When Shakespeare penned the immortal line, "double, double toil and trouble/fire burn and cauldron bubble" in Macbeth he was thinking of a toxic witch's brew but there's another bubble that is laced with even more trouble and toxicity.
Travel bubbles represent one of the greatest, albeit as yet unrealised, hopes during the pandemic for a return to some semblance of normality for the chance to resuscitate economies.
But the trouble with bubbles is that they also represent one of the most profound challenges for political leaders and medical experts.
So far all travel bubble attempts have had all the flaccid buoyancy of a prematurely released child's party balloon, with some tacked-together versions in Europe proving disastrously ineffective and leading to damaging setbacks in the containment of COVID-19.
Back in April, the concept of a trans-Tasman travel bridge excited much optimism and excitement, all dashed by our own series of setbacks, including the pandemic plight of Victoria.
A token, partial bubble, allowing New Zealanders to visit NSW, the ACT and the Northern Territory, has the significant catch of a compulsory 14-day quarantine period for Kiwis on their return and no ability for Australians to cross the ditch.
Illustration: Jamie Brown
A sudden upsurge in community-acquired cases in NSW has probably pushed the launch date of the trans-Tasman bubble even further back. Alongside that, the prospect of the tourism and airline industries' survival, devoid of government bailouts, has become even less certain.
Now Singapore and Hong Kong, two destinations which have performed well in containing COVID-19 and which are hugely reliant on tourism, have, in the conspicuous absence of a vaccine, announced their own quarantine-free, rapid testing-dependent travel bubble which may well pre-empt the trans-Tasman equivalent.
Singapore, South Korea and Japan, with their admirable COVID-19 records, have been identified by Australian Prime Minister Scott Morrison, noticeably short on any detail, as other likely travel bubble partners for Australia.
So, in reality, what will the witch's brew-like ingredients need to be to not merely successfully launch a travel bubble but also to maintain one?
A TRAVEL BUBBLE WILL NEED MORE THAN A DASH OF INTEGRITY AND TRUST
For a bubble to have any chance of succeeding each participant nation must be absolutely assured that the other is not permitting tourists to enter from countries with anything other than an exemplary record in controlling the virus.
That rules out any large number of once-lucrative tourism markets such as the US and Europe, both struggling, or in the case of the former not really trying, to suppress the spread of the virus, for the foreseeable future.
With Australia looking to Asian nations, and vice versa, to erect travel bridges, it may eventuate that Australians and New Zealanders will be the only western tourists allowed to enter participant Asian countries during the pandemic.
As evidenced by the unwelcome entry of New Zealanders into Victoria and Western Australia - which had not agreed to be part of the initial trans-Tasman bubble - in recent days, bubble participants need to be provided with clear, easily-accessed guidelines on what they can and cannot do.
A TRAVEL BUBBLE WILL NEED LASHINGS OF POLITICAL WILL AND COURAGE
Here's one the hardest ingredients to find and add. It could be argued that the better a nation has performed at containing COVID-19 the more difficult it is going to be to relaunch international travel.
There are simply too many hard-won gains over the past, momentous months, to protect and so much political capital to be shredded, threatening future electoral success.
As we've seen with states like Queensland and Western Australia, leaders are not shy of allowing political considerations to directly influence their decision-making in a pandemic.
International travel bubbles will be no less vulnerable to the vagaries of political decision-making; bad news for an already ailing, some say near-death tourism industry.
A TRAVEL BUBBLE WILL NEED ENOUGH STRENGTH SO AS NOT TO BURST
There can be no certainty in such uncertain times but a modicum of it will be necessary to ensure that prospective tourists are confident in entering into it and that the level of risk - and in a pandemic there is no such thing as risk-free travel - is acceptable.
Singapore and Hong Kong have indicated that they will deflate their bubble should there be an outbreak of COVID-19 during it. But in doing so, they may make it much harder to relaunch a bubble as less confident and fearful travellers could be reluctant to participate.
Indeed, one overriding question for political leaders, without an effective vaccine, is how perfect do they want and expect their bubble to be? How many, or any, cases of COVID-19, should a prick, or worse, in the bubble reveal itself, will they be prepared to tolerate and manage from both sides?
A TRAVEL BUBBLE WILL NEED THE CRUCIAL INGREDIENT OF THE RIGHT TIMING
The longer a travel bubble is delayed, the less the benefit for damaged economies. Our nearest neighbours, New Zealand and the various Pacific island nations, including Fiji, are massively dependent on tourism.
In the case of the Pacific, diplomatic and geopolitical considerations may come into play, given China's growing influence in the region.
Tourism accounts for 20 per cent of New Zealand's economy, with internal travel in a country of only 5 million not a sustainable method for propping up a tourism industry in the long-term (take note Western Australia, Queensland and Tasmania).
The situation is even more grave for Fiji where tourism represents 40 per cent of the nation's gross domestic product.
There will come a time when leaders, for the sake of their fragile economies and tourism industries, will need to emerge from their "COVID-19 cul-de-sacs", as they've been described, and commit to a travel bubble or face possible long-term financial ruin.
A TRAVEL BUBBLE WILL NEED TO ADDRESS PRESSING ISSUES FOR TRAVELLERS
One of the many unanswered questions surrounding travel bubbles is whether participants will be able to access travel insurance that will cover them in the event they contract COVID-19 while abroad or on return to their home nations.
Could it be that governments, even with the advent of an effective vaccine, will need to underwrite travel insurance and medical care for its citizens if a travel bubble is to succeed (Australia does maintain reciprocal medical agreements with certain countries and some more may need to be established).
If so, these measures will add to the cost of launching and maintaining a bubble and possibly the cost of travel itself.
They are already likely to prove costly, with mass rapid testing at airports becoming the new and essential international travel burden, akin to the extreme security measures that were introduced after September 11.
Authorities will need to make the process of travel as seamless as possible, lest travellers be deterred from holidaying overseas at all.
Finally, one other serious consideration for travel bubbles is the potential for security lapses as a result of the focus on complex health measures at major airports and elsewhere.
There's no doubt that the world's terrorists will be ready and willing to exploit any vulnerabilities in international travel that may emerge as a result of the pandemic. A witch's brew indeed.
Anthony Dennis is the editor of Traveller in The Sydney Morning Herald and The Age. He wrote the first story on the trans-Tasman bubble in April.