If people in relatively low-prevalence regions start mixing again when restrictions are relaxed, cases could once more begin to rise, researchers warn.

And what happens in the United States — which accounts for almost a quarter of all recorded worldwide COVID-19 cases to date — will have an important effect on the global trajectory. Although the portion of the US population that’s tested positive for antibodies is lower than that in India, in some states, more than a quarter of the people tested had antibodies against the virus in late January, according to the US Centers for Disease Control and Prevention.

But the overall numbers don’t reflect large variations within communities, says Marm Kilpatrick, an infectious-disease researcher at the University of California, Santa Cruz. Immune protection could explain the fall in some communities where people have been very highly exposed to the virus, but the drop in other communities is probably due to people hunkering down since the holiday period in November and December, says Kilpatrick. As some states lift restrictions, people could start to socialize again, he says.

“It worries me that the US is taking a strong step back from controls,” adds Baker.