From social media users to the US president, everyone seems to be speculating on how the weather is likely to affect the spread of Covid-19, but finally here is some peer-reviewed science for us to digest.
According to a new study, an inverse relationship exists between humidity and reported Covid-19 cases in Sydney, both as the number of Covid-19 cases increased and declined, showing that dry air aids the spread of the novice virus.
How The Study Was Conducted
The study which was carried out by Professor Michael Ward of the veterinary and public health at the University of Sydney in conjunction with international colleagues used Covid-19 cases in Sydney for a period of time starting February to May and relied on postcode data to connect cases to the nearest weather station.
The team then compared the data of reported cases with different weather conditions as recorded at the Bureau of Meteorology such as temperature, wind speed and relative humidity all recorded at the same time between 9:00am and 3:00pm, and data of rainfall for the 14 days preceding the reported cases.
Under all other weather variables, relative humidity at 9:00am was considered the best predictor. For every 1 percent fall in relative humidity, they noted a 7-8 percent rise in the number of Covid-19 cases.
According to a separate study by Tim Inglis, a renowned medical microbiologist and head of pathology and laboratory medicine at the University of WA, this was “very plausible” and “interesting” sequence of observations.
“Which, for those in the know who understand the behaviour of respiratory viruses and the environment, is maybe not too surprising,” Dr Inglis stated.
Previous studies of similar respiratory viruses such as SARS and MERS, were also linked to several weather indicators including humidity as well as temperature.
In a different study published by Dr. Ward’s team, Covid-19 was linked to humidity and temperature as the virus ravaged through mainland China, with another study of Sydney Covid-19 cases published in May, also concluding that humidity was the only factor.
Humidity appears to be a constant thread.
How Dry Air Helps Spread COVID-19
The theory explained by Dr Ward and his team states that when air is more humid, the coronavirus aerosol particles become larger and just like droplets, fall from the air.
When the air in the atmosphere is drier, the aerosol particles end up shrinking and remain suspended in the atmosphere for a long time.
For this theory to hold any water, we must admit that Covid-19 continues to spread via aerosols through airborne transmissions.
This is a finding that the World Health Organisation (WHO) downplayed earlier but is now acknowledging that there are cases within closed setups where aerosol transmission can’t be ignored.
According to one study, the coronavirus can remain suspended in the air as an aerosol for about three hours and others for up to 16 hours when placed in laboratory conditions.
“They see the same relationship in the laboratory we’re seeing out in the real world. It gives us confidence that maybe that’s something that’s going on,” Dr Ward said.
Are Our Tropics Off The Hook?
Before the residents of Queensland and those in the Top End become all too cocky, this doesn’t render them immune to Covid-19.
Consider Florida and Indonesia which have recorded thousands of new cases in recent months despite their high humidity.
If we accept the fact that there is a link between low humidity and a surge in aerosol transmissions as the means behind this relationship, that doesn’t limit any of the other ways of transmission.
All the Covid-19 droplets in the atmosphere and the direct contact possibilities exist out there no matter how sauna-like it is.
The outcome in this latest paper only considered Sydney and is directly applicable to the city.
Dr Ward thinks that even during humid conditions, changes in humidity levels over short periods of time could affect the rate of coronavirus infection but lays emphasis on the fact that analysis hasn’t been done.
The tropics could present a completely varied relationship. For instance, flu is not a seasonal respiratory illness in the tropics as it is in the temperature latitudes.
Things To Consider:
These research findings were published in one of the local scientific peer-reviewed journal, meaning it has received the green light signal by others in the field.
However, even with top-notch science, there are several things to be considered.
Most definitely, the weather isn’t the only factor contributing to the spread of Covid-19. And these kinds of observational studies can’t rule out other factors that influence overall outcomes such as behaviour changes.
Dr Ward noted that they tried to “crudely” take into consideration behavioural change by clarifying the study period into before and after coronavirus control measures were put in place. They discovered that the inverse relationship between relative humidity and Covid-19 cases were present in both phases.
The study also faced limitations of access to only home postcode data, not really the location where coronavirus was first contracted.
Similarly, it considers outdoor weather variables at a point when most transmissions are thought to occur indoors.
John Mathews, a professor at the University of Melbourne’s school of population and global health and who did not participate in the study, criticised the data saying that it did not measure the actual spread of the Coronavirus and that when cases are detected through testing, that testing is more based on symptoms.
“I can’t prove that’s true, but it just illustrates how difficult it is to draw valid conclusions from complex data like this.”
Despite his skepticism, Dr Mathews adds that the proposed link between reduced transmission and low humidity is credible.
“There may be alternative explanations.”
So Should You Buy A Humidifier?
Not so fast.
The findings have only shown the existing link between outdoor humidity and the statistics of the Covid-19 cases yet most transmissions are believed to be taking place indoors.
Dr Ward noted that it would be awesome to consider indoor outbreak locations and see if there is any potential relationship with indoor humidity.
The ideal places include meatworks environments which he considers more conducive for aerosol production or in aged-care homes.
“If there is, I think then you might have enough evidence to say, ‘well, maybe we should consider modifying humidity as one of our responses’,” Dr Ward said.
COVID-19 Alert Days
Dr Inglis noted that it would be great to have a warning system in place to caution the public of days with low humidity like those for fire danger and thunderstorm asthma.
“Those are the sorts of things that you’ll see the public health physicians and the people who do the detailed modelling looking at on a wider basis than just New South Wales or urban Sydney,” he said.
Study Supports Wearing A Mask
Ultimately, this study supports the religious wearing of a mask.
“If it’s weather, if it’s humidity, it means it’s aerosol transmission,” Dr Ward concluded.
“If you do have this aerosolised virus in the atmosphere, the only way you can protect yourself from that is by wearing a mask.
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