DETROIT – In a press briefing Monday, the World Health Organization (WHO) made a confusing statement that appeared to contradict the Center for Disease Control and Prevention (CDC).
In the statement WHO noted it appears to be rare that an asymptomatic person transmits the virus.
It seems like a big difference from the guidance by the CDC. On Tuesday, WHO walked that statement back issuing a clarification that better defines what they were referring to when they used the term asymptomatic spread, which they now estimate at around 16 percent of cases.
Although other studies have suggested a much larger number and it is critical for planning and modeling that we are as accurate as possible.
Determining how often people who have COVID-19, but don’t have symptoms can spread the virus is really important.
If someone doesn’t have symptoms but is contagious, it makes symptom screening questionnaires and temperature checks far less useful -- and it makes universal precautions like distancing, hand washing, and masks far more important.
When we talk about asymptomatic people spreading the virus we could really be talking about two very different things.
First off, it is well recognized that people are contagious at least one to two days before they develop symptoms. That’s really known as presymptomatic spread.
Technically in those one to two days, the person is asymptomatic, but they will eventually develop symptoms. This needs to be distinguished from truly asymptomatic but infected people.
In the truly asymptomatic, symptoms will never develop, and the only way to identify these people is by testing.
A final category that can complicate things are people who are minimally symptomatic. Unless those people are carefully questioned about even the tiniest problem, they may not recognize symptoms and become grouped with asymptomatic people.
Circling back to the WHO clarification issued Tuesday, its 16 percent estimate of asymptomatic spread appears to refer specifically to people who are truly asymptomatic.
Notably the CDC, in its guidance for developing COVID-19 pandemic models used an estimate of 35 percent of asymptomatic spread and a study published on June 3 in the Annals of Internal Medicine analyzed 16 other reports of asymptomatic spread and estimated it to be as high as 40 to 45 percent, although they could not completely assess what percentage might have been presymptomatic spread.
In terms of whether these differences in asymptomatic versus presymptomatic spread change anything right now the answer is simple.
No they do not, regardless of the which group an infected person ends up in, the bottom line is it’s well accepted that a significant percentage of infected people can spread the virus when they do not have symptoms, still emphasizing the need for ongoing broad precautions.
- Special section: COVID-19 data