Melbourne’s COVID-19 cliffhanger: how are the numbers tracking against reopening targets?

For Melbourne to move to step three of the state government's road map for easing restrictions on October 19, Victoria's 14-day average for new coronavirus cases has to be driven below five and there needs to have been five or fewer mystery cases, in total, over the previous fortnight.

We are now within that crucial fortnight meaning that the case numbers we see every day are directly determining whether the targets are met and the lockdown is eased.

The graphics on the 14-day average and the mystery case threshold show how the numbers are tracking compared against the two-week reference period the government will use to make its decision on reopening.

The 14-day average target

The Health Department would not confirm precisely what reference period for the 14-day average it would use for making a decision on October 19. But government sources said the relevant period would be between October 5 and October 18. That means the state’s target is to remain under 70 new cases, in total, over that period.

This graph above, which we will be updating daily, shows how daily case numbers are tracking against this 70-case ceiling, as well as the average number of cases that will be needed for the rest of the reference period in order to reach the target. The 15 new cases confirmed from the past day on Tuesday were not the best start to the 14-day reference period.

Those 10 cases above the target average have to be made up some time over the coming days. That means that over the next 12 days, it will take an average of four cases per day to meet the target.

Chief Health Officer Brett Sutton said this week that getting the 14-day average under five was "absolutely a target", but that health authorities would also consider where new cases were being recorded or whether new cases were in people who were close contacts or already self-isolating.

He said health authorities would review the state’s coronavirus situation frequently as the time to make a decision on easing restrictions got closer.

"We emphasise that it is not only the numbers, but the stories behind the numbers that are important," he said.

The mystery cases target

For Melbourne to join regional areas at step three, the whole state also needs to post five or fewer mystery cases in total over the previous two weeks. Mystery cases are infections that cannot be traced to an existing outbreak or case, suggesting the virus is circulating somewhere in the community.

Again the department would not confirm its reference period for this measure either, but government sources said it would cover investigations into the infection source for people who tested positive between October 3 and October 16.

The graphic above shows how Victoria is tracking to avoid this target's tipping point. We will be updating it when the new figures come out each day. It often takes health authorities a couple of days or more to trace how someone contracted the virus, so it will likely be a few days before the most recent days’ cases are fully investigated.

The earliest day of the 14-day reference period for which all the cases have been investigated is October 3. On that day there were 10 people who tested positive statewide, and of those the infection source of one of them could not be traced.

Professor Sutton has started tweeting out a daily breakdown of the infection source of new cases, which shows there are 12 cases being investigated from October 4 onwards. For some of these, the infection source may well end up being a mystery.

What you need to remember about the data

• The day numbers are announced to the public is the day after they are counted towards the reference period. So when the premier announced “six new cases overnight” at his Wednesday press conference the department records those cases as the result for Tuesday (the day the cases were found) not Wednesday (the day they were announced);
• Daily totals are sometimes revised down. For example, earlier this week someone tested positive for COVID-19 in Shepparton, but it later turned out the initial test result was wrong (this can happen sometimes and is called a false positive). That case was removed from the total cases tally; and
• An “unknown source” finding is not final. Sometimes more information comes to light and health authorities are able to solve the mystery, so that total could also be revised down in the coming days.

For 16 people over this same three-day period, it was clear the source of infection was from an existing case or cluster such as the Butcher’s Club outbreak in Chadstone.

"The mystery cases (target) in particular is a really important one to meet," Professor Sutton said this week.

"The successful jurisdictions internationally and within Australia have gotten to five or fewer cases within a 14-day period when they have been able to maintain control into the long-term. Cases that haven’t met that threshold haven’t been able to do that."

He said he wouldn’t rule out Melbourne moving to step three if there were more than five mystery cases recorded over the reference period, depending on the trend and when those case were confirmed.

"If all five of those cases were in the first week of that fortnight and the seven to nine days prior to the 19th that had no mystery cases, that’s a positive obviously."

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