Home Health News People who do not show COVID-19 symptoms can and do transmit it to others; physical distancing and face masks effectively reduce the risk of transmission – Health Feedback

People who do not show COVID-19 symptoms can and do transmit it to others; physical distancing and face masks effectively reduce the risk of transmission – Health Feedback

26 min read

The WHO stated that asymptomatic spread of COVID-19 is “very rare”, therefore physical distancing and face masks are not necessary
Imprecise: The scientific definition of the word “asymptomatic” refers only to a very small subset of infected people who never develop symptoms during the course of their infection. However, the public tends to interpret the word as also including presymptomatic individuals—those who are infected and not yet showing symptoms, but eventually go on to do so. The WHO official was not referring to presymptomatic individuals in her statement.
The WHO uses the scientific definition of “asymptomatic”, which refers to people who never develop COVID-19 symptoms during infection with SARS-CoV-2. It is inaccurate to refer to all people who are not showing symptoms as “asymptomatic”, because presymptomatic people also appear healthy initially yet can still transmit the virus to others while showing no symptoms. As such, practicing physical distancing and using face masks or face coverings remain necessary for breaking the chain of transmission.

FULL CLAIM: The WHO stated that asymptomatic spread of COVID-19 is “very rare”, therefore physical distancing and face masks are not necessary; asymptomatic people are those not showing symptoms of illness


Claims that physical distancing (or social distancing) and using face masks or face coverings are not necessary went viral on Facebook in June 2020 (see examples here and here), following remarks by World Health Organization (WHO) official Dr. Maria Van Kerkhove at a press conference on 8 June 2020.

Van Kerkhove stated that asymptomatic transmission of SARS-CoV-2 is “very rare”. After the comment sparked widespread claims that the virus that causes COVID-19 can only be spread by individuals with obvious symptoms of the disease, Van Kerkhove attempted to clarify that she was using a much narrower scientific definition of “asymptomatic” that refers only to people who never develop symptoms during infection and does not refer to individuals who are presymptomatic. Presymptomatic individuals are those who are currently infected with SARS-CoV-2 but appear healthy and only develop symptoms later.

The word “asymptomatic” has been widely misinterpreted by some media outlets, including CNBC, to include presymptomatic individuals. Because public health measures like physical distancing were implemented out of concern that seemingly healthy individuals could transmit the disease, many have interpreted the WHO official’s remarks to mean that such measures are no longer necessary.

It is inaccurate and misleading to use the word “asymptomatic” to refer to people who are not showing symptoms, as several media outlets have done, because many COVID-19 cases involve people who appear healthy initially and only later develop symptoms (presymptomatic). These are not considered as truly asymptomatic cases by the WHO’s definition.

In fact, presymptomatic transmission is the primary reason that individuals suspected of being infected with SARS-CoV-2 are quarantined for 14 days. This is the estimated maximum number of days that it may take for symptoms to emerge (the so-called incubation period). Individuals can appear healthy during this time, but are still contagious.

Preliminary data shows that approximately 40-50% of individuals who test positive for COVID-19 are not symptomatic (including asymptomatic, presymptomatic, and paucisymptomatic whose symptoms are so mild that clinicians fail to detect the disease) at the time of testing[1]. And the U.S. Centers for Disease Control and Prevention have estimated that about 40% of COVID-19 transmission occurs before onset of symptoms.

Identifying who is uninfected, asymptomatic, or presymptomatic is a difficult task, as it requires repeated testing which demands a lot of time, manpower, and resources. As such, physical distancing and using face masks or face coverings are recommended as an effective stop-gap measure.

In addition to the misunderstanding caused by different interpretations of the word “asymptomatic” by health authorities and the public, Van Kerkhove quickly clarified in a briefing held the day after her original remarks that she had based her statement on only a “small subset of studies”, saying:

I was responding to a question at the press conference, I wasn’t stating a policy of WHO or anything like that. I was just trying to articulate what we know. I used the phrase ‘very rare,’ and I think that that’s misunderstanding to state that asymptomatic transmission globally is very rare. What I was referring to was a subset of studies.

In summary, the WHO official who called asymptomatic transmission “very rare” was not expressing an overarching WHO opinion or policy and was referring only to a very small group of infected individuals who never show symptoms during the course of the disease. Describing asymptomatic individuals as people who don’t have symptoms, as several media outlets have done, is inaccurate and imprecise. Presymptomatic individuals are also symptomless during the incubation period, yet are nonetheless capable of transmitting infection.

Presymptomatic SARS-CoV-2 transmission has been documented in several studies, although the level of transmission reported varies widely. One study in Nature Medicine estimated that 45% of secondary infections among people who had contact with an infected individual occurred during the individual’s pre-symptomatic phase[2] and a Eurosurveillance study reported an estimate of 48%[3], whereas other studies have estimated as low as 6 to 12%[4,5].

This emerging data on the frequency of SARS-CoV-2 transmission from seemingly healthy presymptomatic individuals strongly supports current recommendations from health authorities on the use of physical distancing and face masks as an effective strategy for reducing the spread of COVID-19 by breaking the chain of transmission.


Nina Fefferman, Professor, Department of Mathematics, University of Tennessee, Knoxville:
[This comment comes from an evaluation of a related claim.]
The reporting about the WHO conclusion misinterprets the WHO findings. The WHO narrowly defines “asymptomatic” to mean “never develops any symptoms at all, during the entire course of being infected”. This is not how the public (or the press) interpret “asymptomatic” – instead, the more common interpretation is about whether or not people who are not obviously ill with COVID-19 related symptoms can spread the infection. The WHO data says they CAN, but calls these people “presymptomatic” or else “paucisymptomatic”, meaning presenting few or unusual symptoms. This has VERY different epidemiological importance and it is entirely wrong to interpret the WHO announcement as a criticism of efforts to limit spread from seemingly healthy people who may transmit the virus to others. Those efforts are important and the WHO announcement does not address them directly.

A. Marm Kilpatrick, Professor, Department of Ecology and Evolutionary Biology, University of California Santa Cruz:
Early in the epidemic, there were questions about whether people without symptoms could spread the disease. This is critically important and fundamentally determines whether quarantining based on symptoms will be effective. See a fantastic paper on this by Fraser et al[6].

Abundant data now shows that transmission from people without symptoms makes up a huge fraction (~40-50%) of transmission[2,3,7]. But a key question has been whether transmission is from pre-symptomatic people who develop symptoms later or truly asymptomatic people who never do.

This question is not answered and some studies have shown evidence in support of asymptomatic cases being equally infectious as symptomatic ones, whereas others show milder cases to be less infectious. Here’s a study supporting each case[8,9].

The bigger problem is that contact tracing studies are flawed due to the delays between the time of infection and testing. When a person is tested, they usually have had at least mild symptoms for several days and were infected ~5.5 days before that[10].

As a result, the person that infected them was infectious 8 – 11 days ago. If that person was asymptomatic, there’s a moderate chance they would test negative when traced; unfortunately we don’t know the exact probability of that, as discussed in the thread below:

Data from symptomatic people by Borremans et al. suggest that this probability could be high; ten days after symptom onset, 30% of infected people test negative by swab[11].

It could be higher for asymptomatic people, and even if this is not the case, one could mis-attribute transmission due to delay. For symptomatic people you’d attribute them to be an infector based on the date of onset of symptoms (all studies I know of use an earlier onset date to identify infector vs. infectee).

Given the uncertainty in infectiousness and bias in contact tracing methodology, there is insufficient evidence for the World Health Organization to make this claim for asymptomatic transmission and given the confusion between the definitions of “pre-symptomatic” and truly “asymptomatic”, it’s a misleading PR disaster.

Liam Smeeth, Professor of Clinical Epidemiology, London School of Hygiene and Tropical Health:
[This comment was first published on the Science Media Centre.]
I was quite surprised by the WHO statement, and I have not seen the data the statement is based on.

It goes against my impressions from the science so far that suggest asymptomatic (people who never get symptoms) and pre-symptomatic people are an important source of infection to others. This is the main basis for steps such as self-isolation and lockdown – steps we know, from yesterday’s two Nature papers[12,13], have massively reduced the numbers of people infected and have prevented millions of deaths globally.

What is true is that once we have successfully interrupted community transmission, our main [focus] can shift [to] case finding and testing of symptomatic people and tracing, testing and self-isolation of their contacts.

There remains scientific uncertainty, but asymptomatic infection could be around 30% to 50% of cases. The best scientific studies to date suggest that up to half of cases became infected from asymptomatic or pre-symptomatic people.

Keith Neal, Emeritus Professor of the Epidemiology of Infectious Diseases, University of Nottingham:
[This comment was first published on the Science Media Centre.]
How much of a role asymptomatic transmission plays in the total number of new infections remains unclear but symptomatic people are responsible for most of the new infections of COVID-19.

This reinforces the importance of any person who has any of the symptoms of COVID-19 arranges a test for themselves as soon as possible and isolating [themselves] until they get their test result.

Everyone has a role to play in stopping COVID-19.

Babak Javid, Principal Investigator, Tsinghua University School of Medicine, Beijing, and Consultant in Infectious Diseases at Cambridge University Hospitals:
[This comment was first published on the Science Media Centre.]
In the WHO’s press conference [on 8 June 2020], it was suggested that patients with asymptomatic infection rarely transmit [COVID-19] to others. Dr. Van Kerkhove makes the important distinction between true asymptomatics (never develop symptoms), presymptomatics (don’t have symptoms at the time of testing positive for SARS-CoV-2 but later develop symptoms) and paucisymptomatics (have atypical or very mild symptoms). She describes that unpublished data suggest “asymptomatics” (not further clarified) do not transmit infection. This may well be true. Detailed contact tracing from Taiwan[14], as well as the first European transmission chain in Germany[15], suggested that true asymptomatics rarely transmit.

However, those (and many other) studies have found that paucisymptomatic transmission can occur, and in particular, in the German study, they found that transmission often appeared to occur before or on the day symptoms first appeared (i.e. presymptomatic transmission)[15].

Without having access to the data Dr. Van Kerkhove refers to, it is difficult to make any other assessment. I’m sure those data will become publicly available in due course. In the meantime, other data available from studies in several continents, confirming that presymptomatic transmission does occur, would suggest that being well does not necessarily mean one cannot transmit SARS-CoV-2. However, the important point [made is] that [even some] very mild symptoms that are not ‘typical’ of [COVID-19] (i.e. not having a fever or cough) may still represent someone who can be contagious. This has important implications for the track/trace/isolate measures being instituted in many countries.


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