- Last 7 days
Asymptomatic spread of coronavirus is ‘very rare,’ WHO says
Take away: Dr. Van Kerkhove appeared to refer to only “asymptomatic” individuals and not “presymptomatic” individuals in her statement. Clarification from the WHO, and public availability of the data leading to the claim, is needed for proper interpretation. At the current time, existing published data indicates that a significant amount of SARS-CoV-2 infections are due to individuals who did not have symptoms when they spread the virus.
The claim: According to the WHO, asymptomatic spread of coronavirus is ‘very rare’.
The evidence: This statement is attributed to WHO official Dr. Maria Van Kerkhove during a recent news conference. It deserves greater clarification from the WHO, but Dr. Van Kerkhove appears to make the distinction between “asymptomatic” and “pre-symptomatic” individuals during her comments. This distinction is essential for proper interpretation of her statement. “Asymptomatic” refers to persons who test positive, but who never display symptoms throughout the course of their SARS-CoV-2 infection. In contrast, “presymptomatic” individuals are those with confirmed infection, who do not currently display symptoms, but later go on to develop COVID-19 related symptoms (fever, cough, loss of taste/smell, etc).
Importantly, the distinction between asymptomatic and presymptomatic can only be made retrospectively. From a clinical standpoint, if someone currently has no symptoms, but tests positive, there is no way of knowing at that time if they are “asymptomatic” or “presymptomatic”. Preliminary data estimates that around 20% of SARS-CoV-2 infections are truly “asymptomatic”.
If “asymptomatic” individuals were rarely involved in transmission of the virus, this would be an important finding, but from a practical standpoint if “presymptomatic” individuals still spread the virus (as the data indicates), then the rationale for preventative measures still stands. Early studies   have estimated that up to 40-60% of virus spread occurs when people don’t have symptoms. Preventative measures such as social distancing and universal mask wearing have been implemented to prevent the spread of virus from individuals not currently demonstrating symptoms.
Though important, social distancing could be reduced to one metre instead of 2m
Take away: As with most things in nature, there are always exceptions – transmission occurring at greater distances than 3 ft and evidence of aerosolization have been reported.
Discussion: In scientific terms, this virus is still very new so the data supporting an optimal physical distance to prevent transmission remains scarce. In the absence of data, public health agencies have used what they understand about this virus and similar viruses to infer a “best” answer. Public health agencies try to simplify the recommendation to a single answer, but the reality is much more complex.
According to reports the WHO bases their recommendation for 1 meter (~3 ft) distancing off of an understanding that SARS-CoV-2 behaves like similar respiratory viruses that are primarily transmitted via larger droplets (as opposed to smaller aerosols). Assuming most spread is via droplets, the WHO reportedly follows the results of a 1934 study indicating most respiratory droplets fall to the ground within 3 feet.
However, as with most things in nature, there are always exceptions – transmission occurring at greater distances than 3 ft and evidence of aerosolization have been reported.
The evidence basis for the CDCs guidance for 6 feet of separation is less clear, but probably reflects lower risk tolerance, or greater weight to evidence of aerosolization or wider droplet spread.
Even with further study, there may never be a clear answer for optimal physical distancing. This is because, (1) the area of high risk for transmission is probably dependent on the specific conditions of the interaction (e.g. loud talking, windy environment), and (2) the “optimal” distance is based on risk tolerance. There is no single distance between individuals where risk of transmission drops off precipitously to zero.
All evidence indicates that greater distances are safer but, for example, consider how restrictive a physical distancing recommendation of >50 ft would be. In the end, because we can’t control how far others stand away from us, we ask governments to consider these tradeoffs and deliver a “best” answer to guide their citizenry.
twitter.com twitter.comAlex Berenson on Twitter: "1/ TL:DR - @who published a massive review/meta-analysis of interventions for flu epidemics in 2019, found "moderate" evidence AGAINST using masks. (They actually missed the 2015 Vietnam study, yet another brick in the wall.) 2019. So recent, yet so long ago. What's changed?" / Twitter1
@who published a massive review/meta-analysis of interventions for flu epidemics in 2019, found "moderate" evidence AGAINST using masks.
Take away: In their 2019 report the WHO actually recommended for, not against, the use of masks in severe influenza epidemics or pandemics, contrasting the statement made in this tweet. Further, recent evidence overwhelmingly supports the benefit of masks for preventing the spread of SARS-CoV2, the virus that causes COVID-19.
The claim: Overall the claim here appears to be that masks are ineffective against the spread of SARS-CoV2, the virus that causes the clinical syndrome known as COVID-19. The evidence used in support of this claim is that “the WHO found ‘moderate’ evidence AGAINST using masks” in their 2019 report on the use of non-pharmaceutical interventions for mitigating influenza pandemics.
The evidence: This overall claim is poorly supported by data and the evidence used to support this claim is incorrectly characterized by the claimant. Narrowly, the claim that the WHO recommended against mask use is patently false. In their report, the WHO reviewed 10 separate studies and did conclude that there was scant evidence that masks significantly decreased spread of the flu. However, they found no evidence that masks increased spread, and based on mechanistic plausibility (i.e. masks are barriers that prevent droplets from passing between people) and the low risk/high reward, they made a conditional recommendation for mask use in severe influenza epidemics or pandemics.
While influenza does not behave exactly like the SARS-CoV2 virus, the similarities in mode of transmission make it reasonably likely that masks would also have protective effects against the spread of this virus is well. The best evidence is hard data, and that too increasingly points to the benefit of masks for slowing down or preventing the transmission of SARS-CoV2. A recent summary of that data is available here.
- Jul 2020
When virus levels in the population are very low, the chances of a test accurately detecting Covid-19 could be even less than 50 per cent
Take away: Real-world evidence from countries like New Zealand, that already have very low disease incidence, suggests that the concerns for false positives raised in this article are overhyped.
The claim: "When virus levels in the population are very low, the chances of a test accurately detecting Covid-19 could be even less than 50 per cent..."
The evidence: The author explains theoretical scenarios where, when rates of true COVID infections are low, the rate of true positives (test positive and have COVID) may be equal to or less than false positives (test positive but do not have COVID). The background here is that no test is perfect and every screening test used in medicine has some percentage of false negatives and false positives. Several anecdotes are cited in support, however real world data from countries that already have very low disease incidence, suggests that the concerns or false positives raised in this article are unfounded. New Zealand, for example, has tested an average of 2127 people per day from July 1-22, with an average of 1.2 positive cases identified per day—an average % positive of only 0.07%. In order for the authors assumptions to hold, all of the positive tests reported there would have to be false positives—highly unlikely as New Zealand still has symptomatic patients. Therefore, real-world evidence from standard PCR based COVID testing in low incidence populations suggests that the concern for high rates of false positives raised in this article is overhyped.