<p><img src="https://static6.businessinsider.com/image/60020fdae3d62500185fce4d-2000/COVID Vaccine Line.jpg" border="0" alt="COVID Vaccine Line" data-mce-source="Valerie Macon/AFP/Getty Images" data-mce-caption="People wait in line in a Disneyland parking lot to receive Covid-19 vaccines on the opening day of the Disneyland Covid-19 vaccination site in Anaheim, California."></p><p></p><bi-shortcode id="summary-shortcode" data-type="summary-shortcode" class="mceNonEditable" contenteditable="false">Summary List Placement</bi-shortcode><p>The US's daily coronavirus cases have declined 65% in the last montha record drop in the course of the nation's outbreak. New cases reached an all-time high of 312,000 on January 8. Since then, they've fallen to a weekly average of around 73,000 per day.</p><p>Dr. Martin Makary, a professor of surgery at the Johns Hopkins School of Medicine, suggested in The Wall Street Journal that the most likely explanation for the decline is that the US could be close to reaching herd immunity.</p><p>In <a href="https://www.wsj.com/articles/well-have-herd-immunity-by-april-11613669731">a Thursday op-ed</a>, Makary predicted that COVID-19 would be "mostly gone" by April.</p><p>He wrote that infections have likely been far more widespread than data suggestsso much so, in fact, that the US will soon hit a threshold beyond which the virus won't be able to pass easily from person to person. </p><p>"The consistent and rapid decline in daily cases since January 8 can be explained only by natural immunity," Makary wrote. "Behavior didn't suddenly improve over the holidays; Americans traveled more over Christmas than they had since March."</p><p>He added that vaccines "don't explain the steep decline" since early January, because "vaccination rates were low and they take weeks to kick in." </p><p>But many other doctors and public-health experts continue to caution that herd immunity is still a long way off in the USparticularly as more contagious variants spread.</p><p>"We're nowhere near community immunity or population immunity or whatever people want to call it at this point," Dr. Cindy Prins, an epidemiologist at University of Florida, told Insider. "We're nowhere near that yet."</p><h2>Could 55% of Americans have natural immunity already'</h2><p><img src="https://static3.businessinsider.com/image/5ff38e3b3793140019e284ad-2121/GettyImages-1279787727.jpg" border="0" alt="mask coronavirus herd immunity" data-mce-source="SetsukoN/Getty Images"></p><p>Researchers generally estimate the coronavirus' reproductive valuethat of the original strain, at leastto be between 2 and 3 in the absence of vaccines or public-health measures. That means that to achieve herd immunity, around 50% to 67% of a population would need to have some immunity to the viruswhether through vaccination or natural infection. </p><p>"In theory, the numbers are around 70%some say 65%, some say 75%, 80%but it's generally around those numbers. So it takes a while before you can get there," Dr. Eyal Zimlichman, deputy director general at Sheba Medical Center, Israel's largest hospital, told Insider in January. </p><p>But Makary's op-ed suggested that "observational data" indicates the US is close to the herd immunity threshold.</p><p>Assuming testing only captures 10% to 25% of infections, he said, about 55% of Americans would have natural immunity already, based on the number of tests reported. Add to that 15% of Americans who have been vaccinated so far, he wrote, "and the figure is rising fast."</p><p>Makary didn't respond to Insider's request for comment on this story.</p><h2>Other factors could explain the drop in US cases</h2><p><img src="https://static1.businessinsider.com/image/5fdaac84c910a400192e7f6a-2400/mask shield airport.JPG" border="0" alt="mask shield airport" data-mce-source="Mike Segar/Reuters" data-mce-caption="A couple wear protective face shields and masks as they walk to their flight at Newark International Airport on November 25, 2020."></p><p>Experts say herd immunity isn't the only possible explanation for the US's falling case count. </p><p>California's second lockdown, which started in December, may have partially contributed to the decline. At the peak of its outbreak, California accounted for around 40,000 of the US's daily coronavirus cases, on average.</p><p>Other factors like mask wearing, social distancing, and decreased travel after the holidays likely played a role as well.</p><p>"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, <a href="https://www.theatlantic.com/ideas/archive/2021/02/why-covid-19-cases-are-falling-so-fast/618041/">told The Atlantic</a>. "If you look at mobility data the week after Thanksgiving and Christmas, activity went down."</p><p>Thompson has also <a href="https://www.theatlantic.com/ideas/archive/2021/02/why-covid-19-cases-are-falling-so-fast/618041/">pointed out</a> that certain communities may have greater shares of immunity than others. </p><p>"Immunity is probably concentrated among people who had little opportunity to avoid the disease, such as homeless people, frontline and essential workers, and people living in crowded multigenerational homes," he wrote. "It might also include people who were more likely to encounter the virus because of their lifestyle and values, such as risk-tolerant Americans who have been going to eat at indoor restaurants."</p><p>Even if the US is nowhere near herd immunity, then, it's possible that high levels of immunity among those with frequent social interactions could help slow transmission.</p><p>But there's no concrete data yet to suggest that the majority of Americans are immune to the coronavirus.</p><p>The Centers for Disease Control and Prevention estimates that one-quarter of the US population has been infected with the coronavirus so far. Even in New York City, where the virus spread widely in the beginning of 2020, studies found that <a href="https://www.nature.com/articles/s41586-020-2912-6" data-analytics-module="body_link" data-analytics-post-depth="100" data-ml-dynamic="true" data-ml-dynamic-type="sl" data-orig-url="https://www.nature.com/articles/s41586-020-2912-6" data-ml-id="3" data-ml="true" data-xid="fr1613779571673iii" data-skimlinks-tracking="xid:fr1613779571673iii" data-uri="ac554795f2ee1d26e31554f0e67ec731">22% of the city's population</a> had been infected by April. A more <a href="https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2774584">recent study</a> found that by mid-November, around 14% of the US population had coronavirus antibodies.</p><p>"Even after adjusting for underreporting, a substantial gap remains between the estimated proportion of the population infected and the proportion infected required to reach herd immunity," the authors wrote.</p><h2>Variants make herd immunity a moving target</h2><p><img src="https://static1.businessinsider.com/image/5f6903d9323fc4001e0d7945-2200/covid abbott rapid test swab.jpg" border="0" alt="covid abbott rapid test swab" data-mce-source="Spencer Platt/Getty Images" data-mce-caption="A medical worker conducts rapid, 15-minute coronavirus testing on the new Abbott ID Now machine at a ProHEALTH center in Brooklyn, New York, on August 27, 2020."></p><p>Zimlichman said the very idea that a nation can reach herd immunity with the coronavirus has "never been put to the test." That's because the required threshold is often a moving targetand it can rise as new, more contagious variants spread.</p><p><a href="https://www.imperial.ac.uk/media/imperial-college/medicine/mrc-gida/2020-12-31-COVID19-Report-42-Preprint-VOC.pdf" data-analytics-module="body_link" data-analytics-post-depth="100" data-uri="8affcf31a4c86b470eb148cab7d6f2d5">Studies</a> <a href="https://khub.net/documents/135939561/338928724/SARS-CoV-2+variant+under+investigation%2C+meeting+minutes.pdf/962e866b-161f-2fd5-1030-32b6ab467896't=1608491166921" data-analytics-module="body_link" data-analytics-post-depth="100" data-uri="3b4b300d269a829ec4eb9d50180cfe1d">have shown</a> that the more contagious coronavirus variant discovered in the UK, called B.1.1.7, may increase the virus' reproductive value by 0.4 to 0.9. In that case, up to 75% of the US population would likely need to develop some form of immunity.</p><p>"When you have a new variant of COVID, if the reproductive number is higher, that means that the virus is going to be able to spread even if fewer people are susceptible," Rahul Subramanian, a data scientist at the University of Chicago, recently told Insider.</p><p>With that in mind, he said, "I would hesitate to say that we've reached herd immunity."</p><p>Reaching herd immunity could be even more difficult if vaccines prove less effective against new variants or if people refuse to get shots.</p><p>Some research suggests that vaccines <a href="https://www.businessinsider.com/coronavirus-variant-south-africa-vaccines-2021-1" data-analytics-module="body_link" data-analytics-post-depth="80" data-uri="f615e1e3ed3dba7529281647d3ed2551">may not work as well</a> against the more infectious variant discovered in South Africa. And 13% of adults in the US say they won't get a coronavirus shot, according to <a href="https://khn.org/news/article/poll-nearly-half-of-american-adults-now-want-the-covid-vaccine-asap/">a recent survey</a> from the Kaiser Family Foundation. </p><p>"Obviously you'll have issues with people that refuse to get vaccinated for whatever reason," Zimlichman said, "so getting herd immunity by vaccination is a hard one to achieve."</p><p><a href="https://www.businessinsider.com/johns-hopkins-professor-herd-immunity-us-april-2021-2#comments">Join the conversation about this story »</a></p> <p>NOW WATCH: <a href="https://www.businessinsider.com/covid-19-vaccine-what-its-like-healthcare-workers-nashville-2020-12">What it's like to get the COVID-19 vaccine, according to some of the first healthcare workers to receive it</a></p> Click here to read full news..