The asymptomatic spread is very significant in COVID-19

Norman Swan: Hello, this is Coronacast, a podcast all about the novel coronavirus. I’m physician and journalist Dr Norman Swan.

Tegan Taylor: And I’m health reporter Tegan Taylor, it’s Friday, 17 April.

The following I copied from the end of the transcript!

Tegan Taylor: So one of the things we do on most episodes of Coronacast is talk about a piece of research and that’s what we’re going to do today. And we’ve got one that you want to talk about, Norman, about when people are most infectious.

Norman Swan: Yes, this is a study which comes from China, looking at the shedding of virus. It’s fine to say that you’ve got the virus on board, but are you actually losing the virus into the atmosphere and spreading it to other people? And they did a study in China of about 77 pairs of people who…they knew the transmission path. Just under half of people spread the virus, they were shedding the virus prior to when they were symptomatic, in the pre-symptomatic stages. So it confirms what we know, and we’ve already had one question today acknowledging that, is that the asymptomatic spread is very significant in COVID-19. And so although this comes from earlier on in the epidemic, we are keeping on getting…in science the more reproducibility you get of results, the more confident that you can be. So we were saying maybe two or three weeks ago, four weeks ago it was 50% asymptomatic, that came from Korea I think initially. And now we know from a very specific study really looking at virus shedding that it is around the 40% to 50% range, which means that asymptomatic spread is significant and it’s why people are starting again to talk about masks so that asymptomatic people don’t spread it in public, and that we are careful with social distancing because you don’t know who you’re coming in contact with.

Tegan Taylor: One of the things I found really interesting about this study was it compared SARS-CoV-2 to the original SARS which…you were most infectious with SARS several days into having lots of symptoms, so that made it relatively easy to lock that virus down, whereas this is transmitting before people have symptoms, and so it’s really hard to get a handle on that when people haven’t even shown that they are sick yet.

Norman Swan: That’s right. I think it was day two or something like that with SARS-1, it was where you really began to be infectious and then continue from there. So even if you got somebody in the door and they said, look, I’ve just got symptoms from last night, you had some time here, and then you knew exactly…and that’s why healthcare workers were so much at risk because largely it was healthcare workers who came in contact with these people when they were symptomatic.

Asymptomatic here changes the nature of it, and this is why we are going to have to accept…even if we got to elimination and we started to lift controls, we are going to have to check everybody with a cough and a cold. And you say, well, why would you do that when 50% is asymptomatic? The only way to find asymptomatic people really systematically is find everybody with a cough and a cold, test them, and there’ll be a very small number who are COVID-19 positive, so do tens of thousands of tests for one or two positives. But for those one or two positives we find everybody they’ve come in contact with, whether we use one of these tracing apps or traditional contact tracing. And those contacts are potentially the people who will have asymptomatic spread because you know when they came in contact with them, you know what the average incubation period is and you can either test them or you can put them into strict quarantine and test them as you go along.

But that’s how you prevent asymptomatic spread, is find people with cases, find the people they’ve come in contact with. You always miss one or two people who are asymptomatic in the community, but you’ll get most of them. Isolate the person with COVID-19, strictly quarantine the people they come in contact with, testing, and then you’ll start to really get a handle on asymptomatic spread.

Tegan Taylor: Well, that’s all we’ve got time for in Coronacast today, but, as a reminder, Paul Kelly the Deputy Chief Medical Officer is coming to speak to us next week. Send us your questions for him by going to abc.net.au/coronavirus, and make sure you use the word Coronacast in your questions so that we can find it.

Norman Swan: And if you like the show, give us a rating and please review us at Apple Podcasts, we really like reading them and it helps others find us. Tell a friend about us too, you can find us at Apple Podcasts, you can find us on Spotify and Google Podcasts, as well as the ABC Listen app. We’ll see you on Monday.

Tegan Taylor: See you then.