By Paul Sax
We don't know.
It's also quite possible that the number of cases is actually decreasing; it's not just about reduced testing, and it's happening almost everywhere.
Urban and rural areas. Red and blue states. Places with widespread vaccine rollouts and those with hardly any. North and South America, Europe, Africa, and Asia. Even countries with the B.1.1.7 variant.
Source: 91-divoc.com/pages/covid-visualization/ , downloaded on February 21, 2021.
Let's gather some theories:
1. Seasonality. An attractive hypothesis: Coronavirus infections prior to SARS-CoV-2 definitely show a seasonal pattern.
And several viral illnesses spread through communities in sync with the seasons, especially when school starts or the weather gets colder. Any pediatrician will tell you that.
Keep in mind that the term "seasonality" has always been a bit misleading: it refers to infections that peak within seasons, not throughout them. Think of influenza, how sometimes we have an early seasonal peak, sometimes a late one in the winter.
The problem with this seasonality theory is that the seasons are reversed in the Southern Hemisphere. And didn't cases increase during the summer in many southern US states?
2. Herd immunity. Nearly 28 million Americans have had a confirmed COVID-19 diagnosis reported to the CDC. This represents only a fraction of the actual cases, especially mild or asymptomatic ones, and the CDC estimates that only 1 in 4.6 infections are reported. That could bring us to as much as half the US population with some degree of natural immunity to infections.
Even in mid-January, the CDC estimated the actual number of cases at over 80 million, and it's certainly higher than that now. And keep in mind that in some regions, the true case count may be even higher— 5 to 20 times higher, according to a recent publication.
3. Behavior. We now know much better how this virus is transmitted. Avoiding crowds and poorly ventilated indoor spaces, and wearing masks, reduces the risk. But has our behavior followed suit?
The holidays are over. The Super Bowl was awful. There aren't many matches scheduled for the Australian Open final. Spring break hasn't even happened yet.
One compelling hypothesis, related to herd immunity, is that people who are less likely to follow infection control advice, or who are unable to follow it based on their job or living situation, have already had COVID-19 and are therefore immune.
The others, who weren't 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 may 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 a Monty Python movie.
4. Vaccines. The world is getting vaccinated like crazy. Demand is off the charts. And in most places, we're targeting the people most likely to have a symptomatic or severe illness.
Furthermore, data increasingly suggest that vaccines reduce not only the disease, but also the likelihood of transmission: they reduce infections overall (uninfected people cannot transmit) and infected people have lower viral loads.
While the vaccine rollout is not yet widespread enough to explain the drop in cases on its own, it could be contributing. It certainly could be playing a role in Israel.
5. The virus. Perhaps the virus is doing us a favor and becoming less virulent over time. Perhaps some of these variants, if not B.1.1.7, to gain the ability to transmit, will also cause a less severe illness.
Take the virus's perspective—yes, think like a virus—and consider how this would be evolutionarily beneficial. Milder cases mean a greater chance of spreading its genetic material to other susceptible hosts. That's all viruses care about, right?
6. It's a gem. This brings us to the most likely explanation for the drop in cases: a gem, Yiddish for a mixture of things. (It's pronounced "ga-mish," in case you want to try it yourself.)
It could be all of the above explanations, in varying proportions and different in various regions, plus things that no one has considered.
And the uncertainty about why cases are declining again goes back to this great quote from HL Menkin, which over time has transformed into this profound statement:
Every complex problem has a solution that is simple, straightforward, plausible, and incorrect.
I want to emphasize the importance of being humble about not knowing why cases are declining simply because relying on one factor over another could get us into trouble. For example, this week Dr. Marty Makary, writing in the Wall Street Journal , postulated that we are already close to herd immunity, making this bold prediction:
There is reason to believe the country is moving toward an extremely low level of infection. As more people have been infected, most of whom have mild or no symptoms, fewer Americans remain to be infected. On the current trajectory, I expect Covid to largely disappear by April, allowing Americans to resume normal life.
Warning: If someone confidently tells you they know exactly why cases are declining and have an accurate crystal ball showing that by April we'll be safe from this pandemic, view it with the appropriate scientific skepticism it deserves.
Look, we can hope this optimistic prediction is correct; we all want that. April isn't far off, we'll know soon.
But if there is one thing that the pandemic of a new human disease teaches us, it is that there are many things we do not know.