COVID-19 is a highly infectious, novel disease that has swiftly infected populations globally. Based on our study, there appears to be a negative relationship between temperature/absolute humidity and the number of cases. The higher the temperature and absolute humidity, the lower the case numbers. Furthermore, health infrastructure, such as critical care and ICU beds, was significantly correlated with mortality. We found an unexpected relationship between increased deaths and increased critical care/ICU beds. This could be explained by testing and treatment disparities between countries. High-income countries are more likely to have a greater quantity of ICU/critical care beds and, therefore, have the ability to test and care for more severe patients.(16) As the patients cared for in the ICU are critically ill, we would expect a higher mortality rate. Similarly, many of the ICU’s in high-income countries are overcapacity, which could further result in increased reported mortality rates.(28) As we begin to provide more tests and learn how to better manage patients with COVID-19, this relationship is expected to invert.
Given our more robust data from the start of the outbreak to late March 2020, it’s likely that the SARS-CoV-2 transmits more readily in colder and dry climates. The implications of this effect still need to be studied. Though temperature and humidity may be a factor in COVID-19 transmission, it is not as major of a determinant such as human behavior and hygiene. Despite the significance being found when comparing countries, the relationship was not as apparent when looking specifically at the US. One factor to consider is that there may still be a lag time in testing, especially in low-and-middle-income countries. Many of the countries within the African continent are only recently starting to have confirmed cases. Therefore, the true effect of temperature and humidity on COVID-19 will only be revealed over time.
Prior studies have used preliminary, regional data that shows a slight association between temperature and humidity and cases of COVID-19. Similar to influenza, COVID-19 seems to spread more readily in colder and drier climates.(29-31) This effect has been seen on the coronavirus’ survivability on surfaces as well.(32, 33) It is hypothesized that temperature and humidity not only affect viral transmissibility through respiratory droplets, but that the lower temperatures alone can result in systematically worse immune systems.(34, 35)
There has also been contradictory evidence that the effect of temperature and humidity is minimal to none. The 2003 Severe Acute Respiratory Syndrome (SARS) outbreak dissipation has been credited due to warmer weather.(5) However, the success in reducing SARS cases was really attributable to increased intensive precautions and public health measures.(36) Regardless of the effects of meteorological conditions, it’s unlikely that there will be a significant decrease in COVID-19 cases this coming summer due to temperature or humidity changes alone. As the coronavirus is a novel disease, there is no predictability as to how it will behave, particularly when the entire world population lacks any immunity. At this point, the best response to thwart the virus is to ensure continual isolation, prompt identification of cases and contacts, strict quarantines, travel restrictions, “social distancing,” hand and general hygiene, and other intensive efforts rather than relying on a change in season.(37, 38)
After monitoring the COVID-19 outbreak in Western Europe, further consideration must be given to the preparedness of the US healthcare system. The continued growth of cases is in large part due to the overload of the healthcare infrastructure.(39) The US is already experiencing strain on its limited critical care facilities with mechanical ventilators and a shortage of personal protective equipment (PPE).(40, 41) Healthcare professionals without easy access to PPE have resorted to either reusing/substituting masks or being left unprotected.(42) Given that the US is facing the COVID-19 challenge at a later stage of the pandemic, the global reserves of PPE have largely been depleted.(43) This PPE shortage is further exacerbated by public hysteria and hoarding of masks.(44) If precautionary measures do not improve, the risk to frontline providers will increase and healthcare capacity will dwindle.
In a time of high connectivity and globalization, the spread of COVID-19 is unprecedented. COVID-19 is more infectious than SARS, with a basic reproduction number (R0) between 2.2 and 6.7.(45-48) Even though quarantines and curfews have been instituted, many are unenforceable and allow for the flexibility for both domestic and international travel.(49) Understandably, a true shut down would be too difficult given the monumental financial and social consequences. Supply chain disruptions have already resulted in loss of revenue and supply shortages for businesses and cities as well as major disruptions to the global economy.(50)
A limitation of this study is that the temperature and humidity can vary greatly across a state or country. For ease of analysis, this study generalized the capital city’s temperature and humidity for the entire state and country. Therefore, the true effect of temperature and humidity on COVID-19 may be different. Additionally, it’s difficult to account for the variations in human behaviors and country restrictions in response to this outbreak as people may or may not be participating in the recommended public health guidelines. Lastly, we were unable to obtain data on testing practices and the number of ICU/critical care beds from all affected countries.
As we are still in the early stages of the pandemic, it’s hard to predict how the coronavirus will resolve. The hope is that with summer soon approaching, the global community beginning to enforce more stringent public health precautions, and the use of promising pharmacological interventions, we will begin to curb the transmission of COVID-19.