Clarifying the WHO’s comments on asymptomatic transmission

On Monday, a WHO official, Dr Maria van Kerkhove, said in a statement that asymptomatic transmission of COVID-19 was “very rare.” Subsequent media reports added to the confusion.

CNBC opened a story on this stating that “Coronavirus patients without symptoms aren’t driving the spread of the virus, World Health Organization officials said Monday, casting doubt on concerns by some researchers that the disease could be difficult to contain due to asymptomatic infections.”

While the messaging from the WHO was very confusing, the CNBC paragraph above is misleading. On Tuesday, the WHO walked back the comments and gave some further context but I still saw some people asking for clarificartion on social media.

Where does the confusion stem from?

There is a difference between asymptomatic, presymptomatic and paucisymptomatic:

  • Asymptomatic: is someone that has COVID-19 AND is not currently showing symptoms AND will not eventually have symptoms. These people NEVER have symptoms, and they are very hard to find by most studies.
  • Presymptomatic: is someone that has COVID-19 AND is not currently showing symptoms BUT will eventually have symptoms. Studies might sometimes classify these as asymptomatic if there isn’t a follow up further on.
  • Paucisymptomatic: is someone that has COVID-19 AND currently has symptoms BUT they are so mild that maybe they don’t register (such as a very mild fever or a mild cough that could be confused with alolergy symptoms). They can get classified as asymptomatic in studies.

Right now we don’t know either how many people are TRULY asymptomatic or how big a contribution they have to the spread of the disease. The WHO was indicating that the spread from TRULY asymptomatic people might not be that common, not that just because someone is not showing symptoms that they are not spreading the disease.

A STAT News article on Tuesday described what led to the confusion: “To some, it came across as if the WHO was suggesting that people without symptoms weren’t driving spread. Some studies, however, have estimated that people without symptoms (whether truly asymptomatic or presymptomatic) could be responsible for up to half of the spread, which is why the virus has been so difficult to contain.”

Dr Natalie E. Dean, Assistant Professor of Biostatistics at the University of Florida specializing in emerging infectious diseases and vaccine study design, tweeted this short thread on the topic:

Several other scientists also took to Twitter to clarify. Another good thread on the topic:

The Science Media Centre also gathered reactions from scientists to the WHO announcement.

So what does that mean for an individual? Has anything changed in how we behave or think about risk? Not really and Popular Science had a handy explanation chart:

Some good articles on the topic to delve deeper: