Other officials have pointed to Google mobility data to argue that Americans withdrew into their homes after the winter holidays and hunkered down during the subsequent spike in cases that grew out of all that yuletide socializing. New hospital admissions for COVID-19 peaked in the second week of January—another sign that social distancing during the coldest month of the year bent the curve.
Read: The pandemic’s deadly winter surge is rapidly easing
Our cautious behavior evidently requires the impetus of a terrifying surge. In the spring, southern and western states thought they had avoided the worst of the early wave, and governors refused to issue mask mandates. Then cases spiked in Texas, Florida, and Arizona, and mask-wearing behavior in the South increased. When cases came down again, people relaxed, cases went up again, and the awful do-si-do continued.
The lesson is not to let today’s good news become tomorrow’s bad news, again. Until much of the population is vaccinated, don’t interpret the decline in cases as a green light to resume your pre-pandemic behavior.
2. Seasonality: The coronavirus was perhaps destined to decline this time of year.
Behavior can’t explain everything. Mask wearing, social distancing, and other virus-mitigating habits vary among states and countries. But COVID-19 is in retreat across North America and Europe. Since January 1, daily cases are down 70 percent in the United Kingdom, 50 percent in Canada, and 30 percent in Portugal.
This raises the possibility that SARS-CoV-2, the virus that causes COVID-19, is seasonal. Last year, a meta-study of coronaviruses such as SARS-CoV-2 found that they typically peak in the Northern Hemisphere during the winter, with the most common peak months being January and February. “The apparent seasonality of human coronaviruses across the globe suggests that this phenomenon might be mined to produce improved understanding of transmission of COVID-19,” the authors concluded.
The notion of seasonality is both obvious and mysterious. We know that many respiratory viruses are less virulent in the summer, accelerate in the closing months of the calendar year, and then recede as the days grow longer after December. But as the Harvard epidemiologist Michael Mina told New York magazine, “We don’t fully appreciate or understand why seasonality works.”
What we call seasonality seems to be a combination of environmental factors and the things people do in response to them. Many viruses fare best in cold and dry conditions; they’re not well designed to thrive in warmer, sunnier, and more humid outdoor areas. Each virus is a bundle of genes and protein encased in a fatty lipid molecule. This fatty shell breaks down more easily in warmer environments. You can see this for yourself when you try washing a smear of butter off your hands with cold water versus warm water.