How about zero.
Is zero good for you?
A few weeks ago, I had a quality piece of parental misunderstanding. I had got the impression, in a conversation involving my 9yo son and one of his friends, that he was dissatisfied with our state government retaining tight restrictions in an effort to continue to bring down coronavirus numbers. I later discovered that his concern was over the restrictions not being cautious enough, rather than being too cautious, but anyway… while I was labouring under the misapprehension that he thought we should have opened up more already, I set him and his friend a mental arithmetic challenge:
“What’s 1 times 2?”
“2”
“What’s 2 times 2?”
“4”
“What’s 4 times 2?”
“8”
… and so forth. He and his friend got up to 4096 before I decided it was probably enough powers-of-two practice for the day. I explained that that was — in simple terms — more or less how coronavirus numbers go in the population if you don’t do anything to reduce the spread. Then I asked him another question:
“What’s zero times 2?”
“Zero.”
“Times two?”
“Zero.”
“Times two?”
[cue exasperation] “ZERO!”
“Exactly.”
We’re now on our fifteenth day in a row of zero new COVID-19 cases in Victoria, Australia. Just over three months ago, we had 700 new cases per day. Our success in bringing it down to zero is being held as an example in many countries around the world who are experiencing massive waves of increasing case numbers, and heading back into varying degrees of lockdown as a result.
Only a few weeks ago, when our numbers were trending downwards but staying frustratingly above zero, widespread sentiment (if you listened to the media) seemed to be “we’re never going to be rid of this, we just need to find ways to tolerate ongoing low levels of community spread of this virus”. And a bunch of commentators repeatedly criticised the government for setting out a roadmap that had quite obviously set elimination as the goal (without ever saying that word). The obvious need to accept ongoing low levels of community transmission appears to have been news to all the other Australian states who were emphatically not talking about opening their borders to us while we continued to suffer low levels of community spread, whereas they had enjoyed months of no community spread at all.
And then our numbers collapsed the rest of the way. Zero. Zero. Two (oh well, we didn’t think it would last). Three (no, the other way!). Four (it’s spiking!). One. Zero. Zero. Zero. Zero. Zero. Zero. Zero…
The state government had permitted very cautious relaxation of some restrictions about a week before we first hit zero — kids were back at school, you could meet a small group of friends outside in a park, hairdressers could reopen, outdoor pools could operate with limited numbers — but otherwise, things were still very tightly restricted. Now suddenly, so much more is in reach. Cafes, pubs and restaurants are open for dine-in service rather than take-away only. We can go anywhere in the state. We can even have one or two people over to visit in our homes (very limited, but at least it’s allowed).
And considerable further lifting of restrictions is on the way too: other states are making plans to open their borders to us (although not for a couple of weeks yet), and by early next month we could well be allowed to do genuinely high-risk things, like have parties and concerts and… how is this possible? Because of zero. Because of the infinite (as a multiple) difference between zero and one.
But how can we be sure? Well, first of all, we can’t be… not yet. We know that something like 30–40% of COVID-19 cases are asymptomatic, or are so mildly symptomatic as to go unnoticed (especially in weeks which have seen extraordinarily high grass pollen counts in southeastern Australia). So what if there was an undetected infectious case out there two weeks ago? There could certainly have been. Then, under our current restrictions, let’s say they passed it onto one other person. Who would have been (on average) infectious about five days later. And perhaps might have passed it on to one other person. And around we go again. In the space of just over two weeks, with restrictions that still significantly limit the opportunity the average person has of passing the disease on to others, we might plausibly have sustained community spread through a chain of just four people from one initial case. At a 40% chance of each case going undetected, that’s roughly a 2.5% chance of any one sustained chain of infection remaining undetected with no infections being detected. That risk isn’t additive, mind you — if there were multiple undetected cases at the beginning each leading to a sustained chain of infection, the chance of them all remaining undetected is 0.025^n (where n is the number of four-person chains of infection). But right now, the chance that we have just one such chain is still too big to stake anything high-risk on it. In a couple more weeks, if there have been no further detected cases of local transmission, it would be fair to say that it is almost certain there is no ongoing community transmission in our state. Not certain, but almost certain. When it comes to risk management of this order, that is a meaningful threshold to achieve. It certainly looks like we’re on that track, but we want to be more sure than we are right now.
The difference between zero and anything more than zero is important in another direction, too. I’ve heard people talking with great concern about behaviour of people in “St Kilda last weekend” (for example), worrying that “in two weeks we’ll have case numbers climbing again”. If we’ve genuinely achieved zero local transmission (as it already looks like we have), then we won’t see case numbers climb, no matter how un-COVID-safe everyone was being. Because the virus isn’t magic: it can’t appear out of nowhere. It spreads from someone who already has it to someone (or — occasionally and significantly — dozens or hundreds of someones) who didn’t. Just as you didn’t worry about massive gatherings of people spreading SARS-CoV-2 a year ago, nor should you need to worry about massive gatherings of people in a location where community spread has been eliminated… because if no-one can bring the virus to the party, they can’t give it to anyone else either.
Am I saying that after another couple of weeks, we can all just go back to acting like SARS-CoV-2 doesn’t exist? Absolutely not. We still need widespread public awareness of the importance of getting tested if you have symptoms, no matter how mild. We still need to be absolutely intolerant of people attending work when they are sick (with anything!). We need strong ongoing public health measures to detect any reintroduced cases, to trace them fully and stamp out every possible chain of transmission so that we never need to go through the last three months again. It’s possible we’d need to be ready to temporarily and sharply restrict things if it were to pop up somewhere we weren’t expecting (just like New Zealand did with Auckland in August). And any low-cost things we can do to limit the chances of a sneaky reintroduced case spreading widely, we should just continue to do — continuing to require masks in shared indoor spaces seems like a no-brainer: it’s cheap, easy to do, and moderately effective in reducing risk of spread (at the very least, on public transport and in shared workspaces where it doesn’t interfere with the activity). Improved ventilation in indoor spaces. Continuing to let people work from home if it works for the employee. Keeping good records of who is where, when. These are all things which come at little cost and reduce the risk of an outbreak taking hold before it can be detected, and/or assist in stamping it out if it does.
We also have to be so, so vigilant about the ways the virus could be reintroduced. In Australia, there’s really only one way for that to happen: through international arrivals, either among those people crewing international operations, or Australians returning home from overseas, and anyone who is required to have contact with them during quarantine. I suspect we have learned our lesson on that one, given where Victoria’s outbreak came from, but there’s always a non-zero risk there, and we will need to manage those ongoing threats for months and months to come.
But beyond that — let’s have some concerts! Hell, let’s have some rehearsals! Let’s have team sports, and open-mic nights, and dinner parties, and pub trivia, and… you get the idea. Let’s hug our friends again. Let’s hug our grandparents again. This is what achieving zero can mean for the people who sacrificed so much to get there.
It looks like Victoria may well have done it. Australia may well have done it. But it won’t hurt to wait a couple more weeks, just to be on the safe side.
