On April 3, the Centers for Disease Control and Prevention issued an advisory that the general public wear cloth face masks when outside, particularly those residing in areas with significant severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) community transmission.1 Recent research reveals several factors related to the nature of the virus as well as the epidemiologic spread of the illness that may have led to this decision.
However, controversy still prevails whether this recommendation will alleviate or aggravate disease progression. With many hospitals across America lacking sufficient personal protective equipment (PPE) and scrambling for supplies, universal masking may create more chaos, especially with certain states imposing monetary fines on individuals spotted outdoors without a mask. With new information being discovered each day about COVID-19, it is more imperative than ever to update existing strategies and formulate more effective methods to flatten the curve.
Airborne vs. droplet transmission
According to a scientific brief released by the World Health Organization, there have been studies with mixed evidence and opinions regarding the presence of COVID-19 ribonucleic acid (RNA) in air samples.2 In medRxiv, Santarpia et al., from the University of Nebraska Medical Center, Omaha, detected viral RNA in samples taken from beneath a patient’s bed and from a window ledge, both areas in which neither the patient nor health care personnel had any direct contact. They also found that 66.7% of air samples taken from a hospital hallway carried virus-containing particles.3 It is worth noting that certain aerosol-generating procedures (AGP) may increase the likelihood of airborne dissemination. Whether airborne transmission is a major mode of COVID-19 spread in the community and routine clinical settings (with no aerosol-generating procedures) is still a debatable question without a definitive answer.
We should consider the epidemiology of COVID-19 thus far in the pandemic to determine if transmission patterns are more consistent with that of other common respiratory viral pathogens or more consistent with that of the agents we classically consider to be transmitted by the airborne route (measles, varicella zoster virus, and Mycobacterium tuberculosis). The attack rates in various settings (household, health care, and the public) as well as the expected number of secondary cases from a single infected individual in a susceptible population (R0) are more consistent with those of a droplet spread pathogen.
For measles, the R0 is 12-18, and the secondary household attack rates are ≥ 90%. In case of the varicella zoster virus, the R0 is ~10, and the secondary household attack rate is 85%. The R0 for pulmonary tuberculosis is up to 10 (per year) and the secondary household attack rate has been reported to be >50%. With COVID-19, the R0 appears to be around 2.5-3 and secondary household attack rates are ~ 10% from data available so far, similar to that of influenza viruses. This discrepancy suggests that droplet transmission may be more likely. The dichotomy of airborne versus droplet mode of spread may be better described as a continuum, as pointed out in a recent article in the JAMA. Infectious droplets form turbulent gas clouds allowing the virus particles to travel further and remain in the air longer.4 The necessary precautions for an airborne illness should be chosen over droplet precautions, especially when there is concern for an AGP.