Effect of Latitude on COVID-19

April 27, 2020

Carl  Heneghan, Tom Jefferson,

We recently conducted a rapid review of evidence to explore if climate conditions may influence the spread of SARS-CoV-2.

The evidence suggests that weather conditions may influence the transmission, with cold and dry conditions appearing to boost the spread. 

To look at this further and try and explain why some countries are disproportionately affected (see Six Countries: Three-Quarters of the COVID-19 Deaths) we analysed the effect of latitude on global deaths and cases per million in the Northern Hemisphere and globally.

To do this we download the country data from Worldometer and information on country Latitude. We included 128 countries with > 5 reported  COVID deaths and then plotted the country-specific latitude versus deaths and cases per million. We used a logarithmic trend line to assess the association and R2 value to estimate the fit.

Update of data 29th April

Latitude vs. Deaths/Million Population in the Northern Hemisphere

Latitude vs. Cases/Million Population  in the Northern Hemisphere

Number of Deaths/Million Population Globally

Latitude vs. Cases/Million Population  Globally.

What does this mean?

As of the 28th April, there is an association across 128 countries with latitude and COVID deaths and cases per million population.

  • Death rates (R2 0.30 Northern Hemisphere, 0.20 Globally)
  • Case rates (R2 =0.35, Northern Hemisphere, R2 =0.23 Globally). 

These have not changed since the 26th of April when we last updated  the data on deaths the

  • Death rates (R2 0.29 Northern Hemisphere, 0,21 Globally)
  • Case rates (R2 =0.35, Northern Hemisphere, R2 =0.23 Globally). 

This gradient relationship can also be observed within the same country such as Italy, where the North of the country is more impacted than the South, but not in others. For example, we did not observe an association in America when we analysed deaths and cases by the latitude of the individual states. 

In the previous SARs outbreak, the risk of increased daily incidence of SARS in Hong Kong was higher in days with a lower air temperature than in days with a higher temperature. Lower temperatures and lower humidity were thought  to enable the prolonged survival of SARs on surfaces. 

The lack of viral activity in countries with high temperature and high relative humidity might explain why they do not have major community outbreaks of SARS, and why they have found it easier to manage the SARs-CoV-2 outbreak. 

However, high use of air-conditioning in hospitals or hotels (e.g., Singapore and Hong Kong) facilitated SARs transmission in the 2003 outbreak, and its impact should, therefore, be urgently investigated.

This work has several limitations: the outbreak is not over, the collection and reporting of data from different countries are highly variable and subject to errors. The detection of cases is dependent on the testing regime applied, which varies across countries. Furthermore, countries’ response to the outbreak has differed, which will have affected the rates of cases and deaths.  

The relationship to seasonality and evidence of an association with weather conditions, the concentration of COVID’s impact in a small number of countries and the association with latitude provides evidence that environmental factors impact on the transmission of SARs-CoV-2. 

Other environmental factors were shown to be associated with the spread of SARs in 2oo3 included wind velocity, daily sunlight, and air pressure should be considered along with environmental factors such as air pollution and density.  These factors warrant urgent investigation and verification and should be considered when interpreting what may happen next.

See also: 

COVID 19 – The Widow of Hampstead Revisited

COVID-19 deaths compared with “Swine Flu

Six countries: Three-quarters of the COVID Deaths

COVID-19 Global Charts of Deaths

Disclaimer: This article has not been peer-reviewed; it should not replace individual clinical judgement and the sources cited should be checked.  The views expressed in this commentary represent the views of the authors and not necessarily those of the host institution, the NHS, the NIHR, or the Department of Health and Social Care.  The views are not a substitute for professional medical advice.

Tom Jefferson is an Epidemiologist.  Disclosure statement is here

Carl Heneghan is Professor of Evidence-Based Medicine, Director of the Centre for Evidence-Based Medicine and Director of Studies for the Evidence-Based Health Care Programme. (Full bio and disclosure statement here)