Victoria’s COVID-19 lockdown: Getting ahead of the spread

Today, Victorian Acting Premier James Merlino announced that all of Victoria will be subject to a “circuit-breaker” lockdown for seven days as a result of the most recent COVID-19 outbreak. The outbreak currently contains 26 known cases, with health authorities expecting more to roll in over the next few days.

This has all happened very quickly. The outbreak is believed to have started from a hotel quarantine “leak”, with an individual who had completed their 14-day quarantine stay testing positive in Melbourne on 11 May.

There were no locally acquired cases for 13 days thereafter, with the next four positive cases diagnosed on 24 May, 10 on 25 May, and a further 12 on 26 May.

The situation has moved from a small leak of hotel quarantine with seemingly little consequence, to an outbreak big enough to warrant a statewide lockdown.

An element of “catch-up”

There’s no doubt that detection of these cases is happening quickly, but also somewhat late. Media reports have highlighted that many of the individuals were experiencing symptoms for days before being tested, and by the time one individual in a house tested positive, everyone in the house was positive.

An element of these swiftly-growing numbers is that detection is now “catching up” to the outbreak.

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It’s not just the rapid growth in detected cases that can raise alarm bells. It was announced this morning that more than 10,000 close contacts have been identified and are currently in isolation, with more expected to follow shortly, and approximately 150 exposure sites were identified.

More exposure sites doesn’t always mean more cases – often they represent places where the amount of contact is minimal, but there are a lot of individual contacts – but what it does show is that there’s the potential for the virus to be dispersed geographically across Victoria.

What does the serial interval mean?

A key point raised in this morning’s press conference was that the virus is spreading faster than ever, with a shorter “serial interval”.

The serial interval refers to the amount of time between the symptoms of two cases who infected each other. Thus far, for COVID-19, the serial interval is, on average, five days.

Although the observed serial interval in this outbreak has been short, we must remember that:

• the number of cases with which to calculate this number is small; and

• in small populations with complicated contact patterns (such as in households), the serial interval can be observed to be much smaller than the population average .

There’s little international evidence of shortened serial intervals for COVID-19, even among newly-circulating variants. Nonetheless, Victoria has seen before how quickly an outbreak can get out of hand.

A big focus of the public health response is the variant of COVID-19. The genomic sequencing that has been completed for the cases so far has shown that all are linked, and that all are the so-called “Indian’” variant, B.1.617. There are two subvariants currently circulating globally (named B.1.617.1 and B.1.617.2).

Importantly for Victoria, this strain has been reported as B.1.617.1, which is believed to be the less-transmissible of the two, although is likely at least as infectious as B.1.1.7, the so-called “UK variant”.

Ease of transmission makes the lockdown understandable

It’s important to note that for all of these variants of concern (VOC), there are few reports of individuals developing symptoms faster, but rather that the virus is easier to transmit.

For example, with the original strain of COVID, perhaps a conversation with someone who is physically close is required to transmit a virus; however, with a VOC, simply being in the same room as someone is sufficient.

With all these factors put together – the increasing cases, contacts, and the variant of concern – the decision to enact a lockdown is understandable, if disappointing to Victorians.

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The circuit-breaker lockdown itself is of a different length to Victoria’s previous short lockdown (five days), and other state efforts, which are often only a couple of days. Seven days gives enough time for one generation of transmission and a bit of buffer to detect those new cases.

Importantly, it also gives enough time to stop one generation of transmission, hopefully limiting the number of households that we’re dealing with at any one time.

This length should be sufficient for the public health team to get a handle on the full situation, while simultaneously limiting what’s to come shortly afterwards.

Did the lockdown come too late?

It has been asked why there was no snap lockdown earlier. There are a few reasons for that.

First, although it may feel like a long time, the first problematic cases were only identified three days ago. Second, the first sets of cases were restricted to relatively few households, and when transmission is limited like that, there’s little to worry about.

However, with the explosion in contacts, higher-risk exposure sites and a geographically spread outbreak, it does seem that now might be the best time.

Overall, the latest COVID-19 outbreak has gone from zero to maximum very quickly.

No one wants to be in the position where another lockdown of any length is required, but the alternative situation is clearly going to be worse.

This coronavirus keeps us guessing, but one thing’s for sure – Victorians have done this before, and we can do it again.