Covid-19 and wild swimming

In the UK, about 4 million people swim outdoors in pools, seas, lakes and rivers. I can’t find a figure specifically for rivers, but swimming in the River Cam between Grantchester and Cambridge is certainly popular — the Newnham Riverbank Club has several hundred members, and many more people swim from other points in Grantchester Meadows — and I believe this is true at many other sites around the UK, and in many other countries.

It’s a little surprising that there has been almost no discussion of possible covid-19 transmission risks from wild swimming. Here is one interesting but inconclusive article, focussing mainly on possible risks from sewage. This also contributed to cautious advice during peak lockdown. Nonetheless, government guidance since mid-May has been that wild swimming per se is low risk so long as social distancing is maintained.

It may well be; I hope so. But we’ve learned during the pandemic that there seems to be a high transmission risk in settings ranging from choir practice to meat-packing factories. Afterwards, very plausible explanations have been given, but no one seems to have identified the risks in advance. Might river swimming at popular sites be another potentially risky setting?

I have some concerns, which may not stand up to empirical test but which I haven’t seen considered carefully. My worry isn’t about transmission from sewage: I’m going to assume that swimmers will avoid sewage-contaminated waters. The worry is that person-to-person transmission while swimming in a river might be much more effective, at much longer distances, than is generally true outdoors, or even perhaps indoors.

First, let’s look at the survival of coronaviruses in water. This study looks at other coronaviruses, but I’ll take it as applicable to the covid-19 virus in the absence of contrary evidence. In the most hostile aqueous enironment studied, primary filtered effluent at 23C, 1% of coronaviruses remained active after 1.5 days. The deactivation rate seems to be roughly time-independent, which gives us 10% still active after 18 hours, or ~90% still active after 1 hour. If someone sneezed into your river an hour ago, any coronaviruses they expelled are likely still active.

Ok, but we’re also not sure how long coronaviruses stay active in air or on surfaces. You can keep a social distance — let’s say the recommended 2m — while swimming, just as you would elsewhere, so what’s the problem?

Here’s one possible concern. In the air, droplets fall to the ground, and aerosols disperse in all directions. If someone coughs or sneezes while swimming in a river, droplets go into the water surface. What about aerosols? I don’t know — we need a fluid dynamicist and probably a range of experiments. But rivers have banks, and a direction of flow. Maybe aerosols mostly stay in a mist not far above the river, and over time a fair fraction maybe end up in the river.

And here’s another concern. Almost no one wears a mask while swimming (and it’s not obvious any standard mask would be effective). Few people wear goggles while swimming in the wild. Your eyes, nose and mouth are all close to the water surface — right in the zone where viruses are potentially concentrated. You’re often breathing hard, splashing water into your face, probably inhaling and swallowing some, and of course inhaling any aerosols or droplets above the surface.

Still, rivers are big, sneezed droplets are small. Surely they quickly dilute to irrelevance? Well, let’s try to estimate. A sneeze might emit 200 million viruses; an infectious dose might be 1000 viruses.      So you need to inhale just 1/20000 of the viruses from a single sneeze to be infected.  Let’s first think about the aerial route. Take a 10m river with 1m high banks; suppose the cough/sneeze spreads over 1m along the river, 1m above the river, and 10m across the river. Crudely idealizing, suppose that for some while it moves downriver as a 10x1x1 box.      Your breath has volume ~ 6 litres.     is 1000 litres.     If you breathe in while going through the box, you might inhale of the 200 million viruses, i.e. 12 infectious doses.     For how long is the rectangle model good enough to give roughly right answers?       It’s very hard to say without empirically modelling.      At a guess, if someone sneezes 10m upstream from you, the situation is worse than the model suggests: the viruses won’t spread out across the river by the time they reach you.     At another guess, if they sneeze 1km upstream from you, the situation is better –- maybe much better — than the model says.    But I don’t know; I wonder if anyone does.

Now suppose most of the viral load is in the water.   The River Cam is  ~6mx2m = in cross-section.     For a sneeze with 20 million viruses to dilute to 1 virus per cc, assuming equal dilutions at all depths and across the river, it would have to spread out uniformly over a length of 166m.     If it stays in the top 20cm — maybe more reasonable, for a long while, for a slow river on a calm day — it has to spread over 1.66km. If it stays in the top 2cm, the length is 16.6km. For 1000 viruses per cc, staying in a 2m x 10cm cross-section, the length is 16.6m.   Take a look at the photo above, and ask how confident you are that the river flow will quickly stretch out sneeze particles over that volume. Now 1cc is about a quarter of a teaspoon. How confident are you about not exposing your eyes, nose and mouth to ¼ teaspoon of water while swimming?

My conclusions? Swimming in an uncrowded unconfined area of sea seems a much better bet: there’s much more turbulence and no confinement except the beach boundary. My guess is that dilution is effective enough in the sea if you stay well away from others. I’m not completely confident about this, but personally, I’d take the risk.

If you’re going to swim in a river, I’d try to be upstream of, well, ideally everyone. All else being equal (don’t let a real drowning risk replace a hypothetical infection risk!), you should maybe prefer large faster-flowing rivers to small slow ones. I would keep well away from anyone not in your household if they’re upstream of you. I’ve no idea whether 100m might be a safeish distance in a small river; I don’t see any reason to think 2m is.

For me, regretfully, the unknowns deter. I love Cam swimming, but haven’t indulged this summer. The risks clearly aren’t huge — no clusters of cases have been reported among wild swimmers in Cambridge, or anywhere else as far as I’m aware. But the general infection rates in Cambridge, and most of the UK, have, thankfully, been low over the summer. There may thus have been few or no asymptomatic but infected people swimming in the Cam, just as there may be few or none in any given pub or gym in Cambridge. This leaves a niggling worry that wild swimming, like bar-hopping or gym-going, may nonetheless be a relatively risky activity.

If you can produce more reassuring data or better arguments, I and (at least) one or two other cautious swimmers would be very grateful.