In laboratories viruses are stored and saved in liquid nitrogen, at a temperature of -196 degree Celsius. Laboratory experiments have shown that viruses, including the coronaviruses, do not survive well on surfaces when the temperature and relative humidity are high. At temperatures of 4 degree Celsius and low relative humidity, viruses could last for a month or longer. Reports of the outbreak of COVID-19 among meat packing factory workers in China and elsewhere, where temperature is close to 4℃, therefore is not unexpected. Shouting above the level of the sound of a running machine by the workers and also the way they live also facilitate transmission. Flu and four milder forms of coronaviruses also flourish and cause illness in winter. Huddling by people in winter and spending more time indoors, more coughing and sneezing on each other also have been incriminated. Also, people walk less, ride open rickshaws, motorbikes and ply in congested vehicles more, e.g. buses, which also have been pointed as a source of higher level of transmission of pathogens in winter. Production of less vitamin D which strengthens immunity, as people expose less to the sun due to gloomy weather and availability of less sunlight in winter are a cause of the severity of infections.
Cold and relative humidity are usually lower inside homes. Flu is transmitted more easily in cold, dry weather- inside homes. It is thought that since SARS-CoV-2 has a similar size and structure to that of the flu virus, it may also get transmitted in the same way. In controlled conditions the effect of a cold weather however, may be dampened. Respiratory viruses including the respiratory syncytial virus (RSV) are generally seasonal, probably as viruses that transmit on water droplets do so less well if the droplet dries up faster, due to higher temperature, humidity and ultraviolet (UV) light.Experience from Australia suggests that low humidity might be a more important factor as a risk of increase in COVID-19 than temperature. However, in Melbourne, there was a large outbreak in July coinciding with a spell of cold weather. This led to a strict lockdown, although the situation came under control only in October when the weather started getting hotter. On the other hand, in New Zealand, in the cold weather during July and August, the spread of SARS-CoV-2 was in control, due to an effective healthcare system. Let us also note that face masks are not only filters, these also keep the nose warm, moist and humid, which are lethal to the virus.
Winter is expected to drive indoors the outdoor socialisers, heightening the risk of spreading COVID-19 among family members. Indoor is much more risky than outdoor in winter, since outdoor environment allows air to flow more freely, thereby diluting it, resulting in more per head air volume, with a chance to dilute the viral concentration below the infectious dose. This scope is not available indoors in the winter, as doors and windows remain shut quite tight.
Although the Bangladesh Government and the World Health Organisation advise people to keep a distance of one meter or three feet, termed as social distance, to avoid getting infected, the western countries practice a physical distance of at least six feet. Martin Bazant, a professor of chemical engineering and mathematics at MIT and his colleague John Bush, urge people however, to think “beyond six feet”. Models devised by them assume that pathogens like the SARS-CoV-2 virus are distributed uniformly throughout indoor rooms. “In such well-mixed spaces, one is no safer from airborne pathogens at 60 feet than 6 feet,” Bazant and Bush wrote.
Although even the Center for Disease Control & Prevention (CDC) of the USA does not lend support to the fact that SARS-CoV-2, like other respiratory diseases may also be spread through aerosols, in addition to droplets, Bazant suggests that “Aerosol transmission was previously implicated in the spread of SARS (Cov-1) and measles with less evidence than we have now for COVID-19.” In dry wintery weather this possibility assumes a more cardinal avenue of spread of the virus. While in warm and humid weather the exhaled droplet gets water from the environment, gets bigger and hence falls to the ground quicker. In the wintery dry weather just the opposite happens, i.e. the droplet evaporates, gets lighter and travels longer distances. In a cooler albeit humid environment, on the other hand, the virus travels distances of up to almost 20 feet before falling to the ground.
Studies have shown that infections spread more when the relative humidity falls from 40 per cent and 60 per cent to 20 per cent. This has also been seen in the past outbreaks of flu and MERS (caused by another coronavirus). Case reports on the SARS-CoV-2 pandemic’s early days in China and Seattle, USA observed that the virus stays stable longer and finds the receptors in the airways better suited to infect when the relative humidity is 20 per cent. This is also a reason why more colds and flu are reported in winter. In Hong Kong, COVID-19 cases were 18 times higher in lower temperatures, i.e. under 24.6°C.
Coronaviruses, including the one that causes COVID-19 will thrive in winter for several reasons, e.g. lower temperatures and relative humidity dries up the air that in turn dries up the respiratory tract while dryness in the nose hampers movement of the cilia (tiny hairy structure) in it, weakening the removal of mucus, debris and invaders like SARS-CoV-2 from the nose. Although relation has been established yet, there nevertheless is a theoretical possibility that after exhaling from an infected person, the virus starts disintegrating in warm external weather faster than in cold weather.A study noted that it was spring-like weather when the 2003 SARS-Cov-1 epidemic in China was at its peak. The epidemic died out in the dry weather of July in 2003. Effective public health measures were however, also in place. Regular outbreaks of MERS also were found to occur in the spring (March to May). Epidemiologists however, think that the outbreak of MERS could in fact be linked to the birth of newborn camels in March (camels are a natural source of human MERS).
While some studies in 17 cities in China found a link between the rise in humidity and reduction in COVID-19 cases some others did not find any relation of rise of COVID-19 cases with fall in temperature. For every gram per cubic meter (1 g/m3) increase in absolute humidity, the cases reduced by 67 per cent, after a lag of 14 days between the humidity increase and the number of cases. Association between the number of cases and humidity was also observed in Australia, Spain and in the Middle East for both cases and deaths. On the other hand, researchers reported no effect of temperature itself on COVID-19 transmission or deaths in Australia, Iran and Spain. Higher temperature was however, found to be associated with a lower number of cases in Turkey, Mexico, Brazil, and the U.S.A. Though threshold level of temperature was seen on the number of COVID-19 cases, beyond which any further rise in temperature was not seen to cause further decline in COVID-19 transmission.
In laboratory studies SARS-CoV-2 virus was found to be remarkably unstable at temperatures above 98.6°F (37°C). On the other hand, sunshine was not linked with the number of infections. The ultraviolet (UV) wavelength required to kill viruses and bacteria should be under 280 nanometers (nm). Only the type C of UV has a wavelength between 100 and 280. However, short-wavelength UVC is completely filtered by the atmosphere and does not reach the earth's surface. The medium range UVB (280-315 nm) shows some minor effects and probably explains the findings of lower transmission of COVID-19 in cold and dry conditions at a higher altitude. Other conditions, e.g. expression of cardiovascular diseases, are more common in cold weather. Another explanation of a higher number of COVID-19 cases in winter may be due to a higher reporting of patients to hospitals with medical conditions, who are in turn more likely to be tested for COVID-19 and further identification of the cases.
Superspreader events have been traced to places frequented by people for extended periods, e.g. clubs, gymnasiums, markets, etc. irrespective of the weather. Similarly adoption of protective measures and devices, as we know, would reduce transmission irrespective of the weather.
The author is an epidemiologist, member of the technical committee of the Bangladesh Health Watch and Honorary CEO of the Social Sector Management Foundation (SSMF)