Why the Northern and Southern Hemispheres are Worlds Apart When it Comes to Covid-19 Spread

Although 209 countries across the world have reported Covid-19 cases, a closer analysis of their geographical locations throws up some interesting numbers and conclusions.

A total of 171 affected countries fall in the northern hemisphere while only 36 are in the southern hemisphere – i.e., 82.6% of the affected countries are from north of the equator and just 17.4% to the south.

Also, 54 of the 63 countries with 1,000-plus cases are from the northern hemisphere.

The skew gets bigger when one looks at the number of cases in the two hemispheres – 96.84% of the cases are from the north and just 3.16% from the south.

Further, the northern hemisphere accounts for 98% of the total number of deaths reported while the southern barely 2% (1,765 of 88,640).

This suggests that, by and large, there is a high correlation between the geographical location of a country and the severity of COVID-19 (in terms of number of cases and mortality rate). A classic example is a comparison between the United States and Australia – the world’s third-largest and sixth-largest countries. While the US has recorded more than 4 lakh cases and 14,000 deaths, Australia has just registered 6,104 cases and 51 deaths.

Of course, there are several factors responsible for the severity of COVID-19 in a country – effective measures of lockdown and social distancing, timing of international travel bans, percentage of old age population, etc. But among these, a key consideration seems to be the geography of the nation.

Experts differ on the role played by temperature and humidity and its relation with COVID-19 and there is no conclusive theory yet to make an absolute claim.

Yet, many credible studies suggest a definite linkage.

Francesco Ficetola, PhD in environmental sciences from the University of Milan, says, “A climatic effect is very likely, for instance because during cold periods (winter) we are more susceptible to infections of the respiratory system. Differences in mortality are also probably related to susceptibility.”

This explains the rapid spread of the virus in the northern hemisphere during February-March – the peak winter season for them. As a corollary, it also explains the less severe effect of the virus in the southern hemisphere where it was summer.

Dr Ficetola, and Diego Rubolini, associate professor of ecology at the University of Milan, wrote a comprehensive paper titled Climate Affects Global Patterns Of COVID-19 Early Outbreak Dynamics, in which they highlighted the role of temperature and humidity in the spread of the virus. Their study revealed how the COVID-19 growth rates peaked in temperate regions (north of the Tropic of Cancer) of the northern hemisphere with mean temperature of 5 degrees Celsius and humidity 0.6-1.0 kilopascal (kPa) while they decreased in the warmer regions.

This explains why 10 of the most severely affected countries (in terms of a combination of number of cases and mortality rate) all lie between 32 degrees north and 55 degrees north latitude range – Italy, US, Spain, France, UK, Iran, China, Germany, Netherlands and Belgium.

India lies between 8°4' to 37°6' north latitude, which means most of the country is outside this latitude range.

The study further claims that COVID-19 could be a seasonal virus which peaks during winter. This theory is backed by many acclaimed voices including Dr Anthony Fauci, director of the National Institute of Allergy and Infectious diseases, US, who further adds that there is a substantial increase in the number of cases in the southern hemisphere as countries there head into their winter.

Dr William Schaffner, an infectious disease expert from Vanderbilt University Medical Center in Tennessee, adds that the virus could get into a cycle and return to the northern hemisphere in the next winter season.

Another related and interesting theory is the correlation between the sun’s ultraviolet rays and extent of spread of COVID-19. A study conducted in the last week of March showed how Iceland had a very high rate of per capita infections – located at 64.1 degrees north, the country receives fewer hours of sunshine and less solar intensity than cities farther south. This pattern is likely to reverse and we could see a sharp rise in cases in the temperate regions of the southern hemisphere post April.