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

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, dissection of  the 2021  COVID-19 Delta variant outbreak centred in the Pundit’s home town Melbourne, 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:

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:

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.

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.