Second, if a vaccine does emerge, there is a good chance that leading health organizations like the Bill and Melinda Gates Foundation or the World Health Organization will have a hand in producing or distributing it. If that’s the case, anti-vaccine activists, who have been crusading against these groups for years, will have plenty of material stockpiled to try to discredit them. They are already taking aim at Mr. Gates with baseless conspiracy theories claiming that he created and is trying to profit from the virus. These theories will be amplified, and the attempts to discredit leading virus research efforts will intensify as the vaccine nears.

Third, if and when a Covid-19 vaccine is approved for widespread use, people may be required to take it before being allowed to fly on certain airlines, attend certain schools or enter certain businesses. That’s a good idea, public health-wise, but it would play into some of the worst fears of the anti-vaccine movement.

Mandatory vaccination has been an especially potent talking point for anti-vaccine activists, some of whom have rebranded themselves “pro-choice” when it comes to vaccines. And years of battling states and school districts over mandatory vaccine policies have given them a playbook for creating a tangle of legal roadblocks and damaging publicity campaigns.