GO BACK THREE or four months and America’s fight against covid-19 seemed to be going pretty well. Vaccination rates were rising, covid deaths were falling and the Biden administration was optimistic the pandemic would recede. The reality has since changed. The highly contagious Delta variant has sent cases surging. They now top 150,000 new cases a day on average, the highest level since January. Countries that once lagged America in the vaccination race have now overtaken it: 53% of Americans are fully vaccinated, compared with 59% in the European Union. Even overcrowded hospital wards seem to have little effect on the numbers of people getting jabbed. On September 9th Mr Biden got much tougher, outlining various measures to increase the proportion of Americans who are fully vaccinated. What opposition will the new vaccine mandates face?

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The measures affect firms and employees, and cover 100m people, nearly two-thirds of the American workforce. Employees of the federal government’s executive branch, and contractors who do business with it, will have to be jabbed (with some exemptions on health and religious grounds). The same is true of workers at health-care facilities that get government funding. (That expands a directive in August aimed at workers in nursing homes that get federal funding.) But the most striking move involves the private sector. Mr Biden ordered the Occupational Safety and Health Administration (OSHA), a government agency, to issue a rule requiring firms with 100 employees or more to mandate vaccinations for their workforces, with weekly testing for those who opt out. OSHA may take time to promulgate its directive, but opponents are already staking out their ground: several Republican governors have vowed to take legal action. “South Dakota will stand up to defend freedom. @JoeBiden see you in court,” tweeted Kristi Noem, the state’s Republican governor.

Ordinarily, implementing such a rule could take years. But a procedure known as an “emergency temporary standard” (ETS) allows OSHA to bypass the normal standard-setting process. Instead it will enter into force when OSHA releases it, after which the agency has six months to develop a permanent version. To justify an ETS, the agency must show it is necessary to protect employees from “grave danger”—a standard never clearly defined by Congress. It last issued one in June, mandating safety precautions at health-care facilities. Before then, the agency had only ever issued nine ETSs, the most recent from 1983. Four were fully invalidated by court challenges. Lawsuits surely await this one, too. Those most likely to succeed will probably contend that the agency exceeded the authority delegated to it by Congress.

The Supreme Court has yet to weigh in on vaccine requirements (though last month Amy Coney Barrett, one of the nine justices, denied an emergency request to block a university from implementing a vaccine requirement.) But plaintiffs have sued successfully to scuttle other public-health measures from the Biden administration. In August the Supreme Court invalidated a moratorium on evictions issued by the Centres for Disease Control and Prevention (CDC). And in July a federal appeals court rejected restrictions imposed by the CDC on cruise ships. Both rulings demonstrated the willingness of the judiciary to narrow the government’s public-health authority, says Wendy Parmet of Northeastern University. Even if the Supreme Court ultimately takes a similar view, Mr Biden’s move may yet yield results. Some firms had already mandated vaccinations among their workers, but others may have held off out of concern about employee pushback. The Biden administration’s proposed rule will give them cover to act. The mandate may spur more vaccinations whether or not it ultimately survives.

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