Critics have argued that quarantining in populated city centres amplifies the risk of spread

Australia's hotel quarantine system has been an extremely effective first line of defence against Covid-19.

The country has largely eliminated the virus, often going weeks without a locally acquired infection. Hotel quarantine is credited a huge part of that success.

But a series of isolated local cases in recent months - all from hotel quarantine leaks - have caused alarm.

Since November, three cities have entered snap lockdowns on the back of such infections, aiming to halt outbreaks at their source.

But how does the virus keep slipping through what officials hail as a "world-class" first defence?

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The first time the system failed
Since March last year, Australia has essentially allowed only returning residents to enter the country, provided they complete a 14-day quarantine - usually in city hotels.

So far, more than 211,000 people have gone through this quarantine. It's a costly but effective method that is used in only a few other nations such as New Zealand, South Korea and Singapore.

But if the virus somehow escapes quarantine, the consequences can be severe. Melbourne saw this last July when a quarantined traveller infected a hotel guard, who unwittingly took the virus into the community. The resulting outbreak accounts for over 90% of Australia's 29,000 cases and 909 deaths to date.

Eventually, after a punishing lockdown, Melbourne's outbreak was suppressed. Inquiries led to better infection prevention and tracing efforts in all states. In particular, hotel quarantine was tightened up.

Despite this, further quarantine breaches have caused Australia's only virus scares in recent times: in Adelaide in November; in Brisbane last month, and in Perth and Melbourne in the past week.

In each case, hotel staff unknowingly caught the virus and took it into the community. None of those workers had breached any safety protocols, officials say.

So how is the virus escaping?
At the time of Melbourne's outbreak, standards were lower. Guards were found to have socialised together and worked across different hotels, and there were reports of guards mingling with quarantined travellers.

These days, however, the rules aim to prevent any such contact. Staff adhere to rigorous cleanliness standards, wear PPE and keep their distance.

As a result, many epidemiologists believe only airborne transmissions are causing cases to slip out, most likely through hotel corridors.

Various state governments have acknowledged this is a possibility, but reject suggestions they haven't taken enough action to prevent it.

Covid can spread by airborne transmission, US says
Prof Michael Toole, from the Burnet Institute, adds that with more infectious variants emerging, it's no wonder we're seeing more "spot fires" in hotels.

But even with Adelaide's breach - which pre-dated the more transmissible variants - it's suspected a security guard became infected while sitting in a corridor outside a room.

"They spent hours poring over CCTV footage to find out what that guard did wrong," said Prof Nancy Baxter, head of the Melbourne School of Population and Global Health.

"And the thing that guard did wrong was breathe air. All that person did was walk the halls, breathing the air. Right there. That should have been the clarion call that we need to do something different."

So what could be tried?
Many experts argue Australia should better adapt its quarantine to counter the threat in the air.

"If the best time to have considered that Covid-19 is airborne was 10 months ago, then the next best time is now," added Prof Baxter.

"The failure to acknowledge the airborne nature is making it difficult to create a very important layer of protection."

According to Prof Baxter, it's not just workers stationed in hallways who are at risk - there is evidence of guest-to-guest transmission too.

In Melbourne last week, a woman caught the virus from a family in a room across a hallway. It's suspected the guests opened doors around the same time to pick up their meal.

On Monday, it was reported that a traveller to Sydney tested positive two days after exiting his 14-day quarantine. This also suggested a case of hotel transmission rather than a rare, late-blooming case, Prof Baxter said.

Many experts have called for a nationwide reassessment of air flow in hotels used for quarantine.

"If we removed the word Covid and replaced it with the word asbestosis, we would think about it differently," said Prof Bruce Thompson from Swinburne University.

Current hotel protections work to prevent droplets spreading, but experts argue that aerosols need more barriers. These could include better masks and face shields for workers, upgrading air conditioners, and even picking buildings with better ventilation or windows, balconies, and other openings.

Dr Baxter said: "Some of the hotels may have great air circulation, but I suspect many don't. And the point is I don't know - no-one's talking about it."

What has been done?
On Monday, Victoria - where the latest two cases were found - announced it had implemented new protections.

Workers now had face shields and higher-quality masks, and engineers were reviewing air conditioners and air flows, the state government said.

Hotel staff were also being tested outside shift hours every day, and staggering the delivery of meals to guests.

"We make the necessary adjustments to the programme as new evidence comes to light," said Health Minister Martin Foley.

After South Australia's scare in November, state officials said they were upgrading their hotel ventilation systems and urged other states to follow.

Prof Toole points out that the lack of a national standard in hotel quarantine has exacerbated matters. He says it's other measures - such as contact tracing in the community - that have prevented hotel breaches growing into larger outbreaks.

Should Australia consider quarantining in the desert?
Critics have argued that quarantining in p