Did the Sydney blockade come too late? Here’s why it’s not that simple


LOn Saturday, the New South Wales government announced a two-week closure for Greater Sydney, the Blue Mountains, the Central Coast and Wollongong following an increase in new COVID cases.

It has been generated a lot of comments on whether NSW delayed the blockade and therefore a longer blockade has been caused. Indeed, a modeling study according to researchers at the University of Sydney, published last November, it is estimated that delaying the blockade by three days would extend the blockade by three weeks.

But it’s not that simple.

This type of modeling involves using blocking as the main intervention in which you rely on the same blocking to eliminate transmission. Even if the cases are not tested and recognized, the virus runs out of people susceptible to infection. This is not the case with NSW, which still relies on a testing, tracking and isolation strategy.

If this blockchain is about gaining time to allow contact tracers to deal with the virus by getting all the risky contacts in quarantine cases before they become infectious, a day late is unlikely to add weeks to it. . Modeling is not applied in this scenario.

Whenever we are evaluating a block, there is always an element of “retrospective that is 20/20”. Some of the circuit breaker blockages we’ve had over the past year didn’t change the management of cases and contacts, as all the community transmission that occurred after the blockage was among known contacts that were already in place. in quarantine. But we didn’t know it until after the closing call. Circuit breakers are essentially insurance policies or safety nets (although we should evaluate them to see when they are warranted).

During this pandemic we often make decisions in situations of considerable uncertainty.

Things have clearly changed in our recent outbreaks in Victoria and NSW. Contact tracers have been very effective in finding, linking and documenting the spread of cases. And, in the latest NSW outbreak, health authorities have had the added benefit of discovering the cluster within the first generations of spread. This means that they have been able to collect data on where the virus was and how the transmission occurred in almost real time.

It’s easy to sit here and say that closing earlier would have made a difference. But when should it have been called? When did we know only ten cases last Saturday? Although infections between occasional contacts were worrisome, no one knew 24 future cases he would soon be exposed to a case at a private party of 30 held later that night.

So what works best in situations like this: go early just in case? I would say we would go with the data when you are so close to the vanguard of the outbreak. Evaluate the data in real time and be prepared for a quick change of response.

Last week’s emerging story was of many cases, but almost all are related to the known cluster. By the end of the week, it was clear that at least one branch of the outbreak was lost with several infectious cases in the community for five days or more.

What caused this blockage?

Instead of relying on high-level modeling of transmission risk and projections, we can also construct a detailed picture of the epidemiology of an outbreak as it develops.

NSW has had very detailed transmission data in almost all cases except a handful, which puts them in a strong position.

A potential risk in relying on contact tracking is how quickly things can increase if a major chain of infection is lost. This was the case of the cluster involving one seafood wholesaler in Marrickville, which stretched invisibly and pushed back transmissions a week before they caught it. Sunday, ten of the 30 new announced cases were linked to this cluster.

Another factor was casual transmission. This variant of Delta is much more infectious than previous strains, and some of the first cases in this outbreak occurred only by “fleeting“exposure. Some of these early transmissions occurred in places where health authorities could not rely on being able to track all occasional contacts and it is possible that those exposed may have underestimated the risk of being infected.

Yes, it is true that almost all cases are “linked” to previously known cases, but some of these were related through a complicated and longer path as contact of previous lost cases. This meant that more people circulated while they were infectious for a greater number of days.

Case distribution also played an important role. While it still seems to be largely focused on Bondi, potential cases and exposures have now spread further.

Should NSW have been blocked a few days earlier? It’s hard to say, but it will be important to assess when things are resolved

Many people forget that there were only ten cases in total in this outbreak just over a week ago. There were two new cases a day between June 16 and 20 inclusive, which are figures we all know are manageable for NSW contact plotters.

Should they have been completely closed on Sunday, June 20, when they had a total of ten cases? I don’t think you can defend it epidemiologically.

The cumulative cases went from ten to 25 two days later, to 54 another two days later, on June 24, to 112 on Saturday, June 26, when the closure was announced.

They seemed to be at the top and very close to reaching all contacts before their infectious periods. But they were still probably a day or so behind the virus. Even an infectious day in the community by a few contacts simultaneously is very risky and is added to exhibition venues.

This Delta variant also appears to have reduced the time between cases that are exposed to the virus and become infected themselves. in accordance with Kerry Chant, NSW Health Director.

All in all he painted a very different picture from the previous week and would have contributed to the decision to block.

We need to analyze the data

We now need to assess this outbreak response, along with all the others in Australia, and learn more about how the virus moves through our communities, our weaknesses, our most effective containment measures and the optimal timing of ‘these.

Blocking has not yet played a major role in new cases. But we also know by the time of the cases that the blockage a few days earlier would not have stopped the outbreak of seafood wholesalers, nor would it stop at essential high-risk jobs.

The analysis of the outbreak aims to understand any additional cases that could have been avoided with a previous block, or how many cases will be avoided with the block in place. It will allow us, under various alternative scenarios, to use this rich detailed case data to remove some of the uncertainty next time.

Catherine Bennett, Chair of Epidemiology, Deakin University

This article is republished from The conversation under a Creative Commons license. Read the original article.

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