Urban areas and rural. Red states and blue. Places with broad vaccine rollouts and those with hardly any. North and South America, Europe, Africa, and Asia. Even countries with the B.1.1.7 variant.<\/p>\n
Look:<\/p>\n
![\"\"](\"https:\/\/blogs.nejm.org\/hiv-id-observations\/wp-content\/uploads\/sites\/2\/2021\/02\/91-DIVOC-countries-normalized-UnitedStates.png\")
<\/a>Source: 91-divoc.com\/pages\/covid-visualization\/, downloaded 21 Feb 2021.<\/p><\/div>\n
Let’s round up some theories:<\/p>\n
1. Seasonality.<\/strong> An attractive hypothesis — coronavirus infections pre-SARS-CoV-2 definitely show a seasonal pattern.<\/a><\/p>\n And various viral diseases go through communities synchronized with the seasons, especially when school starts or the weather gets colder. Any pediatrician will tell you that.<\/p>\n Note that the term “seasonality” has always been a bit misleading — it refers to infections peaking within seasons, not throughout them. Think of influenza, how sometimes we have an early, sometimes a late seasonal peak in the winter.<\/p>\n The problem with this seasonality theory is that the seasons are flipped in the southern hemisphere. And didn’t cases surge over the summer in many southern U.S. states?<\/p>\n 2. Herd immunity. <\/strong>Nearly 28 million Americans have had a confirmed COVID-19 diagnosis reported to the CDC. This represents only a fraction of the true cases — especially the mild or asymptomatic ones — and the CDC estimates that only\u00a01 in 4.6 infections are reported.<\/a>\u00a0That\u00a0could<\/em>\u00a0bring us up to half the US population with some degree of natural immunity to infection.<\/p>\n Even as of mid-January, the CDC put the actual case numbers at over 80 million, and certainly it’s higher than that now. And note that in some regions, the actual case counts might be even higher —\u00a05 to 20 times higher,<\/a>\u00a0according to one recent publication.<\/p>\n 3. Behavior.<\/strong> We know much better now how this virus is transmitted. Avoiding crowds and indoor spaces with poor ventilation — and wearing masks — reduce the risk. But has our behavior actually followed suit?<\/p>\n The holidays are behind us. The Super Bowl was lousy. Not many parties for the Australian Open tennis finals. Spring Break hasn’t happened yet.<\/p>\n One compelling hypothesis, related to herd immunity, is that the people least likely to follow infection control advice — or unable to follow it based on work or living situation — already have had COVID-19 and hence are immune.<\/p>\n The others, not yet infected, watched cases surge in December and January and continue to hunker down and stay safe — or again, have the luxury of staying safe. They might be especially vigilant now that a vaccine is in their not-too-distant future — you know, the pot of gold at the end of the rainbow, the light at the end of the tunnel, or the Holy Grail at the end of the Monty Python movie.<\/p>\n (Sorry about that.)<\/p>\n 4. Vaccines.<\/strong> The world is vaccinating like crazy. Demand is off the charts. And in most places, we’re targeting the people most likely to have symptomatic or severe disease.<\/p>\n Plus, the data increasingly suggest that the vaccines reduce not just disease, but also the likelihood of transmission — they reduce infections overall<\/a> (uninfected people can’t transmit), and those with infection have lower viral loads.\u00a0<\/a><\/p>\n While the vaccine rollout is not yet broad enough to explain the case number drop on its own, it might be contributing. It certainly could be playing a role in Israel.<\/a><\/p>\n 5. The virus.<\/strong> Maybe the virus is doing us a favor and becoming less virulent over time. Perhaps some of these variants — if not B.1.1.7 — in order to gain the ability to transmit, also cause less severe disease.<\/p>\n Take the virus’s perspective — yes, think like a virus — and how this would be evolutionarily beneficial. More mild cases, more chance to spread its genetic material to other susceptible hosts. That’s all viruses care about, right?<\/p>\n 6. It’s a gemish.\u00a0<\/strong>This brings us to the most likely explanation for the drop in cases, a gemish — Yiddish for a mixture of things.<\/a> (It’s pronounced “ga-mish”, in case you want to try it out on your own.)<\/p>\n It could be all of the above explanations, in various proportions, and different in various regions — plus things no one has considered.<\/p>\n And the uncertainty about why cases are dropping again hearkens back to this great H.L Mencken quotation,<\/a> which over time has morphed into this profound statement:<\/p>\n Every complex problem has a solution which is simple, direct, plausible — and wrong.<\/p><\/blockquote>\n