservatives a 6-3 majority on the Court last fall, the Court has been extraordinarily aggressive in granting religious exemptions to Covid-related public health orders.

Federal law also restricts state governments’ ability to regulate people with disabilities; the ADA prohibits state and local governments from discriminating against many people with disabilities. It is likely, in other words, that at least some people will be able to get an exemption from a statewide or citywide vaccine mandate if they have a medical condition that precludes them from being vaccinated.

Another question is whether a state could require non-residents who enter their borders to be vaccinated.

As a general rule, a resident of one state who visits another is subject to the laws of a state they are merely passing through. If a resident of Florida takes a trip to New York, they may be prosecuted by New York officials if they commit a crime in New York. That said, the Supreme Court recognizes a constitutional right of all Americans to travel among the states. So an unvaccinated resident of Florida might claim that this right to travel is violated if New York tells them that they must be vaccinated if they wish to visit.

But there’s some recent evidence that even the Roberts Court’s right flank is unlikely to smile upon such a claim. Justice Clarence Thomas recently denied relief to a man who claimed that requiring him to wear a mask while flying on a commercial airline violates his right to travel.

The federal government probably cannot require every American to become vaccinated, but it could make remaining unvaccinated very expensive

To be brief: Neither Congress nor President Biden can likely force citizens to be vaccinated, although the federal government can use financial carrots and sticks to encourage vaccination.

To be longer (and wonkier): In NFIB v. Sebelius (2012), the Court’s first major Obamacare case, the Supreme Court imposed a novel new limit on Congress’s power. Congress may not use its broad power to regulate the national economy in order to regulate “inactivity.” If someone does not want to take a particular action, the federal government’s ability to require them to take that action is limited.

NFIB’s holding on this point, in the words of one very conservative federal judge, had no support “in either the text of the Constitution or Supreme Court precedent,” but lower courts are required to follow the Supreme Court’s decisions even if they are arbitrary or lawless. And NFIB has pretty clear implications for a federal vaccine mandate.

Indeed, this very issue came up during oral arguments in NFIB. Justice Stephen Breyer posed a hypothetical to Michael Carvin, one of two lawyers arguing that the Affordable Care Act is unconstitutional, about what might happen if the Court adopted his proposed legal standard. “If it turned out there was some terrible epidemic sweeping the United States,” he said, would the federal government have the “power to get people inoculated?”

Carvin’s response: “No, they couldn’t do it.”

Yet, even if the courts endorse Carvin’s reading of the federal government’s authority to mandate vaccines, Congress could still use financial incentives to encourage vaccination.

The simplest way to do so would be to pay people to get vaccinated or to offer a tax break to everyone who gets the vaccine. The tax code gives all sorts of benefits to taxpayers who engage in activity that Congress deems desirable — ranging from buying a home to having a child to driving an electric vehicle.

Another option is to require unvaccinated people to pay a much higher percentage of their income in federal taxes in order to incentivize them to become vaccinated. Such a policy might elicit some outrage, but it’s entirely constitutional even under NFIB.

But Congress also has fairly broad authority to attach conditions to federal benefits. It could require everyone who receives health coverage through a federal program such as Medicare, Medicaid, or the Affordable Care Act to become vaccinated if they want to keep those benefits.

One group the federal government could easily impose vaccines on: immigrants. Federal law already requires foreign nationals who apply for an immigration visa or who want to become lawful permanent residents to be vaccinated against certain diseases. The government could add a Covid-19 vaccine to this list.

There is no guarantee that the courts will follow existing law

Having laid out what the law says about vaccine mandates, there is a danger that a judiciary dominated by Republican appointees will ignore that law. As NFIB taught us, the mere fact that a legal argument has no basis in law or precedent is no guarantee that it won’t win approval from five justices.

There’s also a risk that a conservative lower court judge — we’ll call this hypothetical conservative judge “Reed O’Connor” — could issue an injunction blocking any attempt to require people to become vaccinated. Even if this injunction is lawless, and even if it is ultimately vacated by a higher court, that process could take months or even years.

But existing law is clear that employers have broad latitude to require most of their workers to become vaccinated. It is equally clear that state governments may impose vaccination requirements. And, while the federal government’s power is probably less broad, it is broad enough to give every American a powerful financial incentive to become vaccinated.

1905, the year Jacobson was handed down, is one of the most infamous years in the Supreme Court’s history. It is the same year the Court handed down Lochner v. New York, a now-discredited decision stripping lawmakers of much of their authority to ensure that workers are not exploited. Lochner is now widely taught in law schools as an example of how judges should never, ever behave.

And yet, even most of the right-wing justices who joined the majority in Lochner recognized that striking down a state vaccine mandate would go too far. Even they realized that the government must have the power to protect the public health.

There is good reason to hope, in other words, that the current Supreme Court wouldn’t be so reactionary as to strike down a vaccine mandate.