There are now some early hints that COVID-19 may vary with the seasons. An unpublished analysis comparing the weather in 500 locations around the world where there have been COVID-19 cases seems to suggest a link between the spread of the virus and temperature, wind speed and relative humidity. The study has also shown higher temperatures are linked to lower incidence of COVID-19, but also says that temperature alone cannot account for the global variation in incidence.
The coronavirus belongs to the family of so-called “enveloped viruses”, which are coated in an oily coat known as a lipid bi-layer. These viruses are studded with proteins that stick out like spikes of a crown which gives them the name – Corona is Latin for crown. Recent research on other enveloped viruses had found that this oily coat makes the viruses more susceptible to heat than those that do not have one. In colder conditions, the oily coat hardens into a rubber-like state to protect the virus and give it a longer life outside the body, in warmer zones it just melts down, exposing and destroying the virus.
Scientists from the Massachusetts Institute of Technology (MIT) in the USA have accessed data on a number of COVID-19 infections from different parts of the world and compared it with parameters of temperature and humidity. Their findings suggest that warm and humid weather is linked to slower spread of the virus. Accordingly, Asian countries experiencing monsoon may see lesser transmission of the virus. 90 per cent of the Novel Corona virus until March 28 has occurred in regions with temperature between 3 and 17 degrees Celsius.
Iran, which has accounted for about 90 per cent of the Corona virus cases in the Middle East, is unique as it sits on a plateau where winter conditions resemble those of more northerly countries. At the same time, many Southeast Asian nations with close business and tourism links to China have seen surprisingly few cases, even if we assume that their less developed public health systems are not properly detecting them. Thailand, Indonesia and the Philippines have each seen fewer cases than Estonia, Slovenia or Iceland, despite a combined population more than 100 times as large.
Temperature cannot be discounted as being a very important factor in how counter measures should be drawn for the future containment of the virus. Nations with lower temperatures and lesser humidity create a better environment for the spread of the virus, whereas nations with warmer weather may have a better chance of containment because of this factor.
In regar to temperature, the coronavirus can remain intact at 4 to 10 degree Celsius for a longer period of time, with the optimal temperature for the spread being 8.72 degree Celsius. At around 23–26 degree Celsius, the virus is no longer efficiently transmitted, but at 30 degree Celsius, it gets inactivated. In India and other nations with warm summers, the virus could be challenged at temperatures of 35 degrees Celsius and more.
Two scientists, Dr Malik Peiris and Dr Seto Wing Hong from the Hong Kong University have proved that that low temperatures and low relative humidity had allowed the SARS virus, which was a corona virus too, to survive much longer than they did in high temperatures and humidity. The HKU team argued that this was the reason why warm and humid Southeast Asian countries did not have SARS outbreaks, unlike Hong Kong and Singapore where there was intensive use of air-conditioning.
Another intriguing study by scientists in China too suggests the relationship between how deadly COVID-19 can be and the weather conditions. They analysed 2,300 deaths in Wuhan, and compared them to the humidity, temperature and pollution levels on the day they occurred. Although it has yet to be published in an academic journal, their research suggests mortality rates were lower on days when the humidity levels and temperatures were higher. Their analysis also suggests that on days where the maximum and minimum temperature ranges were greater; there were higher levels of mortality. But this work is largely also based on computer modeling, so the exact nature of this relationship, and whether it will be seen in other parts of the world, is still to be explored.
Dr Alan Evangelista, a microbiology and virology professor at St Christopher’s Hospital for Children in Philadelphia found results similar to the other studies. His research showed that as humidity increases, the viral droplet size becomes larger and settles out of the air rapidly. In contrast, in low humidity, there is rapid evaporation of respiratory droplets and they remain airborne for prolonged periods, increasing the time and distance over which transmission can occur.
University of Utah physicists Saveez Saffarian and Michael Vershinin are conducting studies on how the virus's protective outer shell responds to changes in heat and humidity. According to the duo, “Viruses are not able to do anything on their own, as they are simply shells with genetic instructions tucked inside; when a virus invades a host's cells, it uses that cell's machinery to replicate itself, over and over again.
What we know is that the droplets are better at staying afloat when the air is cold and dry, when the air is humid and warm, they fall to the ground more quickly, making transmission harder."
Dr Mohammad Sajadi, an associate professor of medicine at the University of Maryland, too thinks weather might play a role.
Elizabeth McGraw, Director of the Center for Infectious Disease Dynamics at Pennsylvania State University, explained to the Time magazine: “The droplets that carry viruses do not stay suspended in humid air as long, and the warmer temperatures lead to more rapid virus degradation.” Recent comments attributed to Professor John Nicholls from the University of Hong Kong’s Department of Pathology suggests that weather conditions will be a key factor in the demise of the coronavirus.
The 'Global Virus Network ' has predicted that weather modeling can explain spread of COVID-19. Their observation is that the spread of Covid 19 is along a narrow corridor of 30-50” Latitudes at consistently similar weather conditions of 5 to 11 degree Celsius and 47 per cent to 79 per cent humidity. They also suggested that a temperature rise of 12 degrees Celsius or higher, the viral transmission may be difficult.