Thu Feb 18

Tier 1, tier 2, tier 3? Victoria's COVID exposure sites explained

Written by Meru Sheel, Epidemiologist | Senior Research Fellow, Australian National University

As the Holiday Inn cluster in Melbourne continues to grow, the Victorian government and the media are regularly publicising new exposure sites.

An exposure site is a location a person who has tested positive to COVID-19 visited while they were potentially infectious. These locations are generally identified through contact tracing, where the person provides a detailed history of their contacts and places they visited up to three days before they developed symptoms (or tested positive, if they didn’t have any symptoms).

You might have seen these exposure sites classified as either tier 1, tier 2 or tier 3. But what does this mean?

The three Cs

Contact tracers and public health professionals carry out risk assessments to classify exposure sites. These risk assessments determine whether a particular place is a high-, medium- or low-risk setting for transmission.

We know SARS-CoV-2, the virus that causes COVID-19, is more likely to spread in certain environments. Typically, settings that constitute one or more of the “three Cs” are considered high-risk:

crowded places (indoors carry a greater risk than outdoors)
close-contact settings (especially where people have close-range conversations, such as in a bar)
confined and enclosed spaces (this specifically refers to indoor spaces with poor ventilation).

We’ve often seen high-risk settings associated with super-spreading events, such as the Crossroads Hotel cluster in New South Wales last year, where one exposure site led to several new cases.

In determining how to classify a site, health experts will also consider the amount of time the positive case spent there. We know the risk of COVID spread is related to the length of exposure — that is, the greater the time spent in close contact, the higher the risk.

Finally, the risk level of the site may be influenced by the nature of the location and the sort of activity the positive case conducted there. For example, as we saw during the second wave in Victoria, working in abattoir where physical distancing may be difficult, the virus can spread faster and more readily. In contrast, an infectious person walking in a park or in a workplace with no direct contact with others may carry a lower risk.

A tiered system

In Victoria, these high-, medium- and low-risk sites are being classified as tier 1, tier 2 and tier 3 sites respectively.

Tier 1 represents exposure sites where people attending are at greatest risk of catching the virus and passing the infection to others. As such, people who have visited a tier 1 site during the time specified must immediately get tested and isolate for 14 days — regardless of their result.

We would theoretically regard people who have been at a tier 1 site in the relevant time period as “close contacts”.

There have been several examples of tier 1 exposure sites during the current Victorian outbreak. Last night, the Victorian government listed Sacca’s Fruit World in Broadmeadows as a tier 1 site for anyone who visited between 12.30 and 1pm on Tuesday February 9.

People who visited tier 2 and tier 3 sites are at lower risk of being exposed to a positive case, and therefore less likely to contract and spread the virus.

For tier 2, the public health directive is to get tested and isolate until you receive a negative result. People who visit tier 2 sites during the relevant time period could be regarded as “casual contacts”.

The Sunbury Square shopping centre, for example, was listed as a tier 2 exposure site after a case attended multiple stores between 3.40pm and 4:30pm on February 5. The entire shopping centre was placed on alert, possibly to cast a wide net rather than focusing on a few stores.

Tier 3 sites appear to be precautionary and present the smallest risk — and are potentially places where positive cases have just passed through. But they’re still important to prevent onward transmission. If you’ve visited a tier 3 site during the time specified, you should monitor for symptoms and get tested immediately if you do develop any symptoms.

As an example, Broadmeadows Central (that’s the shopping centre where Sacca’s Fruit World is located) has been listed as a tier 3 exposure site between 12.15pm and 1.15pm on February 9.

Classifying exposure sites is an important public health tool

Other jurisdictions use similar approaches to swiftly identify as many people as possible who may have been exposed to COVID-19, and therefore limit the spread of virus.

In New South Wales, contacts are alerted based on exposure sites, and then categorised as: close contacts who must get tested immediately and self-isolate for 14 days; casual contacts who must get tested immediately and self-isolate until they receive a negative result; and people who must monitor for symptoms (similar to Victoria’s tiers 1, 2 and 3 respectively).

Using public health alerts and media outlets to communicate exposure sites to the wider public can hasten the process of contact tracing, increase targeted testing and enable rapid identification and isolation of cases — ultimately to slow the spread of COVID-19.

These strategies are also useful when resources and time are limited. Contact tracers can prioritise actively following up people who visited tier 1 sites first.

Authors: Meru Sheel, Epidemiologist | Senior Research Fellow, Australian National University