Device detects drop in Covid in patients' breath

Cases of Covid-19 have risen around the country in recent weeks, coinciding with, although not necessarily because of the arrival of a couple of new variants of the virus. At the moment scientists and public health bodies like the World Health Organisation are issuing reassuring messages that the existing immunity among the population seems to be strong enough to keep us safe from severe disease, and that seems to be the experience in most healthcare settings: cases, but not consequences. Nevertheless, the UK at least has brought forward its planned Covid booster vaccine roll out as a precautionary measure, and both Moderna and Pfizer, who manufacture the vaccines we're currently using, have updated the recipes in their shots to keep pace with the evolution of the virus and have said these do appear to confer protection against the latest variants.

But one thing that has remained uncertain is when people really are infectious with the disease, and for how long. One way to find out is to test people regularly across their illness and measure how much virus they are actually shedding in their breath in a standardised way. This might help us to understand better how best to do effective infection control and isolation to keep Covid at Bay. From Northwestern University in Chicago, Greg Lane.

Greg - We were interested in how much virus you are exhaling from the moment you start to feel symptoms until those symptoms abate. So we had to come up with a way to measure people's exhaled breath while they were isolating at home. To do that, we developed this low tech, simple device that people are able to use at home to self collect exhaled breath, condensate samples twice a day over the course of their whole infection, and use those samples to create this map of exhaled virus over the course of their sickness.

Chris - We can get into the nitty gritty in a second of what constitutes infectious particles and so on, but, first and foremost, what did you actually find when you did this? When does a person begin to shed virus and when does their infectivity peak and when does it tail off?

Greg - We worked with people from the day of symptom onset, and what we found is that you exhale a lot of virus for the first eight or nine days of infection, and then there's a steep drop off after that. And it was consistent across all of our participants. There are occasional peaks after day eight in some individuals, but they were never as high as what we saw in those first eight days.

Chris - Now, presumably these are people at home, so these are people who are well enough with SARS-CoV-2 infection not to need hospitalisation. So is this a subgroup then, and is therefore only really representative of people with mild infection?

Greg - I think that's right. These were outpatients, in other words, people who came to Northwestern to get tested for Covid but were well enough, as you say, to go home. But this is an important subgroup. It's perhaps the much larger subgroup of people across the world who are infected with Covid-19.

Chris - Is there a possibility then that if you had a highly symptomatic person and a person with very few symptoms side by side, the more symptomatic person is shedding more virus. Or, if you do stratify like that, do they shed the same amount of virus and it's just the way the body reacts that's different?

Greg - That's a good question. And we looked at this, so we did find people who have more severe symptoms tend to be exhaling more virus. But again, there's a lot of variability there. And we did find people with no symptoms at all, resolved symptoms, who are exhaling very large amounts of virus and people who are experiencing severe symptoms who are exhaling very low amounts of virus. So the takeaway here is that there's a lot of variability in the dynamics of exhaling virus, both over the course of your disease and as it relates to your symptoms. You can't rely on your symptoms to tell you whether you're infectious.

Chris - How did you test how much virus was there and were you measuring just the signature of the virus or were you actually measuring its infectivity, its ability to infect another person?

Greg - We use the same method, I think, a lot of people would be familiar with - the more sensitive test for a nasal swab, which is called PCR. So this is a lab-based test that is highly sensitive and it can detect very small levels of the virus, but it's viral RNA. So this is a signature of the presence of the virus, but it doesn't measure whether that viral copy is viable, whether it could infect somebody. So what we're establishing here is that signatures of the virus are on breath and it would be a next step in our study, it's something we're working on now, to determine what portion of that exhaled viral load is infectious and constitutes a risk to the people around you.

Chris - I ask that because a lot of the studies that were done in other ways looking at when people appeared to be most infectious from swabs and so on during the pandemic seemed to hinge on day five or six. So is that because at day five or six they stopped shedding viable virus that's capable of infecting people, but they still shed the residuum of the infection, a signature that you can pick up with tests like you used, and that could account for that disparity?

Greg - It could. This is a very important question and this is why we are focusing on determining this portion of viable virus in your breath. It could be the case that when you measure the presence of the signature of the virus in an oral swab or a nasal swab, that measure may differ from the amount of virus that appears on breath. And the reason for that is that as you imagine swabbing your own nose, you're taking a sample from a very, very small area, but the virus replicates throughout your respiratory tract. And when we measure breath, we're measuring from all of those areas that contribute to these little aerosols and water droplets on your breath. So that's an open question and it's one we're trying to address now.