Timings for saved lives and protection of health from the rapid COVID vaccine rollout in Sydney

 

Key messages

In the 2021 New South Wales COVID-19 Delta variant outbreak:

• Vaccine rollout in New South Wales was just in time.
• There was an early, dramatic decrease in mortality rates from 1.4 to 0.74 percent during August 2021. 
• Decreases in hospitalisation rates and ICU entry rates were more delayed (about the end of August), but provided improved health outcomes and much reduced misery for many thousands of infected Sydneysiders

In the previous post, we discussed how graphs of Australian COVID-19 hospital occupancy reveal dramatic improvement in health protection because of a more complete rollout of vaccination in Victoria by the start of the outbreak in that state. 

The following graph shows the main point from that post:

Even though Victorian diagnosed daily COVID-19 cases were much greater than in New South Wales, hospital occupancy was about 50 percent lower in Victoria.

The Pundit has been continuing to explore what's happening in this epidemic. He's trying to extract insights from detailed weekly NSW government reports showing numbers of new patients entering hospital and ICU beds each week to enable better understanding of how health impacts of vaccination changed during the NSW epidemic.

A useful way of doing this exploration is to plot cumulative disease events versus the cumulative number of COVID-19 cases. This effectively turns curves into straight lines and from these lines revealing rate factors are easily displayed as line slopes and quickly calculated.

It's a trick the Pundit learned a long time ago when he grew bacteria for a living.

Here's such a plot using death and case numbers for the New South Wales epidemic obtained from weekly disease surveillance reports.

Before doing this plot, cumulative death counts (on the vertical axis) have been shifted back 2 weeks to align with the infection number count that causes them.

What the plot reveals is that very early in the outbreak there is a dramatic change in slope of the plotted line.

The Pundit was delighted to make this discovery, which is most obviously revealed by this graphical approach.

The initial slope corresponds to a death rate of 1.4 lives lost per hundred cases, while the subsequent slope is 0.74 percent which appears unchanged for much of the subsequent part of the outbreak.

It implies that continued vaccine rollout, in addition to eventually halting new cases, saved several hundred lives among the cases of infection that got through health protection measures. To see this, just extend the initial slope upward in your imagination.

But there is more.

Hospitalisation rates changed

A similar graph can be done using cumulative numbers of hospitalisations instead of deaths, as follows.

Updated 4 Nov 21

In this graph the count of total hospital entries (vertical axis) has been shifted by one week to allow for the lag between illness onset and hospital entry.

The graph reveals a change of slope after after about 30,000 infections into the outbreak, to a lower value.

Doing some arithmetic on these slopes reveals that initially 19.3 people in 100 infections had to go to hospital but subsequently this dropped to 10.0 percent.

Thus continued rapid rollout of vaccines during the outbreak cut chances of going to hospital in half (and at least partly explains trends in changes in hospital occupancy during the outbreak).

Another way of looking at this same effect is examining weekly totals of cases and hospitalisations. 

Here is such an examination:

Week 45 ends 13 November 2021

The blue bars are weekly total new admissions to hospital of COVID-19 cases.

Thankfully they decreased rapidly after reaching the peak.

But the orange-line tells us something more just decreased numbers of infections is reducing the load on hospitals. It gives to yield of deaths from 100 infections based on a one-week lag between infection and death using weekly total counts of events.

At the start of the outbreak hospitalisation rates per 100 cases were near 15 to 20 percent, but in the last weeks they have fallen to near 7 percent.

Decreased frequency of severe illness outcomes among infected people dramatically reduced the load on hospitals -- and human misery -- late in the New South Wales epidemic.

ICU entry rates also changed.

Here is an analogous graph for cumulative ICU admissions versus case numbers:

Updated 4 Nov 21

Here cumulative ICU admissions are plotted on the vertical axis, adjusted for a 1 week lag.

Again there is a change in slope during the outbreak.

The initial slope corresponds to 1.9 ICU entries per 100 infections, dropping to 1.1 percent in the last 3 weeks.

(The out of line data value in the middle of the curve occurs at the peak of the outbreak may be an artefact caused by some administrative bottleneck and is ignored for interpretative purposes in this post. Cumulative values on either side of this single point form a smooth trend.)

As with hospitalisation, this can be examined through the lens of weekly ICU admissions and rates of ICU entry per 100 infections each week, adjusted for 1 week of lag between diagnosis and ICU entry:

We see this:

Updated 23 November 2021

The pattern is very similar to that in the graph for weekly hospital admissions displayed earlier in this post.

Overall for the ICUs there is the same pattern as seen with hospital entry-- progressively decreasing rate of severe disease per 100 infections, which started around 3 ICU admissions per 100 infections, and ended less than 1 ICU patient entry per 100 infections. Note that this is similar to the changes in mortality rates that were used to start this discussion (namely mortality 1.7 percent changing to 0.74 percent later in the epidemic).

To some up:

So we see interesting changes accompanying vaccine rollout during the outbreak:

• There was an early, dramatic decrease in mortality rates from 1.4 to 0.74 percent during August 2021. 
• Decreases in hospitalisation rates and ICU entry rates were more delayed (about the end of August), but these gave improved health outcomes for many thousands of infected Sydneysiders.

Vaccine rollout in New South Wales was just in time.