A new study provided insights into the effects of environmental factors on SARS-CoV-2, the virus of COVID-19. Researchers determined that warmer temperatures could influence the pathogen, however, the most likely outcome would be a small reduction in new cases.
The effect of warmer temperatures on the spread of COVID-19 was investigated by researchers at Mount Auburn Hospital, a teaching hospital of Harvard Medical School. Their findings confirmed that warmer temperatures could slow down the transmission, something many would look forward to. But the decline in the spread would not be enough to substantially affect numbers. This means the lax in strict health protocols during summer could still lead to more confirmed cases. The results of the study were published in the journal Clinical Infectious Diseases.
Environmental Factors and Viruses
Viruses are some of the greatest mysteries on Earth. This is because viruses are not considered alive or dead. They are particles that exist in the limbo of life and death of living organisms. Outside a host cell, a virus is considered dormant and exists as a virion, an independent particle. Once a virion successfully hijacks a host cell, it becomes a virus and begins to replicate by using the cellular engines of its host. Despite the threat of viruses, each type can only infect a very specific target. For example, bacteriophages can only attack bacteria. Each strain of bacteriophage is restricted to a particular bacterial strain.
Since virions are dormant outside host cells, it is critical to know how to inactivate them before an opportunity for an infection arises. Inactivated viruses are particles that have lost their ability to infect a host cell. In the case of COVID-19, methods to inactivate SARS-CoV-2 can help control the transmission.
According to the World Health Organization (WHO), a specialized agency of the United Nations, the stability and resistance of SARS-CoV-1, the virus of the 2003 SARS, were compiled by major institutions in China, Hong Kong, Germany, and Japan. Scientific evidence showed that the virus is stable in urine and feces at room temperature for one to two days, at least. If the fecal matter is from a patient with diarrhea, the stability is up to four days because of the higher pH level in the stool.
When it comes to temperature, tests confirmed that a reduction in the load of SARS-CoV-1 is minimal at four degrees Celsius and minus 80 degrees Celsius after 21 days. This means colder temperatures are ineffective at inactivating the virus. At room temperature, the viral load may be reduced by one log for two days. But the temperature with the most impact to the viral load is at 56 degrees Celsius, enough to kill SARS-CoV-1 at 10,000 units per 15 minutes. For disinfectants, the infectivity of the virus will wither upon exposure to commonly used disinfectants.
Temperature and SARS-CoV-2
Typically, viruses are prone to warmer temperatures. Most of these microbes lack the protection to withstand the summer heat and high levels of humidity. The microbes normally lose infectivity and become non-threatening to humans and other living organisms. However, the infectivity of SARS-CoV-2 when exposed to warmer temperatures remains a puzzle for health and environmental experts. If the temperatures to disable the virus are identified, it can help these experts predict patterns of future outbreaks.
At Mount Auburn Hospital, researchers conducted a study to determine if warmer temperatures, precipitation, and ultraviolet light could inactive or kill the novel coronavirus. Their insights would be useful in forecasting transmission rates in various cities worldwide. While high temperatures could reduce the incidence rate, the team found such an external force ineffective in decreasing the risk of transmission. Even in summer, the virus could still be spread in human communities.
"To the best of our knowledge, this is probably one of the first peer-reviewed studies that examine the influence that temperature, precipitation, and UV light have in terms of virus transmission in the general population across the United States," said Dr. Shiv T. Sehra, first author of the study and director of the Internal Medicine Residency Program at Mount Auburn Hospital.
In the study, researchers examined the data of environmental effects and their relationship with COVID-19 cases in the US. Their data analysis covered all reported cases from January 22 to April 3, 2020, which were tracked by Johns Hopkins University. Estimates on temperature, precipitation, ultraviolet index were obtained from the National Centers for Environmental Information.
Results showed that the incidence of COVID-19 slowed down when the daily temperatures climbed to as high as 52 degrees Fahrenheit, about 11 degrees Celsius. But beyond that temperature, any increase was no longer impactful on the transmission rate. The UV index was identified and correlated to the reduced rate of new cases, though, its impact on the overall situation remained moderate. Finally, precipitation was not detected as an influential factor in the transmission. Precipitation was not observed as a factor in the increase or decrease of COVID-19 cases.
After that examination, researchers conducted a series of experiments to model different scenarios involving various temperatures. Five scenarios were modeled that covered temperatures of less than 30 degrees, between 30 and 40 degrees, between 40 and 50 degrees, between 50 and 60 degrees, and more than 60 degrees Fahrenheit. Based on their model, the lowest rate of new cases was observed in days with temperatures higher than 50 degrees Fahrenheit, while the highest rate of cases was found in days with temperatures below 30 degrees Fahrenheit, about minus one degree Celsius.
Researchers concluded that their results are aligned to the predictions of the US Centers for Disease Control and Prevention, in which cases may worsen during colder seasons. However, they clarified that there are several limitations and considerations in their research. One of the major limitations was the temperature applied, wherein the maximum daily temperature did not exceed 70 degrees Fahrenheit or 21 degrees Celsius. During summer, the US usually experience temperatures of more than 75 degrees Fahrenheit or over 23 degrees Celsius.
As of June 1, 2020, the total global cases of COVID-19 reached at 6,057,853. While the total global deaths due to the pandemic peaked at 371,166. The Americas remained with the highest confirmed cases at 2,817,232, but Europe had the highest confirmed deaths at 180,592, according to the WHO.
If temperature and humidity truly influence SARS-CoV-2, it may explain why certain countries with warmer temperatures have lower cases or fewer outbreaks. The influence can be the much-needed break in affected cities from COVID-19. Regardless, people should not grow complacent just because the summer season may lower the incidence rate.