Part 3 in a series. Death rates in the beginning of the 2021 Melbourne COVID-19 Delta 2 outbreak are lower than rates at start of the Sydney 2021 COVID-19 outbreak

Updated 3-Oct-21 and 4-Oct-2021 as fatality trends in Victoria became clearer.

There are twists to this story. Double twists. And more to come undoubtedly. But the good twists in Sydney bode well for Melbourne.

The Pundit is tracking indicators of severe health damage in COVID-19 outbreaks in Australia.

One of these is deaths per 100 infections, or the infection fatality ratio.

Starting August 4 2021 there has been rapid spread of a Delta variant outbreak in Melbourne.

Here is a graph of cumulative cases and cumulative deaths over 43 days

For this graph, accumulated case numbers have been divided by 226 before plotting on the vertical axis.

The horizontal axis is days since August 4.

Cases divided by 226 tracks very closely with numbers of total deaths after a lag adjustment of 11 days. (The median time for death after becoming ill is 11 days.)

This suggest an infection fatality ratio of 1/226 or 0.44 percent

Its interesting to compare this evolution with that of the Sydney outbreak:

For this plot, the cumulative cases have been divided by 100 before plotting. They track closely with cumulative deaths from COVID-19 in Greater Sydney.

This close alignment of cases and deaths suggest that the infection fatality rate in Sydney in this 45 day period was 1.0 percent.

This is higher than seen in the early Melbourne outbreak days.

Vaccine coverage may explain these early differences in death rates

The vaccine coverages at the start of the two respective outbreaks were very different from one another.

12 days before the Sydney outbreak started, in Sydney there were only  2 percent double dose injected persons per 100 people.

When the Melbourne second wave hit, they were much further advanced in vaccine rollout.

On 23 July https://www.covid19data.com.au/vaccines records 12 percent double dose injected persons per 100 people for Victoria. Much more coverage of Sydney pre-outbreak

This might explain why initial outbreak death rates were much higher in the Sydney than in Melbourne.

But wait, there are twists to this story.

Sydney vaccine coverage caught up quickly with Melbourne. By 1st September double dose New South Wales coverage was 30 percent, similarly for Victoria on 4 September.

Remarkably, by the second week in September both states had approximately equivalent vaccine protection 

The subsequent evolution of cases and death curves for the Sydney took a turn for the better around the 15000 cases into the outbreak, during the last week of August:

Between 45 and 91 days the two curves for Sydney -- cases and deaths-- diverged from one another.

This reveals that  infection fatality ratio is dropping in NSW, possibly because the virus is encountering people who are more likely to be vaccinated because of rapid vaccine rollout or younger people.

A plot of deaths against cases enables death rate to be calculated:

The graph plots cumulative outbreak deaths, adjusted for a 11 day lag, against cumulative diagnosed cases.

The slope after the first 50 lost lives yields an infection fatality ratio of 0.70 deaths per 100 diagnosed cases. That is somewhat hopeful.

But the twists to the plot in Melbourne were different.

After extra days wait in Melbourne we see, plotting lives lost versus cases:

This trends show a change in the overall trend with a mortality rate of near 0.78 percent emerging for the recent period for Victoria. The initial rate estimate of 0.44 was misleading. 

In both Melbourne and Sydney mortality rates are currently well below the severe rates (2.7 percent) seen in Australia during 2020.

Targeted vaccination of the elderly and follow through immunisation coverage of the broader population are saving many lives in Australia. Halting of the COVID-19 epidemic in Melbourne will save more, and this depends on continued vaccine rapid rollout in Melbourne. As of 3 October, it is clear that vaccination has halted the Sydney epidemic, but that dramatic event is discussed in other posts.

( See The Trifecta is on in the Land of Oz. Victoria will follow NSW this coming week and halt the COVID-19 epidemic (Part 5 in a series))