Part 4. What does Portugal's success with vaccine protection say about the future of COVID-19 in Australia?

 Key message: vaccines decrease both the chance of infection, and the chance of death if you are infected

This post continues a theme investigating COVID-19 harm reduction in the major Australian population centres — Sydney and Melbourne.

These cities are in the middle of rapid vaccine rollouts in the midst of surging coronavirus infection numbers.

Portugal have been there already, with their marvellous progress highlighted in the Financial Times 28 September 2021 by Peter Wise.

Portugal emerges as Europe’s vaccination frontrunner after public health drive

A former submarine commander has instilled confidence in jab programme that had a faltering start

Here are graphical summaries of what has happened in Portugal from the superb Our World in Data website:

After the first moderate wave of cases in March-April 2020 there was a large surge in winter 2021, followed by a smaller surge in cases in the late summer of 2021.

Deaths are one measure of the severity of these waves of infections:

The graph show the mortality impact of the winter 2020-21 case surge was awful, with around 14,000 people losing their lives. But the late summer infection wave impact on loss of life was relatively marginal.

This means that infection fatality ratios (deaths per 100 infections) changed  significantly for the better between these waves.

A direct way of calculating how deaths per 100 infections changes is to examine the relations ship between deaths and case numbers in a graph. The infection fatality ratio is directly related to the slope of lines in such a graph.

For the Portugal outbreaks using numbers for cumulative deaths and cumulative cases from Our World in Data, we get this graph:

11 day lag adjustment applied to deaths Black arrow indicates onset of fully protected immunisation

The most distinctive feature of this graph is the change in slope just after 800,000 cases, indicating a dramatic drop in mortality per 100 infections.

The slope between 400,000 cases and 600,000 cases gives a death rate of 2.54 deaths per 100 infections.

The slope from 900,000 cases to 1,060,000 cases is 0.46 deaths per 100 infections

Its useful to also check the rollout of full vaccination coverage in Portugal:

Comparison of this graph with the previous one shows that high vaccine coverage of the population was not needed to cause a drop in mortality rates. Probably this was achieved by preferential immunisation of the elderly early in the campaign, and some protection from just one jab of vaccine.