US coronavirus cases have dropped 70% in the last six weeks. Experts chalk it up to more immunity and better behavior.

• Average daily coronavirus cases in the US have fallen roughly 70% in the last six weeks.
• Experts attribute this to two main factors: a buildup of immunity and fewer social interactions after the holidays.
• Other factors, including state lockdowns and increased mask wearing, could have also played a role.

Many public-health experts got a surprise this month: US coronavirus cases have fallen roughly 70%, on average, in the last six weeks.

The US has recorded an average of around 68,000 daily cases over the last week. The last time the country saw weekly averages that low was in October. Yet with just 47 million Americans vaccinated so far, many experts assumed it would take longer for infections to plummet.

So scientists have posited a few theories as to why cases fell: Among the most likely is that many people who are at high risk of exposure or illness have acquired some form of immunity, either through vaccination or natural infection. Cases were also bound to drop after the holidays, once people stopped traveling and congregating indoors as much.

"I think the major reason is that we were just on such a high before, with the surge from Thanksgiving and Christmas, that we're finally dropping off from that," Leana Wen, the former health commissioner of Baltimore, told Insider.

A few other factors may have contributed: California was responsible for nearly 14% of US cases from November through January, and the state's lockdown helped slow transmission there. Increased mask compliance could also have some influence, and a decline in US testing could mean fewer cases are getting recorded.

Still, experts say there was no magic change that brought cases down.

"I'm not sure that we have learned any lessons other than what we've known all along, which is when people congregate, the virus replicates and spreads and we have higher levels of infections," Wen said.

She also cautioned that the US isn't out of the woods.

Indeed, daily cases have risen in the last few days, from a recent low of 52,500 on Monday to around 75,500 on Thursday. (Some of that might be delayed test results from the recent winter storms.) Over time, pandemic fatigue and the spread of more contagious variants could still reverse the US's positive trend.

"We'll see a substantial decline in infection numbers by May," Wen said, "but my worry would be: Will people become complacent and then will we see a substantial rebound come the fall?"

Social interactions declined after the holidays

Case counts typically reflect infections contracted about two weeks prior. That means the effects of transmission that occurred over Thanksgiving were likely recorded around the second week in December. Sure enough, average weekly cases in the US rose 25% from November 26 to December 10.

A similar pattern followed Christmas: Average weekly US cases rose 30% from December 25 to January 8. That's when the nation recorded its highest daily case count ever: more than 312,000.

Then cases began to go down.

"If I were ranking explanations for the decline in COVID-19, behavior would be number one," Ali Mokdad, a global-health professor at the University of Washington, told The Atlantic. "If you look at mobility data the week after Thanksgiving and Christmas, activity went down."

News of high case numbers and overstressed hospitals may also have encouraged people to social distance, avoid crowds, or wear masks more.

"People adhere more to public-health mitigations when they see the cases going off the ceiling, so that's what probably happened," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told Pod Save America's Jon Favreau on Monday.

Fauci added that "we haven't vaccinated enough people to have an impact on the kinetics of the outbreak."

Could immunity be more prevalent than scientists thought?

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 closer than previously realized.

Andrew Noymer, an associate professor of public health at the University of California, Irvine, recently told Insider that the US may be underestimating the prevalence of asymptomatic infections. Indeed, a recent model suggests that just 13% to 18% of COVID-19 cases are symptomatic. Given that asymptomatic people are less likely to seek a test, this could indicate that official case counts are missing a lot of infections.

What's more, many people who have the highest chance of exposure - including essential workers and residents of homeless shelters and nursing homes - have likely been exposed already, gotten vaccinated, or both.

"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.

A buildup of immunity - even if not to a herd-immunity threshold - could help slow transmission.

"It's very likely that acquired immunity is playing a huge role in the falling cases in California and elsewhere," Marm Kilpatrick, a professor of evolutionary biology at the University of California, Santa Cruz, told Insider.

But experts cautioned that this is still a fragile balance.

"While we might be seeing some effects of immunity now, it's because a substantial fraction of the population is continuing to distance and isolate," Dr. Kate Langwig, an infectious disease ecologist at Virginia Tech, told Insider.

She added: "If those people suddenly decided that because cases are going down, they felt more comfortable eating inside at a restaurant or socializing outside their pods, we could potentially erase the reductions that have been made over the past few weeks."

California's lockdown, and others, may have slowed transmission

At the peak of its outbreak, California accounted for around 40,000 of the US's daily coronavirus cases, on average. But the state's cases declined considerably in late January, after seven weeks of stay-at-home orders.

Similar restrictions across other states may have slowed transmission as well. New Mexico, for instance, implemented an 18-day stay-at-home order starting November 13.

In December, North Carolina also imposed a nighttime curfew that required residents to stay home from 10 p.m. to 5 a.m. The restriction lifted on Wednesday.

We've gotten better at wearing masks

Langwig said consistent advice on mask wearing and more high-quality masks available could also be helping.

The Centers for Disease Control and Prevention recently outlined several tips to make face coverings more protective, and even suggested that people double mask.

Around 73% of US adults now say they wear a mask every time they leave the house, according to a recent poll from the Kaiser Family Foundation. That's a considerable increase since May, when only 52% of US adults said they wore a mask every time.

We're not testing as much

Average daily coronavirus tests have declined 27% in the last six weeks. That could indicate that the US isn't confirming some new coronavirus cases - which might mean infections haven't dropped as dramatically as it seems.

"I worry that it's at least partly an artifact of resources being moved from testing to vaccination," Eleanor Murray, a professor of epidemiology at Boston University School of Public Health, wrote earlier this month.

But the number of daily positive tests has declined since early January. At the peak of the outbreak, the percentage of coronavirus tests coming back positive was nearly 14%. Now that number is less than 5% - an acceptable level to start loosening restrictions, according to most scientists.

Plus, if cases were still high, experts would expect hospitalizations and deaths to remain high, too. In the last six weeks, however, average daily hospitalizations have fallen 57%, while average daily deaths have fallen 39%.

Andrew Dunn contributed reporting.