By NORTIN HADLER, MD (2)
We are all reeling from the language of the COVID-19 pandemic: fatality rates, spread, PPE, distancing, pneumonia, anti-viral drugs, ventilators, second wave, vaccines, serological testing, etc. All of this focuses on the infection with the SARS-CoV-2 coronavirus, and for good reason. People are sick and dying from this infection. However, there is murmuring that will grow in volume as COVID-19 subsides: Will this happen again? Is the annual flu season to become more alarming in the future? Will we forever be waiting for vaccines and anti-viral drugs with bated breath?
If we step back and view this nightmare from the perspective of infectivity rather than infection, there may be a way out of this conundrum. Coronaviruses are one of several categories of virus with a proclivity for human respiratory infection. Coronaviruses join Influenza, Ebola, SARS and MERS as respiratory riboviruses. These viruses are tiny bundles, virions, containing a little RNA and not much else. Left alone, little happens. But if they manage to get into a target cell, they commandeer the cell’s metabolic machinery to do their malevolence. They get to their target cell because we inhale them as virions that are aerosolized or carried in the droplets that are produced when infected people exhale. Droplets that are not inhaled settle on surfaces, including surgical and N95 masks, where water evaporates freeing virions to aerosolize unless they are somehow bound to the surface. They are so tiny that even the N95 mesh is an inefficient barrier. One would need filters with such tiny pores that they would impede inhaling.
Infectivity of these riboviruses and probably severity of disease are dose dependent. The more virions one inhales the more likely one is infected. That’s the reason physical distancing is advised. However, these riboviruses retain infectivity for many hours on surfaces during which air currents can launch them and keep them afloat. That’s of little concern out of doors where they are diluted, but not indoors. Modern architectures and civil engineers are well aware of issues that relate to stagnation in built environments. HVAC systems are designed as a compromise between the need to recirculate air for the sake of efficient heating/cooling while turning to filters and air exchanges for the sake of air purity, particularly with regard to inert particulate materials. Concerns about airborne illnesses are seldom primary. In most hospitals, certain rooms are designed as “reverse flow” rooms for patients with contagious diseases with exhaust mechanisms so that the room’s air is released to outside the building, not back into general circulation in the building. Some operating rooms, particularly orthopedic surgical suites, have lamellar flow ventilation to decrease the likelihood of introducing an airborne pathogen along with orthopedic hardware. But otherwise hospitals and other workplaces are not designed with regard to infectivity.