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18 Aug 2020

How low do we have to go?

Written by University of Melbourne Professor Jodie McVernon, Director of Epidemiology at the Doherty Institute.

Victorians anxiously anticipate the daily Tweet and press conference announcing the number of COVID-19 cases and deaths in the last 24 hours, looking to see the longed-for downward trend. The good news is, this level of restrictions really does work and the numbers are steadily declining. Outbreaks are fewer, new reports of health care worker infections are reducing, and cases have less contacts on average to be followed up.

But now for the next question – how low do we have to go? As ever, no one seems to be giving a straight answer but it’s not that they’re being deliberately evasive, just that it’s not as simple as it seems. There’s not a single indicator to look at, and how case numbers are interpreted will vary according to context. So what are the principles that decision makers will consider when determining whether restrictions can be eased?

The effective reproduction number has become part of everyday language. It’s the average number of secondary infections produced by each case. The purpose of the combined public health response and current social measures is to make that number less than one. The smaller the reproduction number the faster newly identified cases will decline, so the lower it gets the better, but it’s not an end goal in itself. The main reason it’s so low right now is because of the Stage 4 restrictions. As soon as they’re lifted and we can move more freely in society the potential for infections to spread will definitely increase again.

That’s why getting case numbers as low as possible before easing restrictions is highly desirable. If we anticipate that lifting restrictions will bring the reproduction number back up to one, from a starting point of 10 infections we would anticipate 10 new infections in a week’s time. If that starting number is 100 or 1,000 – there will be likely will 100 or 1,000 new infections appearing in a week’s time and so on. As case numbers add up every day, it’s clear that the total number of active cases in the community over the course of a week will be vastly different depending on where we’re starting from.

It’s been observed globally that SARS-CoV-2 has the potential to cause explosive outbreaks. One of the tragic features of the recent Victorian epidemic has been the large number of outbreaks in residential aged care facilities, sadly resulting in many deaths. From the UK and elsewhere we’ve seen clearly that the number of such outbreaks at any given point in time is directly related to how much virus is circulating in the community. In other words, the more active cases there are out there, the more likely it is that one of them will occur in an aged care worker or visitor who can introduce the virus into a vulnerable setting. So once again, the lower you can go the better.

Cases in health care workers are rightly also a focus of special attention. Workers on the front line of health delivery are at the highest risk of anyone in our community of being exposed to people with COVID-19 who present for clinical care in primary care or hospital settings. SARS-CoV-2 is very contagious and regrettably despite stringent attention to infection prevention and control this virus can cause outbreaks in clinical environments. Health workers are also people with lives beyond their work, so when there’s a lot of COVID-19 around they have the same risk as others of being infected at home or in the community. Ensuring that health workers are adequately protected is critical for their own and all of our benefit.

And then there are the ‘mystery cases’ – those that turn up without known links to other infected family members, work or social contacts or outbreaks. These cases are our best indicator that there is ongoing community transmission, which is why the ‘disease detectives’ work as hard as possible to find links between reported infections. Lockdowns are very effective at limiting the random contacts that enable community transmission. The aim is to bring us back to a situation where the links in transmission chains can be identified, making us confident we can break the links to further infections. There’ll be particularly close attention to minimising the ongoing proportion of mystery cases as the overall numbers fall further.

The end is in sight. Victoria’s state of emergency declaration officially ends on the 13th September, and our Premier is quoted as being ‘cautiously optimistic’ that measures will ease after this date. We can all do our bit by just holding out for the next few weeks to help make sure that happens.