Humans, viruses and climate

Finally some of our natural priorities are coming into focus, namely well-being and survival of humans. Unfortunately, first we have to face new, unknown danger to remember basic facts. Nowadays, everyone in Europe is invoking higher temperatures which should hopefully end the current epidemics (1). Computer models predicting future climate may eventually be useful, but for here and now there are still some lessons we should learn and remember from the past. I would like to add some points to the discussion about humans and climate. 

Somehow, the results from Gasparrini et al. (2) are being somewhat neglected. The authors investigated over 74 million deaths in the period between 1985 and 2012, of which 7.71% were attributed to non-optimum temperatures. Only 11.1% were attributable to combined extreme cold and hot temperatures (0.86% of the total number). I rearranged the order of countries in the table, from hottest to coldest, and got the next graph.

temp – mean annual temperature (oC), tr – temperature related deaths (%), c – „cold“ deaths (%), w – „warm“ deaths (%)

The following graph shows the ratio of „cold“ to „warm“ death cases.

There are some conclusions that can be made:

1) Higher annual mean temperatures cause less temperature related deaths

2) Three to thirty times more people die from colder weather (temperatures) than average

3) Extreme hot and cold temperatures make a small fraction of the total number and are not situations to be concerned about

China and USA included locations with more heterogenous climate. Even in tropical and subtropical countries (Thailand, Brazil and Taiwan) there are far more deaths from temperatures lower than usual and Canadians are a special sort of people adapted to whatever nature throws at them. There is another medical research paper that agrees well with the global data analysis in the above mentioned research. Almeida et al. (3) analyzed seasonal dynamics of influenza in Brazil for the period 2010-2016 and correlated incidence number with latitude. The authors divided 27 Brazilian states in two groups; states with and without seasonal influenza pattern.

Number of influenza cases in horizontal, Latitude in vertical (0 is the equator), results for states with annual seasonality in coral, and non seasonal in gray

For both data sets there is a simple enough relationship. Influenza incidence in states with seasonal pattern is less than one in hundred thousand inhabitants near equator and over twenty at 30o southern latitude. Similar relationship is valid in the states with non seasonal occurence.    One can translate latitudes into temperatures. Two simple examples are Boa Vista, a town near equator with annual mean temperature of 27.2 oC, and Porto Alegre which lies exactly on 30 o S with the annual mean value of 19.5 oC.

Map of Brazil, B-Boa Vista, P-Porto Alegre

The results of both papers show clear negative corelation of mean annual temperature and related number of death cases or influenza incidence. Of course, there are other factors that play role in the above statistics, but still the general rule applies. Less pronounced differences in season temperatures and warmer climate have beneficial effect on humans regarding virus diseases.

There are more theories about the winter virus outbreaks. One of them points to the lack of sunlight hours and related lower levels of vitamin D and melatonin (4) thus compromising human defense mechanism. There is a lesson from the past that we tend to forget. In her research E. Oster (5) studied the relationship between weather conditions and trials for witchcraft and found that up to one million people were executed for crimes of witchcraft between the thirteenth and nineteenth centuries. When compared to mean temperatures, there is clear negative correlation presented in the next graph, meaning that lower temperatures were associated with more trials.

From Oster, 2004

The period presented above is what we call Little Ice Age and was an era of low economic growth, hunger and high mortality rates from hypothermia and lack of food. Someone had to be guilty for all the misfortunes humankind experienced.

Generally, I wanted to show that humans are creatures who (like most of living things) like warm and not cold weather conditions so, if there is some warming, we should be enjoying it and not be afraid of it.

sources:

1) https://www.newscientist.com/article/2233249-will-the-covid-19-coronavirus-outbreak-die-out-in-the-summers-heat/

2) Gasparrini, A. et al.: Mortality risk attributable to high and low ambient temperature: a multicountry observational study, The Lancet, Volume 386, ISSUE 9991, P369-375, July 25, 2015

3) Almeida, A. et al.: Seasonal dynamics of influenza in Brazil: the latitude effect, BMC Infectious Diseases, 2018; 18: 695.

4) http://sitn.hms.harvard.edu/flash/2014/the-reason-for-the-season-why-flu-strikes-in-winter/

5) Oster, Emily. 2004. “Witchcraft, Weather and Economic Growth in Renaissance Europe .” Journal of Economic Perspectives, 18 (1): 215-228.)

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