Thread by @joel_c_miller

COVID-19 has proven itself to be very explosive. It's particularly dangerous because people who don't (yet) feel sick can transmit.

However, I'm going to argue that it's perhaps easier to eliminate than we might expect (at least in Australia)

We've seen success in New Zealand in elimination. Most states in Australia appear to have eliminated it.

In fact there's reason to suspect that Australia would have successfully eliminated it except for a leak from quarantined travelers.

And Australia's official policy wasn't elimination. It was just suppression.

What if they now actively chase elimination?

First, it's not clear that the current outbreak can be controlled. So the response right now is to try to get things under control before we decide whether to push for elimination.

But I’m a modeller, so I have some room to think about what if scenarios that may not be relevant for 4 weeks.

What I want to speak about now is why that explosiveness is actually a weakness for the virus when it comes to elimination.

The R for SARS-CoV-2 is somewhere in the 2-5 range. Probably on the low end. With significant movement restrictions in place it's below 2, probably below 1.

Despite having a relatively moderate value for R, it's been very explosive. This is because there are Super-Spreading Events.

Every once in a while the virus gets lucky and 10, 20 or even more people get infected at once.

But the average is something like 1

What that means is that many/most people don't transmit, or only transmit to 1 person.

Even if contact tracing misses some people we're not at high risk.

What about those super-spreading events?

Some of them don't happen because that person didn't go to the wedding/birthday party.
Some happen, but are much smaller than they would have been.

But some still happen. (this is probably what happened in Melbourne).

When one of these events happens, and case counts are low it's much more likely to be caught than most transmissions because of robust testing.

So many of the others in that cluster are also found.

Infections are more likely to be found if they are in a large cluster.

And SARS-CoV-2 relies on large clusters more than most diseases.

So test/trace/isolate is more effective for SARS-CoV-2 than it would be for most diseases.

This is a case where heterogeneity in the disease may be important - it is easier to eliminate this disease than a simple homogeneous model would predict.

Whether we get the disease back under control is largely up to the choices the public makes.

And whether we can get that final push to eliminate it will depend on decisions by policy makers and whether the public is willing to tolerate restrictions as numbers fall.

But there's genuine reason to believe that if the public is willing to tolerate it, we can repeat that success we almost had.

Many states successfully eliminated it.

It may yet happen again. The public would need to be willing to accept restrictions while case counts are low.

@timsmithMP: if the decision is made to do this, please resist the temptation to undermine it.