The science behind COVID-19 — and how to guard against infection

Since the onset of the pandemic many have wondered why New York, Milan and Wuhan were overwhelmed with COVID-19 hospitalizations in short spans of time while other large cities were mostly spared the “surge.” Part of the explanation may be in dose exposure and viral loads.  Since COVID-19 replicates in the nasopharynx and respiratory tract and spreads by aerosol, droplet and direct contact, the virus can jump from one person to another and the number of viral particles that make the jump may determine how severely ill the next person will become. 

While the concept of viral load as a determinant of illness severity is controversial, considering this aspect of viral transmission makes sense in understanding the severe hotspots. In New York City, health care workers described entire waiting rooms of patients with COVID-19 coughing and struggling to breathe. These severely ill patients invariably passed large viral loads to their families and other close contacts, first responders and others on the way to the hospital, creating even more severe cases.  

Large viral load transmission on planes must have played a role in global spread. Air travel is one activity that most individuals will undertake with a severe illness given the outlaid costs, personal/business obligations of the trip, and the consequences of canceling. So it is not hard to imagine an airline passenger suffering through a flight with a “cold” in February — only later to understand it could have been COVID-19, with every sneeze or cough being a heavy blast of viral particles to an unsuspecting neighbor.   

The converse is true among COVID-19 asymptomatic or minimally symptomatic individuals who have considerably less viral shedding and, hence, lower numbers of viral particles to jump from one person to another, creating another asymptomatic or mildly symptomatic case. In support of this hypothesis, there have been reports of relatively large proportions of the population having antibodies directed against COVID-19, indicating prior exposure, possibly to low viral loads, and resulting in community spread without hospitalization.

Another key concept in COVID-19 illness is susceptibility. Major reports on the pandemic have indicated that both hospitalization and mortality is associated with older age and prior medical conditions such as diabetes and lung and heart disease. Additionally, we have become acutely aware of the susceptibility and vulnerability of nursing home residents. The senior center outbreaks are working examples of how an asymptomatic/minimally symptomatic employee can transmit COVID-19, possibly in a low viral dose, to a susceptible, frail senior and result in hospitalization or death.  

The final consideration is repeated viral exposures and self-dosing. The outbreaks on cruise ships and the USS Theodore Roosevelt are examples of likely repeated exposures among individuals kept in close proximity to one another over a prolonged period of time. The same may be true in some of the health care worker deaths where repeated exposure to multiple critically ill patients, even with personal protective equipment, may have allowed enough cumulative viral transfer to create a fulminant disease. Because COVID-19 is exhaled with each breath as a bioaerosol, it is possible to breath the same viral particles back into lungs, promoting an even worse infection. This could explain why enclosed high-rise apartments in New York, Milan and Wuhan were fostering severe cases as infected patients sheltered in place without access to fresh air. It may also explain why there were relatively few severe cases in rural areas of any country, or in places where moderate temperatures and open windows allowed aeration.

How can these principles be put into practice for everyday life in the COVID-19 era? The first principle is to reduce one’s own viral load by hand-washing, use of hand sanitizer, and at every instance open windows and go outside for fresh air. In Texas, I advise my patients that the COVID-19 virus does not like dry windy days, so get outside without a mask when distancing from others or in a car with the windows down. The benefits of fresh air have been applied to past viral pandemics and, in my view, should be emphasized with COVID-19. The daily presidential briefings could set a wonderful example if moved to the White House lawn, allowing the media and the Coronavirus Task Force to enjoy fresh air and reduce their risk in the crowded briefing room. 

The second principle is to wear a mask of any type indoors when in close contact with others to avoid the “big blast” of viral particles in a sneeze or cough or a cloud of bioaerosol left in closed spaces such as a public restroom or elevator. Several airlines have announced they will hand out and enforce the use of masks for passengers — a very prudent measure. The third principle is to understand that the most important form of social distancing is to stay away from a sick household member. Case contact studies have indicated that the most common form of transmission leading to symptomatic cases occurs in the home. That means moving out to be away from the ill family member, or isolating in a room with windows open to ventilate and reduce the concentration of viral particles in the air. In my view, we should not trust air-handling systems to cleanse the air. We have nice weather in most of the country now, so it is time for fresh air. 

Finally, unless an ill patient confirmed with COVID-19 is in close contact with others, I do not advise a mask for the patient.  The mask will only facilitate rebreathing the bioaerosol and serve to potentially amplify the infection. Staying close to the window or out on the porch alone without a mask is the best way to self-quarantine and  ride out the virus, in my view.   

As we emerge from lockdowns and other restrictions, we should work to avoid the “big viral blasts” from sick persons and be wary of low-level transmissions to the elderly or infirm. These two scenarios of transmission are likely responsible for most of the hospitalizations and mortality. Conversely, low-dose transmission through a variety of means in healthy populations is likely to continue and lead to self-limited community spread. While it would be ideal to arrive at zero cases, this is unlikely since COVID-19 is highly communicable. Thus, we are in a new normal that should be manageable with understanding these principles.

Peter A. McCullough, MD, MPH, is vice chairman of medicine at Baylor University Medical Center and a professor of medicine at Texas A&M College of Medicine in Dallas. An internist, cardiologist and epidemiologist, he is the Editor-in-Chief of “Cardiorenal Medicine” and “Reviews in Cardiovascular Medicine.” He has authored over 500 cited works in the National Library of Medicine. Follow him on Twitter @McCulloughBHVH.