Enlarge /. WHO technical director and epidemiologist Maria Van Kerkhove attended a virtual press conference on COVID-19 from WHO headquarters in Geneva on April 6, 2020.
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An expert from the World Health Organization made brief comments on Monday about the asymptomatic spread of COVID-19, which triggered a firestorm of confusion, backlash, and criticism.
Some public health experts quickly turned to the organization for poor messaging. Others tried to clarify what the WHO expert might have been trying to say. Still others have quickly indicted evidence-based strategies to fight the pandemic virus.
On Tuesday, WHO responded with a live Q&A on social media to address confusion and ongoing questions about the broadcast. In it, the WHO expert, who made the confusing comments on Monday, tried to clarify the problem and add context and reservations. But the answer can still be confused and frustrated.
Here we will try to clear the air of what has been said, what should have been said and what we are doing about the transmission of the new SARS-CoV-2 coronavirus and what is not.
What we know
First and foremost: people who are infected with SARS-CoV-2 but have no symptoms can – and do – transmit the virus to other people. In other words, people who appear healthy and do not have classic COVID-19 symptoms may actually be infected and can still pass the virus on and infect other people.
We know that this is possible and we know that it happens. No public health expert says otherwise – not even at the WHO.
We also know that there are two scenarios in which this can happen.
Some people who are infected with SARS-CoV-2 never develop symptoms from their infection – that is, from the time they are infected with the virus to the time the virus is no longer in theirs Cells multiply and are therefore no longer infected. The WHO regards these cases as "asymptomatic". If asymptomatic people transmit the virus to another person during their silent infection, this spread is considered "asymptomatic transmission".
In the other scenario, people who are infected and have no symptoms initially develop symptoms later, sometimes very mild. In fact, data suggest that a person can test positive for the infection one to three days before the symptoms appear. Symptoms usually develop between five and six days after exposure to the virus, but the process can take up to 14 days. Studies have shown that virus excretion – how much infectious virus particles emerge from an infected person – appears to be highest in the few days around the first day of symptoms.
This means that infected people can test positive for the virus and spread the virus to others before their symptoms develop.
So far, so good
This is where it gets confusing.
Regardless of when an infected person develops symptoms in the course of an infection, it is assumed that they have one symptomatic case overall.
However, if they test positive while showing no symptoms and then develop symptoms, they are classified as "pre-symptomatic" at the time of the test. If the case cannot be followed up to determine if they ever develop symptoms, they can be classified as "asymptomatic".
If you transmit the virus to another person while you are "pre-symptomatic", this can be considered "pre-symptomatic transmission" or "asymptomatic transmission" because you were asymptomatic at the time of transmission.
I use "could" in these sentences because different experts and studies use these terms differently. Do you see how confusing that is?
Then of course there are direct symptomatic cases that we understand very well. These cases indicate that SARS-CoV-2 spreads in large breath droplets that come out of the mouth or nose through sneezing, coughing, speaking loudly, or breathing hard. We know that these droplets tend to be no more than a meter or two from an infected person. The most effective way to prevent this type of transmission is to keep your physical distance from other people who may be infected. If physical distancing is not possible, experts recommend a face mask.
To summarize it:
Symptomatic case Someone who is infected and has symptoms at some point.
Asymptomatic case = Someone who is infected but never develops symptoms.
Pre-symptomatic = The phase of a symptomatic infection in which a person may test positive for the virus and / or spread the virus but has not yet developed any symptoms.
Pre-symptomatic transmission = Spread of the virus from a symptomatic case during its pre-symptomatic phase.
Asymptomatic transmission = Virus spread from an infected person with no current symptoms. This transmission can come from a pre-symptomatic person or a really asymptomatic case, depending on how the terms are used.
The WHO has only applied the asymptomatic transmission consistently if it is really asymptomatic cases.
What we don't know
There is a lot about transmission that we don't know.
For one, we have no clear picture of how many infected people have symptomatic cases and how many asymptomatic cases have. Estimates of the percentage of asymptomatic cases vary widely, with some ranging between 4 and 45 percent.
From there, we are not sure which types of cases transmit the infection and when – that is, we do not know what percentage of the asymptomatic cases transmit the infection to others. Some data suggest that a small portion – say 6.4 percent – of the asymptomatic cases transmit the virus, while other model data estimate that 40 percent of all pandemic transmissions come from asymptomatic cases.
In symptomatic cases, we do not know how much of the infection spreads before symptoms appear. There are also a large number of estimates here.
The WHO found that data showed that symptomatic cases still appear to be behind the majority of new infections, regardless of whether they occur in the pre-symptomatic phase or not. Therefore, the organization is pushing for the best strategies to isolate symptomatic cases and to track, quarantine and test contacts.
What the WHO said originally
Amidst all this uncertainty and confusion, the issue of asymptomatic transmission was raised in a regular WHO press conference on Monday June 8th.
A Reuters reporter found that a Singapore health official had reported that almost half of the new cases there appeared to be asymptomatic. The reporter asked WHO "whether it is possible that (asymptomatic cases) play a larger role than originally thought by the WHO to spread the pandemic and what political implications this could have."
WHO's technical director, COVID-19, Maria Van Kerkhove, provided the notorious answer (transcript here).
Kerkhove – an epidemiologist – first tried to define the above case terms – asymptomatic, pre-symptomatic – and to explain that the WHO does not use the term "asymptomatic" at face value. But her answer was confused and fragmented.
“When we return to and discuss with a number of countries – first, how are these asymptomatic cases identified? Many of them are identified by contact tracking, which we would like to see you have a known one. Hopefully, if you find your contacts, they are already in quarantine and some of them are being tested, ”she said. “Then take in people who may have asymptomatic or no symptoms, or even mild symptoms.
"The other thing we find is that when we go back and say how many of them were really asymptomatic, we find that many have really mild illnesses, they are not COVID symptoms, which means they can They have never developed, may not have a noticeable cough or shortness of breath, but some may have a mild illness. "