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Science & Technology

Symptomatic, Asymptomatic, Presymptomatic: Who Can Spread The Coronavirus?

MARY LOUISE KELLY, HOST:

Yesterday, a World Health Organization official said that it is, quote, "very rare" for the coronavirus to be transmitted from asymptomatic carriers. Today she walked that statement back. Maria Van Kerkhove, a top epidemiologist at the WHO, went on Facebook Live to clarify what she meant.

(SOUNDBITE OF FACEBOOK LIVE VIDEO)

MARIA VAN KERKHOVE: 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. And in that, I used the phrase very rare. And I think that that's misunderstanding to state that - that asymptomatic transmission globally is very rare.

KELLY: We've got NPR global health correspondent Jason Beaubien here to try to help us understand what she is saying now. Hi, Jason.

JASON BEAUBIEN, BYLINE: Hi, Mary Louise.

KELLY: May I ask you just to cut to the chase? Can people who do not have symptoms transmit the virus or not?

BEAUBIEN: Yes, they can. There's documented evidence of people who are not showing any symptoms transmitting this virus to others. And there are studies in laboratories that have also found that there's transmissible virus in infected people who have not yet gotten sick or may never get sick with COVID.

KELLY: So for those of us still confused by her clarification today, why was she saying it is very rare for people who don't have symptoms to spread the virus?

BEAUBIEN: So this comes down to the terms here. And let's first start with asymptomatic. Van Kerkhove, she's the scientist, and she's using asymptomatic to refer to something very specific here - coronavirus carriers who are infected but never end up getting sick. And there have been some cases where they have passed on the virus to others. And then there's what Van Kerkhove calls pre-symptomatic. These are people who've been infected, haven't yet gotten sick but will get sick. And they definitely can spread the virus, and there's plenty of evidence of this.

KELLY: But if both these groups have got the virus and both can spread the virus, why is the distinction between asymptomatic and pre-symptomatic so important from a transmission point of view?

BEAUBIEN: Because the pre-symptomatics potentially pose a far greater danger to transmit than the asymptomatics. But until one of them starts showing symptoms, you can't tell them apart. And what Van Kerkhove had been saying yesterday at the press conference - saying it was very rare to see documented cases of an asymptomatic person transmitting the virus on to someone else. That doesn't mean it doesn't happen. They are probably less likely to transmit it, but that hasn't been proven. And Van Kerkhove was trying to say this in the context of - if we could just go after the symptomatic cases, contain those, we can bring these outbreaks under control.

KELLY: How common is it, by the way, to have a carrier who is asymptomatic?

BEAUBIEN: So again, we don't have a solid answer on that either. The virus is new. Studies are ongoing. There's different studies that are coming up with wildly different rates of potentially asymptomatic carriers. But just today, the CDC actually released results of an investigation that it did on that outbreak on the USS Theodore Roosevelt. And they had roughly a thousand servicemembers test positive on there, and that was one that they had to pull into Guam. In that study, the CDC investigators found 18% of the sailors who tested positive for coronavirus - had the virus in their bodies - but 18% of them showed no symptoms at all. The CDC, however, did not look at whether those asymptomatics did or did not transmit the virus on to others.

KELLY: Fascinating - so much still to learn about this virus.

NPR's Jason Beaubien, thank you.

BEAUBIEN: You're welcome. Transcript provided by NPR, Copyright NPR.