Experts say the US has turned a major corner in its coronavirus outbreak, with cases at their lowest point in 4 months

• Daily US coronavirus cases have fallen roughly 74%, on average, in the last six weeks.
• Experts are hopeful that the US outbreak has turned a corner.
• Economist Ian Shepherdson predicted the "effective end of the US COVID crisis" before May.

As coronavirus vaccinations ramped up in the US at the start of this year, the hopeful progress was overshadowed by fears of variants. Scientists worried that B.1.1.7, the more contagious variant discovered in the UK, would keep coronavirus cases high through the winter even as more people got shots.

"The restrictions applied across the US right now, on average, are not tight enough to control B.1.1.7," Ian Shepherdson, chief economist at Pantheon Macroeconomics, wrote on February 12. He added that the US was "in a race between B.1.1.7 and the pace of vaccinations."

But this week, Shepherdson changed his tune: "If B.1.1.7 cases don't accelerate markedly over the next month, it will become realistic to call the effective end of the US COVID crisis - at least in terms of the case and hospitalization numbers - by the end of April," Shepherdson wrote on Monday.

Daily US cases have dropped by roughly 74%, on average, in the last six weeks. The country recorded fewer than 53,000 cases on Monday - its lowest daily count since October. On Tuesday, however, daily cases rose to nearly 68,000.

Daily deaths have also declined 38% over the last six weeks, while daily hospitalizations have fallen 55%.

"The balance is more optimism and less caution than four weeks ago," Andrew Noymer, an associate professor of public health at the University of California, Irvine, told Insider.

Such sharp declines in infections and illnesses "were not necessarily predicted by people like me," he added.

Many experts are now hopeful that the US outbreak has turned a corner - at least for the next few months.

Why have cases dropped so suddenly?

Scientists don't fully understand why cases have fallen so dramatically in the last six weeks.

Shepherdson suggested that B.1.1.7 might not be spreading as rapidly as some epidemiologists feared.

"We see no signs yet that the spread of the more infectious B.1.1.7 variant is slowing the rate of decline, even in Florida, where it appears to be most prevalent," he wrote.

Leana Wen, a visiting professor of health policy and management at George Washington University, told Insider that the major reason is likely that the holidays are behind us.

The US was "on such a high before, with the surge from Thanksgiving and Christmas, that we're finally dropping off from that," she said.

Another contributing factor could be that decreased testing means fewer cases are getting recorded: Average daily tests have declined 30% in the last six weeks. But that doesn't explain the considerable drop in hospitalizations and deaths.

Weather can also influence the dynamics of coronavirus outbreaks, since some studies have shown that warmer conditions might slow coronavirus transmission. But on the whole, the US didn't see dramatic temperature changes from January to February.

"It's probably a combination of things - a combination of there being enough people who are already infected and people getting vaccinated, so there's some level of protection," Wen said.

That combination of natural and vaccine-derived immunity, some experts think, may even be pushing the US closer to a herd-immunity threshold.

Is herd immunity closer than we realize?

Most scientists don't believe the US has reached herd immunity - the threshold beyond which the virus can no longer spread easily from person to person. But a few experts think we could be close.

Dr. Martin Makary, a professor of surgery at the Johns Hopkins School of Medicine, wrote in The Wall Street Journal last week that "the consistent and rapid decline in daily cases since January 8 can be explained only by natural immunity."

Based on that, he predicted COVID-19 would be "mostly gone" by April.

Shepherdson similarly suggested on Monday that herd immunity "won't be far off" by April.

Scientists calculate herd immunity based a virus' reproductive value: the number of people that one sick person infects, on average. Researchers generally estimate the coronavirus' reproductive value - the original virus, not a variant - to be between 2 and 3 without interventions like vaccines or public-health measures.

That means around 50% to 67% of the US population would need to have some immunity - whether through vaccination or natural infection - to achieve herd immunity.

But studies suggest that B.1.1.7 may increase the virus' reproductive value by 0.4 to 0.9. In that case, the threshold for herd immunity would be higher: up to 75% of the US population would likely need to develop immunity.

"I don't think there's any chance that we're going to be able to vaccinate 80% of Americans by July, so we're not going to reach herd immunity by that point," Wen said.

She added, though, that the US will likely see a "substantial decline" in infections by May, and a similar decline in hospitalizations in deaths to follow.

This period of lower infection rates, Wen said, "is our opportunity to get as much immunity in the community as possible" via vaccines.

Shepherdson expects that as the US gets nearer to herd immunity, more businesses will reopen.

"The gradual reopening of the services sector now underway will accelerate over the next two months," he said, "with most of the economy open in plenty of time for Memorial Day."

Andrew Dunn contributed reporting.