Considerable attention was paid in the early days of the Covid-19 pandemic to its spatial distribution in the hope that environmental factors might be found to play a key role in influencing its spread in two ways: by restricting it to a narrow band of countries with specific environmental factors; and hoping that a rise in temperature in the summer would kill it off.
- Researchers at Maryland University (Sajadi, M.M. et al., 2020) thus used maps of the early stages of Covid-19 to suggest that it spreads more easily in cold, damp climates, and that its highest incidence would be between latitudes 30-50 N. At the time, I suggested on 3rd April that there were too many anomalies for this to be valid, that it was only based on limited data (where the coronavirus had spread by early March 2020) and that it was necessary to understand better the actual physical processes involved. However, the idea that there might be environmental factors that will control Covid-19 still persists.
- Likewise, in the early days of the pandemic there was much optimism that the new coronavirus might act in similar ways to some of its predecessors and be seasonal in character, waning in the summer months when it gets warmer. Again, this was in part based on the timing of its outbreak (in China in December 2019 ) and its rapid spread through Europe with an approximately similar timing to seasonal flu. However, many experts were cautious about this possible scenario (see Jon Cohen in Science, 13th March 2020, and Alvin Powell in the Harvard Gazette, 14th April 2020).
Nevertheless, the much more rapid spread of Covid-19 in Europe and North America than in Africa and South Asia has led some to continue to argue that the devastating impact of lockdown in countries nearer the equator, particularly on the lives of some of the poorest people living there, may be un-necessary if this pattern can indeed be explained by environmental factors. The lockdown has already been partially rolled back, for example, in countries such as Pakistan (with some factories reopening on 12th April , and congregational prayers at mosques durong Ramadan being permitted from 21st April) and South Africa (with initial steps being taken to reopen the economy on 1st May). Clearly, the rate and distribution of the spread of Covid-19 is influenced by many factors, including government policies (with the UK performing especially badly, see my recent post), demographic characteristics (with the elderly being particularly vulnerable), population distribution (spreading slower in sparsely settled areas), characteristics of the several strains and mutations of the Sars-Cov-2 coronavirus (summary in EMCrit), and the inaccuracy and unreliability of reported data about infections and deaths (see my comments here).
The role of environmental factors remains uncertain, despite a considerable amount of research (see systematic review by Mecenas, P. et al., 2020 – thanks to Serge Stinckwich for sharing this) which has sought to draw conclusions from the distribution of cases in parts of the world with different climates, and has suggested that cold and dry conditions helped the spread of the virus whereas warm and wet climates seem to reduce its spread. A more recent study by Jüni et al. (8th May 2020) has claimed that epidemic growth has little or no association with latitude and temperature, although it has weak negative associations with relative and absolute humidity. Unfortunately, very few studies have yet sought to do experimental research that actually measures the survivability and ease of spread of Sars-Cov-2 under different real-world environmental conditions. Moreover, if as appears likely, most infections actually occur indoors, it is not the external climatic conditions that will influence rates of infection but rather the artifical environments created indoors through heating and ventlaltion systems that will be of most significance in influencing its spread.
Two related approaches to this challenge are necessary: identifying its survivabililty in a range of different environments (and surfaces), and analysis of the effect of different environments on the distance that it can be spread by infected people.
Research on the survivability of Sars-Cov-2 in different contexts
Several reported studies have explored the stability of the new coronavirus on different surfaces. In a widely cited study, van Doremalen et al. (13th March 2020) suggested that the stability of HCov-19 (Sars-Cov-2) was very similar to that of Sars-Cov-1 (the SARS outbreak in 2003), and that viable virus could be detected as follows:
- in aerosols up to 3 hours after aerosolization
- up to 4 hours on copper
- up to 24 hours on cardboard and up to 47-72 hours on plastic and stainless steel.
This important study has subsequently been used as the standard estimate for the survivability of the coronavirus. However, it was undertaken in the USA under very specific relatively humidity (for aerosols at 65%; for surfaces at 40%) and temperature conditions (for both at 21-23o C) (See also more recently, van Doremalen et al. 16 April 2020). A rapid expert review of Sars-Cov-2’s survivability under different conditions (Fineberg, 7th April 2020) notes that the number of experimental studies remains small, but that elevated temperatures seem to reduce its survivability, and that this varies for diffferent materials. Nevertheless, Fineberg emphasises that laboratory conditions do not necessarily accurately reflect real-world conditions. In referrring to natural history studies, he also emphasises, as noted above, that conflicting results have emerged because such studies are “hampered by poor quaity data, confounding factors, and insufficient time since the beginning of the pandemix from which to draw conclusions” (p.4).
If a better understanding of Sars-Cov-2’s survivability in different parts of the world is to be gained, it is therefore essential urgently to undertake real world studies of its viability on similar surfaces in various places with different temperature and humidity profiles.
The dispersal distance of Sars-Cov-2
The standard advice across many countries of the world is that people should maintain a minimum distance of 2 m (in some countries 1.5 m) between each other to limit the spread of Covid-19 (see, for example, Public Health England). This is double the WHO’s advice for the public, which is to “Maintain at least 1 metre (3 feet) distance between yourself and others. Why? When someone coughs, sneezes, or speaks they spray small liquid droplets from their nose or mouth which may contain virus. If you are too close, you can breathe in the droplets, including the COVID-19 virus if the person has the disease“. The 2 m figure was adopted early by some CDCs, and appears to be more of an approximate early guess (based on the previous Sars-Cov-1 outbreak) that has taken root, rather than an accurate scientifically based figure.
Since then, more rigorous research has been undertaken, much of which suggests that 2 m may not be enough. Setti et al. (23rd April) thus note that Sars-Cov-2 has higher aerosol survivability than did its predecessor, and that a growing body of literature supports a view that “it is plausible that small particles containing the virus may diffuse in indoor environments covering distances up to 10 m from the emission sources”. They also conclude that “The inter-personal distance of 2 m can be reasonably considered as an effective protection only if everybody wears face masks in daily life activities”. A particularly interesting laboratory based study a month previously by Bourouiba (26th March 2020) provides strong evidence that the turbulent gas clouds formed by sneezes and coughs provide conditions that enable the coronavirus to survive for much longer at greater distances: “The locally moist and warm atmosphere within the turbulent gas cloud allows the contained droplets to evade evaporation for much longer than occurs with isolated droplets“. She concludes that the “gas cloud and its payload of pathogen-bearing droplets of all sizes can travel 23 to 27 feet (7-8 m)”. Furthermore, another study by Blocken et al. (9th April) noted that the 1.5 m – 2 m distance was based on people who were standing still, and that there could be a potential aerodynamic effect for people cycling and running. For someone running at 14.4 km/hr the social distance in the slipstream might be nearer 10 m.
Such studies have been controversial (for a summary, see Eric Niiler in Wired, 14th April), but they highlight that in practice:
- the “safe’ distance between people is unknown;
- there is little strong scientific evidence for the 1 m – 2 m recommendations for social distancing; and
- this distance is highly likely to vary in different environmental contexts.
No enough conclusive reseach has yet been undertaken on the extent to which environmental factors, such as humidity, pressure, altitude, wind and temperature actually affect how far Sars-Cov-2 will disperse, and at what infectious dose (see Linda Geddes, NewScientist, 27th March 2020, where viral load is also discussed; see also ECDC, 25th March 2020). It seems likely, though, that dispersal will indeed vary in different conditions, and thus in different parts of the world. We just don’t yet know how great such variability is.
Towards a research agenda
This post has emphasised that we actually know remarkably little with certainty about how Sars-Cov-2 physically survives and disperses in different environmental contexts. This has hugely important ramifications for the spread of Covid-19 in different parts of the world, and thus the mitigating policies and actions that need to be taken. If, for example, Covid-19 does not survive in hot humid conditions, and is also dispersed over shorter distances in such circumstances, then it might be possible for governments of countries where such conditions prevail not to have to impose such stringent social distancing requirements as those that have been put in place in Europe.
Urgent experimental research is therefore required in real-world environments on:
- the survivabililty of Sars-Cov-2 in a range of different physical environments (and surfaces), and
- the effects of different environments on the distance that it can be spread by infected people.
A standard protocol and methodology for such research should be created that could then be used collaboratively by scientists working in different parts of the world to address these crucial issues. Contrasting environments that would warrant the earliest such research (given the high number of economically poor countries therein) would include: high altitude savanna (as in the Bogotá savanna, and the much lower montane Savanna of the Angolan scarp), tropical and subtropical savanna (as in parts of Brazil and Kenya), tropical rainforests (as in Indonesia and Brazil), semi-arid and arid landscapes (as in much of northern and south-west Africa, the Arabian peninsula, and parts of South Asia). It is also very important to undertake such resaerch both in urban and rural areas, and indoors as well as outside. If scientists can indeed co-operate to provide a swift answer to the questions raised in this post, then it would be possible to provide much more tailored advice to governments concerning the mitigating measures (including the use of masks) that they should be taking to protect the highest number of people while also maintaing essential economic activity.
[Updated 8th May, 12th May and 30th May 2020]