Vaccination dramatically shifted the burden of infection in Victoria, Australia to young children and teenagers.

Lines for COVID-19 testing in Melbourne early in the pandemic 

Moving on from the Sydney COVID outbreak to the Melbourne COVID outbreak.

The previous post documented changes that occurred in infection burden during the 2021 New South Wales Delta variant outbreak of coronavirus. As promised there, this post continues along that path, which is a dissection of the 2021  COVID-19 Delta variant outbreak that was centred in the Pundit's home town of Melbourne, a city in the Australia state of Victoria.

Both these outbreaks occurred while vaccination programs were being rolled out. In Victoria, vaccination started 18 May 2021 and was approximately 50 percent delivered by mid October 2021. A similar rollout occurred in Sydney. The epidemic started earlier in New South Wales, so effective protection during the peak of cases was more complete in Victoria. As a result of these timings, the Melbourne outbreak differed from Sydney by starting later and posing a much lower load on the hospital system, as shown in the graph 1 below:

Graph 1. Comparison of Sydney (NSW) and Melbourne (Victoria) COVID-19 hospital occupancy. The vertical axis is numbers of occupied beds.

To better understand these events, the Pundit has classified every diagnosed COVID-19 case between 25 Jan 2020 and 23 November 2021 to examine the timing of infection and the age of the person affected.

 These classifications are plotted in Graph 2:

Proportion of diagnosed cases in different age-groups as a percent of total cases in three different calendar intervals. Blue is the period 25 January 2020 till 17 May 2021, where no vaccination occurred. Orange is for all the vaccination period, 18 May 2021 to 23 November 2021. Grey bars are for the last weeks in this interval only (14-23 November 2021).

A comparison of the blue bars with the red bars reveals that vaccination protected people who were over 50 years of age from infection, and also protected 20-year olds (as indicated by decreased relative numbers of infections as a percent of the total with the passing weeks of the outbreak). The grey bars, that correspond to infections in the last few weeks of the outbreak period, demonstrate even more emphatic evidence for protection from infection in the 20 to 29 age-group. (This group  were late on the list for prioritised vaccine rollout).

The Unvaccinated are different.

The age group that got no vaccination, 0 to 9 year-old children, behaved very differently during the progression of the outbreak, with their proportion of total infections rising over time.

Clearly these patterns mean that very careful attention to infection control in junior school is needed for the coming new year.

A recent tweet from Mike Honey echoes these findings:

It document the categories of current active clusters of cases, and the most numerous are in educational settings. This fits with the previous graphs showing increases in the fraction of case in young children.

It also fits with gossip in the Pundit's street. Bob down the road just told me there are three COVID-19 cases ("kids sick") in the local Catholic school.

Further note 2 Dec 2021

Adam Carey. The Age 1 December 2021.

COVID-19 still causing classroom chaos as schools head list of cluster sites

"COVID outbreaks continue to disrupt schools across Victoria, with fresh clusters forcing unplanned closures as teachers isolate en masse and supply of rapid antigen tests is stretched..."

See also a later GMO Pundit post:

Dec 2. We shouldn’t lift all COVID public health measures until kids are vaccinated. Here’s why

Take home messages

As seen for  New South Wales, very young unvaccinated people are important participants in chains of coronavirus infection.

Vaccination is protecting several age-groups in Victoria from infection, especially vulnerable older people.

The strong beneficial vaccine impact on 20-somethings is a pleasant surprise.