The study findings were published on a preprint server: Research Square, and is currently being peer reviewed.
We have only analyzed data from the initial wave of the pandemic. Various factors, including lockdown and travel restrictions, could affect the spread of infection. Nationwide lockdown introduced on the 23rd of March dramatically changed movement patterns in the UK.
Travel was permitted for key workers and for essential purposes (shopping for food within a 5-mile radius, exercise close to the house). Key workers could therefore contract and transmit the virus back to the areas they are travelling from. However, any such transmission is unlikely to affect downwind areas alone. Infections within care homes were a major problem during the first wave.
It was not possible to obtain reliable data on the number and distribution of care homes in different areas, so we are unable to account for this. Though we cannot rule out the possibility of a local outbreak just after a wind change, the probability of local outbreaks causing the observed changes in all the downwind regions investigated is small (see figures 1C, 2C, 3C, 4C and 4D).
Since changes to the testing policy and reporting definitions could affect the number of confirmed cases, data for all analysis were collated on one day (28/06/2020). Our dataset was hence collated prior to the removal of duplicates on the 2nd of July. Though the issue was UK wide, this is may or may not have an impact on case rates downwind.
Since the areas analyzed are from England, changes in testing strategies between countries should not have an impact. We have not analyzed the data specifically for rainfall or temperature. However, having looked at the pattern of rain during the periods of sustained wind and the following days, we do not think rain had an obvious impact. Given that we are looking at downwind areas, in a 14 day period following wind change, temperature is unlikely to be an important factor.
The increase in cases downwind is likely to be due to the wind carrying virus particles from a hot spot to the adjacent area. Whether the change is easily identifiable probably depends on factors such as the population density in the two areas, the difference in prevalence between the two areas and the duration of the wind change.
A change in wind direction of two days or more was observed all four areas analyzed. Increasing case rate trends in the hotspot were associated with increasing (Staffordshire, Northumberland) or plateauing (Surrey and WosWar) of case rate trends in downwind areas. In the only instance where a wind change did not lead a significant difference in the slope (London vs Surrey – second wind change), case rates in Surrey (downwind area) were higher than that of London (the hotspot) when the wind change happened.
Since the flow of wind across the UK does not begin or end at the edge of a county, it is likely that places outside of the areas analyzed could also have an effect. On initial analysis, we expected Staffordshire case rates not to be affected by viral particles carried from West Midlands (being upwind) and therefore to have a similar pattern as West Midlands.
However, case rates in Staffordshire increased from the 7th of April and behave very differently to West Midlands and WosWar (Fig. 1C). Staffordshire is ~50 km southwest of Sheffield, a known hotspot we choose not to include for analysis initially in the assumption that the Pennine Hills could affect wind currents.
A check on Ventusky suggested that a northeasterly wind from Sheffield would blow into Staffordshire and hence we collated data from Sheffield and included it in the analysis (Fig. 2A-C). Unlike the West Midlands, case rate trends in Sheffield were constantly high from the beginning of lockdown (23rd of March) until the 1st of May after which case rates gradually reduced (Fig. 2C).
The increasing case rate trends in Staffordshire from the 7th of April and the long duration of sustained transmission is temporally associated with the wind blowing from Sheffield into Staffordshire (Fig. 2C). There were also 4 days of southerly wind when the wind would have blown from West Midlands into Staffordshire (4th-7th of April). This, along with the second northeasterly wind from Sheffield into Staffordshire could have contributed to the increase in case rates seen in Staffordshire on the 18th of April.
The production of droplets from coughs and sneezes are well known. Small droplets and aerosols are also generated while breathing 17 and speaking 18-20. People infected with influenza viruses (A/B/dual infection) also shed fine aerosols (≤5µ) during normal breathing and speaking apart from coughing 20. Infectious influenza virus could be cultured from 39% of these individuals pointing to the important role aerosol mediated transmission of influenza 20.
Droplets ≤100µm are the largest fraction generated during breathing 21,22, coughing 22 or sneezing 22. The distance a droplet spreads depends on its size, temperature, humidity and air currents 22-24.
As per the Wells evaporation-falling curve. larger droplets >100µm in diameter take very little time to drop 2 meters and fall close to the individual 25,26.
Droplets ~100µm in diameter settle at 30 cm/second and those around 10µm in diameter settle at 0.3 cm/second, taking ~10 minutes to fall 2 meters 25,26. The estimated time for falling two meters is 6 seconds for a droplet of 100µm and 600 seconds for a 10µm droplet.
At 18˚C, in unsaturated air, particles that are 100µm and 50µm in diameter evaporate to become “droplet nuclei” in 1.7 seconds and 0.4 seconds respectively 25.
Hence, in normal conditions, a large proportion of particles under 100µm are likely to become droplet nuclei. The estimated time to settle for a 1µm particle being 16.6 hours 25,26, these droplet nuclei are likely to be carried long distances in the presence of air currents. We now know that aerosolized SARS-CoV-2 is twice as stable as influenza 9 and aerosolized SARS-CoV-2 in artificial saliva more stable at higher relative humidity (68-88% – decay rage: 0.40% min-1) compared to moderate relative humidity (40-60% – decay rate: 2.27% min-1) 9.
The average relative humidity is stable across the UK (Edinburgh: 81% over 6-years 27; England and Wales: 80.3% over 10 years 28). This suggests that environmental conditions are conducive for the virus to survive in the UK. Rapid inactivation of the virus exposed to sunlight calibrated for 40˚ North latitude (Equivalent to Madrid / Philadelphia) in under 20 minutes has been reported 29.
However, areas situated above 50˚ North latitude (London – 51˚ North) are unlikely to get the high-intensity of sunlight used for the experiment and will probably get the mid-intensity levels at noon, at the height of summer, in the south of England 30,31.
COVID-19 transmission indoors via aerosols is known. In a choir practice, 87% (53 individuals) of those exposed to a symptomatic individual were infected 32. Data from a cruise ship outbreak of COVID-19 highlight the importance of aerosol mediated transmission 6. After quarantining measures were introduced in the cruise ship, fomite transmission was reduced and short-range transmission (i.e., within cabins) increased.
The contribution of aerosol transmission was similar to droplet transmission pre-quarantine (median: 60% vs 40%. p=0.32). However, the contribution of aerosol transmission increased significantly during quarantine (median: 85% vs 15%. P<0.0001), suggesting transmission via air currents in the cruise ship 6.
The reduction in other common respiratory viral infections following the introduction of lockdown and social distancing measures at a time of increased COVID-19 transmission suggests that of SARS-CoV-2 can spread from a distance 33.
Apart from aerosols generated from the respiratory tract, the importance of aerosol generated from stool and urine also needs to be considered. Aerosol mediated transmission of the SARS coronavirus via vertical soil stacks is known 4. These aerosols rose through air shafts and were then blown through open windows to cause transmission in three blocks of flats downwind 4.
SARS-CoV-2 is shed in high concentrations in stool and urine 34, with many having diarrhea associated with COVID-19. The virus in urine and stool is also viable and infective as shown by infection experiments in ferrets (urine: days 11 and 13; stool day 15) 34. Aerosols are generated from toilets on flushing, with a significant increase in particles <3µm in size 35.
Hence a large amount of virus must be flushed from infected households, residential homes and hospitals generating plumes within the toilet which could then be vented into the atmosphere either through open windows, exhaust fans or home ventilation systems. Soil pipes are also vented to the outside atmosphere, and in the UK, soil pipe venting to the outside air can be seen on the roof. This offers a direct route for aerosols generated by flushing to be vented into the surrounding air.
The data hence suggests that small droplets are generated during normal day-to-day living, irrespective of whether someone is coughing or sneezing. Since most small droplets are likely to evaporate and become droplet nuclei before they settle, they are likely to be transported by wind.
We also know that individuals also shed the virus when asymptomatic (either pre-symptomatic or truly asymptomatic). These individuals are likely to be outdoors, increasing the possibility of droplet nuclei being produced and transported by wind. The recommendation from the NHS is to keep windows open to reduce transmission of COVID-19 to household contacts 36. Hence, a proportion of droplet nuclei generated indoors could also be transported outside by the wind.
We know that dust particles, bacteria, fungi and insects are carried across the globe by wind 37. Aerosols are also carried by the wind over hundreds of miles. Aerosolized sea salt left behind when sea water spray evaporates is carried inland and affects chloride levels in inland water.
When measured, the lines of equal chloride content was almost parallel to that off the shore off Massachusetts 25. Sea salt aerosols have been detected hundreds of miles inland, having travelled 250-300km in 10-24 hours 38 demonstrating the long distances small droplet nuclei can be transported.
Our manuscript is the first to report the impact of wind on case numbers downwind. Others have reported the impact of wind on local case numbers (i.e., in the hotspot and not downwind). In Suffolk County, USA, warmer temperatures (16-28˚C) and low wind speeds (≤8.5km/hour) were significantly associated with increased COVID-19 incidence following multivariate analysis 39.
The authors also report that COVID-19 incidence appeared to increase slightly on cooler, windier days as a function of wind speed, but this was not significant on multivariate analysis 39. The study however does not investigate the directionality of wind. Suffolk county is surrounded by the water on three sides.
Wind from the South and East will blow from the Atlantic Ocean. Wind from the North will blow from Connecticut across kilometers of sea and that from the West will blow from New York. Hence, it is possible that a significant difference might be observed if wind direction is included for analysis. A negative association between wind speed and COVID-19 cases has already been reported 40,41.
In 14 day lagged analysis, an increase in wind speed by 5Km/hour is associated with a 6% drop in local COVID-19 cases; a positive association was also seen with maximum temperature (5˚C increase = 7% increase in COVID-19 cases) and absolute humidity between 5-10% g/m3 (23% increase in COVID-19 cases) 41.
Absolute humidity was consistent even on 7 day lagged analysis 41. An analysis of global weather suggests positive association between wind, other meteorological parameters and increase in local case numbers 42. An inverse association between wind speed and secondary cases of SARS CoV was reported from China and two local regions within China 43.
Unlike SARS CoV, lower wind speeds are associated with lower incidence of MERS CoV 44. These reports suggest that wind might be an important factor in viral transmission, and its impact on different viruses may need to be investigated in detail.
The possibility that SARS-CoV-2 can be transmitted by the wind has significant implications in stopping viral spread. Apart from vaccines, the available options are reducing the production of aerosols, contact with aerosols and prophylactic measures which could protect post-exposure to aerosols.
Facemasks have a role in reducing viral shedding and in the inhalation of viral particles. Masks helped reduce secondary transmission to family members in China 45. Countries in Asia that enforced mask wearing over lockdown measures had lower total cases of COVID-19.
Cloth masks are sufficient to reduce the spread 46, and possibly reduce the burden of virus that infects a person in the community 46 allowing critical PPE to remain available to healthcare workers. UK guidelines initially required a 2-meter distance from others in public spaces.
After lockdown was eased, this was reduced to 1 meter+ and people advised to wear a mask when in proximity to others. Since the data suggests that virus is carried long distances, compulsory mask-wearing is likely to be an important control measure, both to stops aerosols from an infected person and possibly to reduce the chance of infection in a susceptible individual.
Since most individuals are unlikely to know if they were exposed during the day, it is important that we make use of the innate antiviral immune response in cells as the first line of defense against respiratory viral infections 47. In this mechanism, infected cells produce hypochlorous acid with which they fight viral infections.
Supplying chloride ion locally to nasal epithelial cells via saltwater helps the cells clear viral infections quicker and reduce viral shedding 48. Others have shown the efficacy of local saltwater application (nasal irrigation/ sprays) in preventing viral upper respiratory tract infections in adults and children 49,50. A prophylactic study to stop URTI including COVID-19 is urgently needed.