Last updated: 2021-10-22 09:22 AEST
Updates daily near 9am Melbourne time

Progression of the COVID outbreak in Victoria

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New: Progression of the COVID outbreak in New Zealand

New: Progression of the COVID outbreak in the Australian Capital Territory

See also: Progression of the COVID outbreak in New South Wales

See also: Australian vaccination rollout

See also: The road to a COVID-free Victoria (old 2020 second wave plots)

Contents

  • Reff estimate from all cases
  • Projected effect of vaccine rollout
  • Animated projections over time
  • Reff in individual LGAs
  • Disclaimer on trends
  • Methodology
  • Data sources and contact
  • Reff estimate from all cases

    Victoria is currently experiencing an outbreak of COVID, due to the delta variant. Since August 6th, metropolitan Melbourne has been under lockdown restrictions, and regional Victoria since August 20th.

    How have Victoria's restrictions affected the spread of the virus? The below plot shows how the effective reproduction number of the virus, Reff has changed over time in Victoria, as well as how the daily cases have changed over time. A trendline shows the approximate trajectory daily case numbers would follow, were the reproduction number of the virus to remain at its current level.

    The same plot is shown twice, the first with cases on a linear scale, and the second on a log scale—the latter showing how consistently the caseload has followed exponential growth (which forms a straight line on a log scale).

    Projected effect of vaccine rollout

    The trendlines in the above plots simply project forward case numbers assuming that Reff remains at its current estimated value. What about the effect of vaccines? Below is a plot with the projected trend taking into account an estimated reduction of spread due to vaccination levels increasing in Victoria through the remainder of the year. This projection is a stochastic SIR model, and thus also takes into account immunity due to infections, which would decrease transmission once a non-neglibible fraction of the population has been infected.

    The rate of vaccination assumed in the model is 1.4 doses per 100 population per day in September, and 1.8 doses per 100 population per day from October onward, up to a maximum of 85% of the population. See below for plots of how these assumed rates compare to the vaccination rollout in Victoria so far. The vaccines are assumed to reduce Reff by 40% per dose per capita—that is, overall spread is reduced by 40% for a partly-vaccinated individual and 80% for a fully vaccinated individual.

    This projection shows what might be possible if all other factors affecting Reff—such as restrictions—are held constant. If Victoria eases restrictions, however, then Reff will likely increase and the effect of vaccines in reducing spread will be delayed compared to this projection. On the other hand, if contact tracing is successful in decreasing Reff, case numbers may decline faster than this projection.

    The same plot is shown twice, the first with cases on a linear scale, and the second on a log scale.

    Below are plots of the vaccination rollout in Victoria to date, in terms of daily and cumulative doses per 100 population, with the assumed future rate used for the above projections also shown.

    Animated projections over time

    How have the above projections changed over time? Below are animated versions of the above projection, one with cases on a linear scale, and one with cases on a log scale, run on old data to show how the projections have changed over time. Note that these are not 100% identical to the projections actually made on previous days, as there have been some slight methodology changes - but they should be very close.

    Reff in individual LGAs

    Below are plots of daily case numbers and estimated Reff values in the ten Victorian local government areas that with the highest number of cases in the last 14 days, sorted by number of cases in the last fourteen days.

    The data in these plots is one day out of date due to the data being released later in the day, after statewide case numbers.

    Note: case numbers on these plots are shown on a log scale.

    Disclaimer on trends

    The plotted trendlines are simple extrapolations of what will happen if Reff remains at its current value. This does not take into account that things are in a state of flux. As restrictions take effect, the virus should have fewer opportunities for spread, and Reff will decrease. If restrictions are eased, it may increase. Contact tracing may suppress spread to a greater or lesser degree over time. The above plots specifically showing the effect of vaccines do take into account a reduction in Reff as vaccination coverage increases, but ignore any other possible future changes in Reff.

    Furthermore, when case numbers are small, the random chance of how many people each infected person subsequently infects can cause estimates of Reff to vary randomly in time. As such the projection should be taken with a grain of salt—it is merely an indication of the trend as it is right now.

    Methodology

    Smoothing, calculating Reff and projections

    Daily case numbers have been smoothed with 4-day Gaussian smoothing:

    Nsmoothed(t) = N(t) ∗ (2πTs2)-1/2 exp(-t2 / 2Ts2)

    where Ts = 4 days and ∗ is the convolution operation.

    Before smoothing, the daily case numbers are padded on the right with an extrapolation based on a exponential fit to the most recent 14 days of data.

    Reff is then calculated for each day as:

    Reff(ti) = (Nsmoothed(ti) / Nsmoothed(ti-1))Tg

    where Tg = 5 days is the approximate generation time of the virus.

    The uncertainty in Reff has contributions from the uncertainty in the above-mentioned exponential fit, as well as uncertainty in daily case numbers. The latter is considered to be Poisson noise scaled by a constant, chosen so as to make the reduced chi squared between raw and smoothed daily case numbers equal to 1.0.

    The extrapolation of daily case numbers is based on exponential growth/decay using the most recent value of Reff and its uncertainty range:

    Nextrap(ti) = Nsmoothed(ttoday) Reff(ttoday) (ti - ttoday)/Tg

    Vaccination model

    Note 2021-07-30: I have changed the model used for the projections of the vaccine rollout to a stochastic SIR model, and the description below is out of date. I haven't yet documented the new model on this page, but the code can be seen here in the meantime. The projected outcomes with the new model are very similar to the old model, which was valid in the limit of infections only reaching a small fraction of the total population.

    To model vaccines taking effect after a delay, daily vaccine dose numbers V(t) are convolved with a Gaussian offset 1.5 weeks in the future, with standard deviation 0.5 weeks:

    Veffective(t) = V(t) ∗ (2πσ2)-1/2 exp(-(t - μ)2 / 2σ2)

    where μ = 10.5 days, σ = 3.5 days, and ∗ is the convolution operation. This causes the effect of a vaccine dose to over the course of the second week after it is adminsitered, reaching nearly full effectiveness approximately two weeks after administration.

    I assume that one dose of any vaccine reduces spread by 40%, and two doses reduce spread by 80%. The proportion of the population susceptible to the virus is then:

    s(ti) = 1 - 0.4 × D(ti)

    where D(ti) is the cumulative number of doses per capita on each day.

    Reff is then estimated for any future date as:

    Reff(ti) = Reff(ttoday) × s(ti) / s(ttoday)

    And case numbers extrapolated from one day to the next, beginning with Nsmoothed(ttoday), according to:

    Nextrap(ti) = Nextrap(ti-1) Reff(ti) 1/Tg

    Data sources and contact

    Source for case numbers: covidlive.com.au and Victorian Department of Health

    Plots/analysis by Chris Billington. Contact: chrisjbillington [at] gmail [dot] com

    Python script for producing the plots can be found at GitHub.