The number of confirmed COVID-19 cases and fatalities is much higher in temperate, mid-latitude countries (cool and dry regions) than those in tropical (warm, humid regions). This is mainly due to the big difference in temperature (T) and relative humidity (RH) in temperate countries (T=3°C to 10°C and cooler, RH=40 percent) compared to tropical countries (T=25°C to 35°C and warmer, RH=65 percent).
The confirmed accumulated cases as of May 10, 2020, of each European (temperate) country (United Kingdom, Spain, Italy) is fourfold, and the number of fatalities is fifteenfold compared to the total Asean (tropical) number of cases and deaths. For the United States versus Asean, the ratio is 22:1, and 42:1 for accumulated cases and fatalities, respectively. It is 3:1 for accumulated number of cases and 14:1 for accumulated deaths for New York City alone, compared to the whole Asean statistics.
Why is this so? Coronaviruses are a family of so-called “enveloped viruses,” i.e., coated in an oily coat, known as a lipid bilayer, studded with proteins that stick out like spikes of a crown, helping to give them their name—corona, which is Latin for crown. Early researches suggest that:
1) This oily coat makes the viruses more susceptible to heat than those that do not have one. In colder conditions, this oily coat hardens into a rubber-like state, much like how fat from cooked meat will harden.
2) High temperature and relative humidity reduce the transmission of COVID-19 both with 1 percent significance levels (“High Temperature and High Humidity Reduce the Transmission of COVID-19,” by Jingyuan Wang, et al.). The higher the temperature and humidity, the shorter the virus survives. Reports also quoted William Bryan, acting head of the US Department of Homeland Security’s Science and Technology Directorate, saying that sunlight (UV radiation), heat (high temperature), and humidity (moisture) weaken the coronavirus.
3) Other coronaviruses can survive for more than 28 days at 4°C.
4) Experiment shows UV rays destroy COVID-19 both on surfaces and in the air. On nonporous surfaces such as stainless steel, the new coronavirus takes 18 hours to lose half its strength in a dark, low-humidity environment. In a high-humidity environment, that half-life dropped to six hours, and when the virus was exposed to high humidity and sunlight, the half-life dropped to two minutes.
5) The virus dies quickest in the presence of direct sunlight; in a dark room, the virus maintained half its strength for an hour. When exposed to sunlight, it lost half its strength in 90 seconds.
Conclusion: Geographically, the higher concentration of COVID-19 cases is at north of latitude 30°N, (temperate countries) where the winter/spring temperatures could be between 3-10°C and below, compared to areas south of 30°N (tropical countries), where the temperatures could be between 25-35ºC and warmer. In short:
6) There is a tendency for warmer climates to slow down the transmission of COVID-19.
7) Areas with significant community transmission of COVID-19 had distribution roughly along the 30-50°N corridor, and much lower along the tropical region-envelop of 30°N to 30°S.
L.A. Amadore, Ph.D.,
professorial lecturer,
Institute of Environmental Science and Meteorology,
University of the Philippines Diliman