When Ebola spread through West Africa in 2014, a global network of researchers faced what appeared to be an impossible task.
Not only would they have to develop and test an Ebola vaccine (something that had never been done), but there was the problem of distribution.
The deadly virus had reached villages with no power and no access beyond narrow dirt roads that would be impassable in the rainy season.
The vaccine they planned to use would have to be kept very cold: about -80 degrees Celsius, or four times more chilly than a household freezer.
They would have to find a way to safely store the vaccine for up to a week without electricity.
And they had three months to find a solution.
In the end, they invented what is essentially a giant space-age thermos that can maintain the temperature of an Antarctic winter for weeks on end.
The Arktek is an esky with such good insulation that it could be packed with ice and left in the blazing desert sun for up to a month without dropping a degree.
As Australia prepares to distribute the recently approved Pfizer vaccine at -70C, the story of how these researchers overcame technical obstacles to successfully roll out a vaccine could also hold vital lessons.
Building a better esky
By late 2014, the Ebola situation was dire. Thousands had died in West Africa and the virus had spread to the US and the UK.
But there was some good news: a vaccine would be ready within months.
In December 2014, the World Health Organization asked a company of inventors, Intellectual Ventures (IV), to solve the problem of how to distribute the vaccine without power, while also storing it safely at -80C.
Daniel Lieberman, a mechanical engineer at the company at the time, was part of the team that assembled in a building near Seattle.
"We turned to the Arktek," he said.
Intellectual Ventures originally developed the Arktek a few years earlier for the distribution of polio vaccines in places without power in response to a challenge from Bill Gates, who nicknamed the invention "the keg of life".
The design uses vacuum barriers, fibreglass and shiny internal coatings to reduce heat transfer between the warm outside and cold inside.
Blocks of ice frozen in semi-circular containers are used for cooling.
"Our design was extremely leak tight," Mr Lieberman said.
Unfortunately, it was only good for vaccines stored between 2 and 8C.
Having to make it work at -80C was an "absolutely massive" problem, Mr Leiberman said.
In December 2014, they set to work. They had until March.
Solving the cooling problem
The researchers focused on finding a better way of cooling the Arktek — what they called a "thermal battery".
In the original Arktek, this battery was simply frozen water.
Frozen carbon dioxide or dry ice, which has a surface temperature of -78C, was an option, but there wasn't enough in West Africa to make this work.
Stumped and desperate for a solution, they turned to a relatively obscure class of chemicals called phase-change materials (PCMs). These are synthetic molecules designed to freeze or thaw at specific temperatures.
Experimenting with different ingredients, they found a combination of denatured alcohol and water froze at -75C — good enough for their purposes.
"It turns out that you can create ice packs of almost any temperature you like, you just can't use water, you have to use something else," Mr Lieberman said.
Within months, modified Arkteks packed with PCMs were deployed to West Africa.
Between April and December 2015 health workers vaccinated 8,000 people against Ebola.
Three months later, the epidemic was declared over.
Largest roll out of an ultra-cool vaccine in the world
Only two years later, Ebola returned — this time in the Democratic Republic of the Congo (DRC) in Central Africa.
Authorities acted fast — cases had been recorded in the city of Mbandaka, not far from the capital of Kinshasa, a city of 14 million.
With 16 Arkteks and a few ultra-cold freezers to supply the PCM cold bricks, health workers inoculated more than 300,000 people in a bit over two years.
When the roads were flooded and cars could not get through, Arkteks were reportedly strapped to canoes or the backs of motor bikes.
The DRC's program, which brought the epidemic to an end, remains the largest rollout of an ultra-cold vaccine, according to GAVI, an alliance of governments, drug companies and charities that arranges global vaccination campaigns.
"The heroic efforts of Congolese vaccinators, logisticians and health workers meant over 300,000 people in the DRC and neighbouring countries consented to and received the vaccine," a spokesperson said.
"In an equatorial region with intermittent electricity, poor roads and few health clinics, this could have been an insurmountable challenge.
"[DRC's success] shows it is feasible, at least at this scale, but also hugely complex."
Esky upgraded for COVID vaccine distribution
With the roll-out of Pfizer's COVID vaccine at -70C, the Arktek may be deployed again.
Mr Lieberman, who is now at the non-profit GH Labs, has modified the device to use dry ice, which he says is a better thermal battery and easy to access in many countries affected by COVID.
"There are many, many, countries that have reliable carbon dioxide, and dry ice mass-production capacity," he said.
Pfizer is already using dry ice to transport its ultra-cold vaccines, but its cooler boxes are not as sophisticated as the Arktek, according to Mr Lieberman.
Made from cardboard and foam, Pfizer's 'thermal shippers' can keep doses for 15 days so long as they are refilled with dry ice every five days.
The Arktek, by comparison, lasts up to four weeks between refills, Mr Lieberman said.
"It has a heat leak rate 10 to 15 times less than something like the Pfizer cool box — that kind of rule of magnitude," he said.
A backup solution for remote areas in Australia?
Whether the Arktek will be needed for distribution of ultra-cold COVID vaccines in Australia or elsewhere in the world remains to be seen.
Earlier this week, Australia approved the Pfizer vaccine for use, with the first jabs in February.
Distributing the Pfizer vaccine to remote areas will be a challenge — the 'thermal shippers' each hold 5,000 doses that have to be thawed together.
Once thawed, they must be kept in the fridge and used within five days.
If a box was to be unthawed in a small town or community, most of the doses would be wasted.
Health authorities plan to avoid the worst of the logistics problems by only distributing the Pfizer vaccine from 30 to 50 urban and regional hospitals.
The really remote places, such as some communities in the Northern Territory, are more likely to get the Oxford-AstraZeneca vaccine, which can be stored in a regular fridge.
Even so, GH Labs is working with GAVI and the Bill and Melinda Gates Foundation to look at ways the Arktek could be used to distribute the Pfizer vaccine more broadly.
Mr Lieberman said one idea was using the Arktek as part of a 'hub and spoke' model.
"The Pfizer box would arrive at a central location and then you'd use other tools like the Arktek to distribute more regionally within a country," he said.
Some countries have ordered Arkteks for their COVID vaccine roll outs.
Aucma, the Chinese company that manufactures the US$2000 devices, said it would double production this year to meet the higher demand.
Rural Doctors Association Australia President Dr John Hall said the Arktek would "absolutely" be useful in the final stage of delivering the Pfizer vaccine to remote areas.
"If there's no backup solution to delivering Pfizer, we could see significant disadvantages for rural and remote Australia," he said.
"All it takes is for a major producer to have a fault and we could see delays of weeks or months."