Experts warn of risks of platforms hosting bets tied to US public health - including vaccine mandates and drug trials

The sign was pasted on a construction fence next to an elementary school in the Brooklyn borough of New York City. “Vaccines,” it proclaimed in bold capital letters against a blue star-spangled background - appearing, at first glance, to be like any other public health message on the importance of getting vaccinated.

But at the bottom of the sign, in smaller letters, it read: “Will any state end vaccine markets?” It was accompanied by the logo for Polymarket, an online prediction market where users can log in to wager on the outcomes of nearly any event - including, now, public health.