How risky is community transmission in NSW right now? Reflections #36

Insight

While the risk of my choir going back to meeting is not very risky for us as thirty individuals, the risk of all choirs in NSW singing without restrictions is much bigger. I don’t think our choir should be singing together just yet.

Cases in Australia have been bumping along at around 10 new cases a day in the last few weeks. Today (28 May) there were 18 new cases, but yesterday there were six. How many of those are community transmissions?. Here is the best link on that – in the past week around 3 per day of those new cases (or around a third) have come from an unknown source (after investigation) – almost entirely in NSW and Victoria. So there are cases out there in the community that haven’t been identified.

So, how many unidentified cases are there? It’s very hard to tell, but given how much testing is going on, I would be surprised it there were more than 100 unidentified cases sitting out in the community – say 50 each in Melbourne and Sydney (there are probably some in other states, but not many).

So how risky would it be for my choir of 30 people to meet? We would be worried about one person unknowingly infecting everyone else (given the mixed evidence that choirs are superspreading events, we want to be cautious). The chances of one of my choir being one of those people is pretty small – simplistically one of us being one of the 50 out of the around 5 million people in Sydney.  My calculation is that is about 0.03% – a 3 in 10,000 chance.

But it isn’t quite as simple as that. If we were the only people getting together, the risk to us as individuals is tiny. But the government, to let us meet as a choir, isn’t just looking at a one-off event. They are looking at all of the choirs in Sydney getting back together, and equivalent sporting events, pub quizzes, etc, etc. So the asymptomatic carriers of the virus (who don’t know they are infectious) would be increasing their number of contacts and getting out and about.

In easing restrictions on weddings, funerals and religious services (to allow up to 50 people at religious services), the government advice in NSW was quite clear:

“Communal singing and chanting should not occur because of the high risk of transmission of the virus. Instead, measures such as one singer standing at least 3 metres away from others would be safer.”

The right way to think about this is about the whole population. If one of those 50 asymptomatic carriers in Sydney is at an event with 30 people, and becomes a superspreader to half of them, do we have the resources set up to trace all of their contacts quickly enough to stop the pandemic bursting out again? There is evidence of a big variety of level of spread – while the average person in an unconstrained society spreads the virus to 2-3 other people, this is an average, and some transmit to many people, some to none or one. The whole point of social distancing is to reduce the chance of people spreading the virus, particularly superspreading events. And while the evidence is mixed on whether choirs are superspreader events, the mechanics of singing do seem to point that way. Humming while wearing masks would be much less risky, but I’m not sure whether that would be that much better than a zoom call.

Right now in Sydney, the risk to an individual, even a susceptible one, of catching the disease from most activities (even choir) is very very low. But the problem is that if we all go back to normal, the nature of exponential growth means that that risk will grow very fast. If one of those 50 people managed to infect 30 others, and then those thirty went out to the shops, and maybe a sporting event, then we are back to serious growth in cases. So unfortunately, in choosing not to meet as a choir, we are doing our bit for the community. Even though our own individual contribution by not singing together is a vanishingly small reduction in the overall community risk, we are playing our part in continuing to reduce risk for everyone.

Links

This fascinating (preprint) study looks at whether investigating sewage can help to predict outbreaks of Covid19. And it is an impressive match! The study was done in New Haven, Connecticut, USA.

In conclusion, the SARS-CoV-2 RNA concentrations in primary sewage sludge were tracked throughout a COVID-19 epidemic and compared with traditional outbreak epidemiological indicators. SARS-CoV-2 RNA concentration in primary sludge closely followed the  epidemiology curves established by compiled COVI substantial policy implications.

This graph is amazing. It shows that measuring #SARSCoV2 levels in municipal sewage almost perfectly predicts forthcoming #COVID19 cases with a full week’s notice (R=0.994).

Life Glimpses

I’ve been enjoying watching dress codes on the various videoconferences I’ve been at. There is a subtle difference in dress codes when the meeting becomes more formal (such as a Board meeting), but it is much more subtle than it was when we were all in the same room.

In the last 10 years in Sydney, ties for men have become much less common, even at peak formality. Women’s dress codes are much more subtle than men (with much more of a sliding scale of formality), so the tie is the easier measure. Although I’m beginning to think that the measure is now whether the shirt can be ironed…

When you move the Board meeting to a virtual videoconference, ties are pretty uncommon. I think I might have seen one, possibly two out of the twenty or thirty possible attendees at meetings I’ve been at in the last few weeks. The most formal dress, though, has been at the two government inquiries I’ve watched – the Federal Government’s standing committee on economics, and the Royal commission into National Natural disaster arrangements. All the witnesses that I knew personally were as formally dressed as I’ve ever seen them.

Bit of beauty

I missed National Sorry Day, but it is still Reconciliation Week and I wanted to remember and acknowledge the mistreatment of Aboriginal and Torres Strait Islander people who were forcibly removed from their families and communities, which we now know as ‘The Stolen Generations’. Today’s bit of beauty is a picture from a beautiful ceremony of acknowledgement and reconciliation I went to on Australia Day a few years ago. This data on school suspensions shows we still have a way to go to close the gap for aboriginal students.