The easy answer is that we should take the exit ramp when the virus no longer presents a clear and present threat to the Australian public. To my mind this yields three logical exit points:
- a therapy or pharmaceutical measure is been found that substantially reduces the fatality risk associated with having COVID19,
- a vaccine is found that can be applied to sufficient Australians to confer herd immunity, or
- sufficient Australians have caught and recovered from COVID19, and that experience confers sufficient immunity for herd immunity to be achieved.
Herd immunity
Let's deal with the last one first. I view this as the failure scenario. Ultimately, if we fail to control the spread of the virus we will achieve herd immunity. At that point, there is no need for public health measures.While estimates vary, the broadly accepted basic reproduction number for SARS-CoV-2 is around 2.5. That means on average, someone with SARS-CoV-2 will pass it on to 2.5 other people. For a virus to propagate, it must have a basic reproduction number greater than 1. If a disease has a basic reproduction number less than one it will burn out.
The basic reproduction number is also affected by the proportion of the population that is susceptible to infection. We can divide the population into three cohorts, those who are susceptible to infection (S), those who are infected (I), and those who are resistant to (re)infection (R). We talked about this SIR model yesterday. A key insight from the SIR model is that as more people fall into R-category (resistant to (re)infection), this effectively limits the capacity for the virus reproduce. Herd immunity occurs when enough people are resistant to (re)infection that transmission to the susceptible population is less than 1 on average. By a neat quirk of mathematics, this occurs when the proportion of the population that is susceptible to infection is less than the reciprocal of the basic reproduction number.
$$For\ Herd\ Immunity:\ Susceptible < \frac{1}{R_0} = \frac{1}{2.5} = 40\%$$
In plain English: if more than 60 per cent of the Australian population has had COVID19, and if having had the disease confers immunity, the disease is likely to die out. But there is a cost. If we assume a case fatality rate of 0.9 per cent then this outcome would have seen almost 138,000 Australians die.
And this is not as easy as it seems, Left to run wild, my model suggests that the virus would far overshoot that herd immunity level before it burns out. Indeed the virus would have such a momentum, it would not burn out until around 90 per cent of the population had caught the virus. But note: no-one is suggesting we should let the virus run its natural course.
We will come back to the challenge of getting to herd immunity without overshooting.
In my view, we are most likely to end up here - with the virus spreading in the community - if we declare victory too early. If we decide that only a handful of people have the virus, so there is no need to continue with social distancing, then we could see things get away from us.
Another pathway here is if the collective cabin-fever of social distancing overwhelms us and people give up on the practice. Let's see how people feel about social distancing in three or six months.
Vaccination
The theoretically easy way to achieve herd immunity is vaccination. There are a lot of people around the world working hard on this. While many people are optimistic about the development of a safe and effective vaccine, it is not guaranteed. Two previous coronaviruses - SARS and MERS - do not have a vaccine, notwithstanding many attempts. But to be fair, these viruses came and went quite quickly.Therapeutic interventions
The third exit pathway identified above is a yet to be developed therapeutic intervention. It could be a treatment based on the blood serum of a previously infected person. Or it might be the discovery that a legacy medicine is also effective in treating COVID19. Or it might be something new. Again, while many people are optimistic about the development of a safe and effective therapeutic intervention, it is not guaranteed. This brings us to a fourth exit point the government needs to consider.V1/T1
When pilots prepare to fly, they identify several speed thresholds. For example, V1 is the speed beyond which a takeoff can no longer be aborted. While it is not a perfect analogy, the government needs to give thought to the point (I will call it T1) at which it is no longer sensible to continue hibernating the economy, in the hope that a vaccine or therapeutic intervention will be found.And when Australia reaches this point, what does the Government do. Does it transition to a national based economy, with strict and compulsory quarantine for all people coming to (or returning to) Australia? Or does it let the virus spread at this point, in a measured way, to reach herd immunity?
My suspicion is that the answer to this second question - what will the government do once it reaches T1 - will depend on whether we have eradicated SARS-CoV-2 within Australia.
Eradication in the short run
An ideal outcome in the short run would be the eradication of SARS-CoV-2 within Australia. If we can stamp out the virus locally, and the latest growth numbers are encouraging that this may be possible, restarting the local economy with strong borders becomes possible. But it is not the perfect solution. Some sectors of the Australian economy would not fully recover under this scenario. The affected sectors would include international education, and tourism (both into and out from Australia).Furthermore, we also need to be honest with ourselves. Notwithstanding China's early success, it is still seeing periods of new case growth. And notwithstanding South Korea's early success, it faces a steady stream of new cases every day (albeit a declining stream). Taiwan might be closer, but it is yet to have a sustained period without new cases. Japan and Singapore started well, but now look like they are losing control. Complete eradication within Australia could take some months, necessitates continued social distancing, and there is a risk it will elude us.
And perhaps, then back to herd immunity
The ugly exit point, should no vaccine or therapeutic intervention be found, and particularly if eradication within Australia proves impossible, could involve the managed propagation of the virus. This would need to protect the most vulnerable (older Australians, those who are immuno-compromised, etc.) It will also need to make sure we do not overshoot herd immunity (as this would involve unnecessary deaths).One approach I have considered to virus propagation management is a series of pulses, where the Government clamps down and then relax the public health measures - to allow a slow spread of the virus in a way that does not overwhelm the health system. The model charts for different periods of relaxation follow (to show how finely tuned and challenging a task this is).
Conclusion
There are two positive exit points, which I am sure we all hope for:- a therapy or pharmaceutical measure has been found that substantially reduces the fatality risk associated with having COVID19, or
- a vaccine is found that can be applied to sufficient Australians to confer herd immunity.
There is one exit point, which suggests failure:
- sufficient Australians have caught and recovered from COVID19, and that experience confers sufficient immunity for herd immunity to be achieved.
And finally,
- there is an exit at the government's choosing when the ongoing costs of economic hibernation continue for too long. The government should give thought to when this point of time might be, and in particular, what it should do if there are no vaccines nor therapeutic interventions available at that time and the virus has not been eradicated.
Can you modify your exit strategy to include higher isolation restrictions fot those over retirement age. It seems 80% that have it are 20-70 but none die and few crirical care.
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