TORONTO -- A new study looking at countries in the Americas in the first wave of the pandemic has found a correlation between higher altitudes and lower incidences of COVID-19.
The results were published Tuesday in the journal PLOS One, and describe how regions that are more than 1,000 metres above sea level seemed to have a lower rate of COVID-19 than regions at lower altitudes, even when taking into account population density.
The study focused on 23 countries in the Americas, including the U.S., Canada, Bolivia, Brazil, Colombia, French Guyana, Panama, Argentina, Belize, Chile, Costa Rica, Cuba, Ecuador, El Salvador, Haiti, Honduras, Mexico, Paraguay, Peru, Puerto Rico, Dominican Republic, Uruguay and Venezuela.
Researchers only looked at epidemiological data from these countries up to May 23, 2020, meaning the data is focused on the first few months of the pandemic only.
Data regarding the number of cases in different cities or regions within these countries was collected through publicly available data posted by local governments. Researchers then cross-referenced these case numbers with the different altitudes of the cities they’d looked at.
Under 1,000 metres above sea level, there was no real correlation with COVID-19 case levels, the study found. But once the altitude of cities and communities passed 1,000 metres above sea level, the correlation only grew stronger with increased altitude — meaning that the higher the altitude was, the fewer cases there were.
“Our results show that a clear turning point in the incidence of the disease occurs at 1,000 masl,” the research stated.
It’s important to note that there is no suggestion in this research that altitude in of itself provides a protective effect. The mechanisms behind this correlation would require further research to understand.
Only 14 countries out of the original 23 had cities or regions with COVID-19 cases reported above 1,000 metres above sea level. Researchers found that “significant negative correlations” — lower case rates in higher places — were found in nine of those 14 countries, including Canada and the U.S.
While the study itself did not expand on which cities in Canada it looked at, numerous communities in Alberta and B.C., including Calgary, Banff, Canmore, Kimberley and Elkford, are higher than 1,000 metres above sea level. However, Calgary is the only major city in Canada higher than 1,000 metres above sea level, and its elevation is between 1,045 - 1,083 metres above sea level, making the correlation less significant in Canada.
Because different countries have had such vastly different strategies to tackle COVID-19, researchers also randomized the data and normalized it by population density to double check regions located higher than 1,000 metres above sea level, and still found the same results.
In addition, researchers took a closer look at five countries — Argentina, Bolivia, Colombia, Ecuador and Peru — in order to asses transmissibility and severity of the coronavirus at higher versus lower altitudes.
Using an epidemiological model called SEIR (Susceptible-Exposed-Infectious-Removed), they estimated transmissibility in different regions and observed that lowlands (below 1,000 metres above sea level) had a slightly higher possibility of COVID-19 transmission compared to highlands (higher than 1,000 metres above sea level).
The largest difference was in Peru, which had a 3.9 per cent chance of transmission in lowlands versus a 2.7 per cent chance in highlands.
In order to measure the severity of the virus in specific regions, researchers looked at the percentage of patients who recovered, as well as the mortality rate.
“A lower mortality rate per case and a higher percentage of recovered patients suggest a lower severity of the disease,” the research pointed out.
Researchers wrote that they “found a significantly higher percentage of recovered patients in the highlands.”
This research builds on other work by this team, which previously theorized that COVID-19 incidence rates would be significantly reduced higher than 2,500 metres above sea level. Other studies have also looked into this phenomenon, including a study published in January that focused on the U.S. only and found that per capita deaths due to COVID-19 were significantly lower at high elevations compared to lower ones.
Researchers in this recent study note that their exact figures are based on theoretical models, and “should be interpreted carefully.”
This study doesn’t mean we should all go running for the literal hills, but researchers believe it supports more research into this correlation.
In its conclusions, the study suggested that the physiology of those who live at higher altitudes might play a part, adding that this is a theory they specifically believe warrants more research.
