This article is quoted from quora, author Tirumalai Kamala (immunologist, Ph.D. Mycobacterium Biology).
Virus dose per exposure, frequency of exposure and mode of transmission matter most in estimating infection risk.
Prevailing evidence suggests that SARS-CoV-2 largely spreads through air.
Plenty of evidence now that indoor crowding and poor ventilation facilitate its spread.
Different individuals have different degrees of risk due to different exposure dose and frequency.
Minimizing virus exposure dose and frequency is a realistic goal. Eliminating exposure altogether isn’t.
A few case reports and anecdotal data suggest a variety of face masks may be protective against SARS-CoV-2. This implies they could minimize exposure risk.
These features and assumptions together suggest that even face masks other than N95 could be potentially life-saving.
Protection measures should flow from what's known about how SARS-CoV-2 spreads. Virus dose per exposure, frequency of exposure and mode of transmission matter most in figuring out how best to protect against any virus.
Prevailing evidence suggests that SARS-CoV-2 largely spreads through air. Since it's a respiratory virus, it largely spreads from the nose/throat of an infected person. Respiratory projectiles come out as either larger droplets or smaller aerosols. Key distinction between the two is size and length of time they can stay airborne(below from1,2,3).
Plenty of evidence now that indoor crowding and poor ventilation facilitate SARS-CoV-2 spread. Whether SARS-CoV-2-laden respiratory projectiles stay afloat for hours as aerosols is still being hotly debated. Regardless, examples such as the Skagit county choir (4) or the Guangzhou restaurant (5) suggest it easily spreads indoors in poorly ventilated spaces,
From coughing, sneezing or panting (that violently emit respiratory projectiles) to talking or even just breathing (that non-violently emit respiratory projectiles) indoors at close quarters,
In restaurants (5).
In gyms/sports or exercise facilities (6).
At call centers (9).
In shopping malls (12).
In buses (13).
At conference venues (14).
From singing indoors at close quarters in choirs (4).
From talking loudly, shouting or yelling indoors at close quarters in noisy workplaces such as meatpacking plants (26, 27, 28, 29, 30) or in vacation spots such as cruise liners (31) or at social events in bars and night clubs (32, 33).
It seems to spread less from contaminated surfaces though even there face masks protect from hand-to-face transmission.
These features specific to COVID-19 outbreaks suggest that a face mask that fully and tightly covers the mouth and nostrils could greatly minimize exposure if an infected person in one's vicinity is spewing virus. Universal masking would minimize exposure risk across the board by reducing viral output into the environment from the infected as well as reducing exposure in the uninfected.
Different individuals have different degrees of risk due to different exposure dose and frequency.
Risk of exposure to the SARS-CoV-2 virus obviously runs the gamut if we consider just two extremes,
On the one hand, an ER doctor in a COVID-19 hotspot who sees a steady stream of seriously ill patients every day for weeks on end.
On the other hand, someone working from home who only occasionally ventures out for groceries and other essentials.
Obviously risk of virus exposure is going to be wildly different between these two extremes. What an ER doctor needs to do to protect themselves from SARS-CoV-2 doesn't apply to someone working from home.
Minimizing virus exposure dose and frequency is a realistic goal. Eliminating exposure altogether isn't.
Goal isn't and shouldn't be to stop each and every virus particle. That's practically impossible even with an N95 mask.
Instead goal should be to minimize virus exposure dose and frequency each and every time out of the house, especially in a place with rampant community transmission which implies anyone could be infected with no way to know for sure hence all the more reason to assume anyone outside one's own household bubble could be infected. Such self-preserving caution becomes all the more necessary because evidence suggests
COVID-19 asymptomatics and presymptomatics are likely just as infectious as the symptomatic.
SARS-CoV-2 viral loads peak in the days before symptoms begin and some evidence suggests even merely talking might suffice to spread infectious droplets.
While certain comorbidities such as diabetes, obesity, chronic heart or lung disease represent greater risk of COVID-19 severity or death, they don't explain all severity or death. Indeed there are reports of those without presently known pre-existing conditions who have had severe disease or died.
This in turn implies that face masks other than N95 might not just be better than nothing, they could even be life-saving.
All sorts of face masks have been found to be pretty good at blocking droplets and that's nothing to sneeze at since blocking droplets could help block the bulk of viral load someone might be spewing at close quarters. In fact, studies show that even not particularly well-fitting homemade masks are quite effective in blocking droplets (larger size) and even some aerosols (smaller size) (34).
A few case reports and anecdotal data suggest a variety of face masks may be protective against SARS-CoV-2.
Universal surgical mask wearing was associated with zero nosocomial (hospital) transmission of SARS-CoV-2 in a Hong Kong hospital (35).
In February 2020, all 41 healthcare workers in Singapore who were exposed to a patient with severe pneumonia before COVID-19 diagnosis were protected against infection. 85% wore a surgical mask while the rest wore N95 masks (below from 36),
“On the basis of contact tracing, 41 health care workers were identified as having exposure to aerosol-generating procedures for at least 10 minutes at a distance of less than 2 meters from the patient. The aerosol-generating procedures included endotracheal intubation, extubation, noninvasive ventilation, and exposure to aerosols in an open circuit (4). All 41 health care workers were placed under home isolation for 2 weeks, with daily monitoring for cough, dyspnea, and myalgia and twice-daily temperature measurements. In addition, they had nasopharyngeal swabs scheduled on the first day of home isolation, which could have been day 1, 2, 4, or 5 after last exposure to patient, and a second swab scheduled on day 14 after their last exposure. The swabs were tested for SARS–CoV-2 by using a PCR assay. None of the exposed health care workers developed symptoms, and all PCR tests were negative (Table).”
A man flying from China to Toronto was later diagnosed COVID-19+. He wore a face mask throughout the flight and the 25 seated closest to him or who attended to him all tested negative and no one from that flight was reported COVID-19+ (37).
A county-level analysis of US COVID-19 cases between March 31 and May 22, 2020, found greater declines in daily case growth rates in states that mandated face mask use in public compared to those that didn't (38).
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