Early on in the pandemic, conventional wisdom dictated that the summer months could slow the spread of COVID-19. After all, other, similar viruses all but disappeared during the summer, due to light, warmth and higher humidity. However, COVID-19 didn't follow this predictable pattern. Instead, cases spiked, with recent information from Johns Hopkins University of Medicine placing global cases at just under 30 million, with the U.S. taking the lion's share at 6.6 million.
As the year moves toward autumn and early winter, there are new concerns that the cooler weather will allow the virus to gain steam and infect more of the population. However, research on COVID-19 is currently limited, and most of these predictions can only be based on the behavior of previous viruses. Also, much is still unknown about how COVID-19 is actually transmitted, making transmission theories based on warmer or cooler weather even more challenging. However, even amidst all of the uncertainty, steps can be taken to limit contagion this winter.
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What We Know About COVID-19 and Cold Weather
Given the relative newness of COVID-19, research on how it behaves is still in its very early days, with much of the speculation based on influenza and the 2003 SARS outbreak. One 2010 lab study focused on viruses similar to the current SARS-CoV-2 indicated that lower temperature and humidity conditions aid in the cells' survival. Additionally, the flu and other respiratory viruses tend to surge during the winter months due to colder, drier air, as these conditions allow easier transmission of airborne viruses.
However, the Centre for Evidenced-Based Medicine indicated that, while early evidence points to the idea that cooler conditions can aid viral spread, "all estimates are subject to significant biases, reinforcing the need for robust public health measures."
Atmospheric and lab studies aside, human behavior also aids in virus transmission during the colder months. "All respiratory viruses tend to spread more during colder weather," noted Timothy Brewer, MD, MPH, who is a Professor of Medicine and Epidemiology at the University of California, Los Angeles. People tend to stay inside more, in close proximity to one another, which encourages viral transmission.
Additionally, "immunity becomes weaker during the winter, which could cause an increased rate of infection," said Waqas Ahmad, MD, a family physician, who is also in charge of Insurecast's Medical Advisory Board.
The takeaway? Previous viruses have survived well in colder, drier weather. But right now, researchers are only guessing when it comes to COVID-19 behavior.
Can COVID-19 Be Transmitted Through HVAC Systems?
Researchers are also divided about how COVID-19 is transmitted, with the two most prominent theories being droplet transmission and aerosol/airborne transmission.
- Droplet transmission — Through this method, virus-laden particles of breath or saliva come from an individual's mouth or nose when he/she speaks, coughs or sneezes. Droplet transmission is considered the most common method of viral spread.
- Aerosol/airborne transmission — In this scenario, an infected individual expels microscopic infectious particles, which linger in the air and are spread via air currents. Aerosol transmission is of particular concern when it comes to heating, ventilation and air conditioning (HVAC) systems.
Again, research is very limited on the topic of viral spread, with one question focused on whether air conditioning systems can suck in coronavirus particles, then distribute them elsewhere. Studies point out that measles, tuberculosis, influenza and the 2003 SARS have spread through HVAC systems, increasing concerns that COVID-19 could also be transmitted via this method. Specifically, "any air circulation system that recirculates indoor air without filtering, disinfecting or mixing in clean, outdoor air could increase the chance of respiratory virus transmission," Brewer commented.
A report issued by the European Centre for Disease Prevention and Control (ECDC) listed studies suggesting that COVID-19 droplet and aerosol emissions could be blown around by air conditioning airflow, facilitating the spread of droplets within indoor spaces.
"We didn't focus on ventilation as much early on as we probably should have," Harvard Medical School researcher Abraar Karan told NPR in an interview.
With the weather getting cooler, the question now is whether heating systems could aid in, or prevent, the spread of COVID-19. Ahmad explained that heat at 56 degrees Celsius/132.8 degrees Fahrenheit could be hot enough to kill the virus. However, "neither systems nor people can support such a heat level," he said. According to the World Health Organization, whether an HVAC system is on "hot" or "cold," clean air filtration is the best bet to lessen the potential of viral spread.
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Limiting COVID-19 Transmission During Fall and Winter
While questions abound concerning coronavirus transmission during cooler weather, individuals can take common-sense steps to limit viral transmission. The main piece of advice is to ensure the circulation of fresh air throughout homes and office buildings whenever possible. The U.S. Environmental Protection Agency (EPA) indicated that it's good to keep windows open when possible. The EPA went on to suggest that if an HVAC system has an outside air intake, keep it open as well.
Brewer also suggested that HVAC systems with HEPA filters, UV lights and those that bring outdoor clean air into indoor spaces will help reduce virus transmission. Also suggested? A flu shot.
"It won't prevent COVID-19," Brewer said. "But it's important for preventing the spread of disease from influenza, that also circulates during winter months." Both Brewer and Ahmad also advised the standard protective tips of frequent hand washing, social distancing, facial coverings, hand sanitizer use and keeping fingers away from touching the mouth, nose and eyes.
The Bottom Line
While researchers continue to study the extent of COVID-19 spread in cooler weather, experts agree that one way to ensure a safer indoor environment is to include as much fresh air as possible. Additionally, the usual mitigation tips of social distancing, face coverings and hygiene can help curb winter transmission.
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