A stir was caused when World Health Organization (WHO) expert Dr Maria Van Kerkhove said that cases of asymptomatic transmission of COVID-19 are “rare”. She has since retracted her statement, as modelling studies have shown that up to 40% of transmissions are attributed to carriers who show no symptoms.
In the same breath, Kerkhove maintains that real world data shows that this is still a rare event. When she made her statement, she was not referencing modelling studies.
Kerkhove made her original statement at a WHO press briefing on Tuesday June 9, and did not at the time stipulate that her comment on the rarity of COVID-19 asymptomatic transmission was based on two or three unpublished studies, as well as unpublished data collected from countries across the world.
“I used the phrase ‘very rare’ and I think that it’s a misunderstanding to state the asymptomatic transmission globally is very rare. What I was referring to was a subset of studies. I was also referring to some data that isn’t published,” she said to clarify her comments.
While Kerkhove can confirm that there have been asymptomatic infections, the frequency of this is still one of the facets scientists are investigating about the coronavirus. One complication is that some people who appear to be asymptomatic may, in fact, be pre-symptomatic. This is a limited time frame where an infected individual spreads the virus to others before showing symptoms – and these are often mild.
“They may not register that they are sick,” Kerkhove said.
One of the critical questions is how long the pre-symptomatic period lasts, as well as just how infectious the carrier is in the days before they display symptoms of the virus.
“It is not only who is transmitting to others, but when,” she said. “The data is very preliminary. It appears that people have more virus in their body around the time they develop symptoms.”