BURKINA FASO: The chain breaks here, in a tiny medical clinic in Burkina Faso that went nearly a year without a working refrigerator.
From factory to syringe, the world's most promising coronavirus vaccine candidates need non-stop sterile refrigeration to stay potent and safe.
But despite enormous strides in equipping developing countries to maintain the vaccine "cold chain," nearly 3 billion of the world's 7.8 billion people live where temperature-controlled storage is insufficient for an immunization campaign to bring COVID-19 under control.
The result: Poor people around the world who were among the hardest hit by the virus pandemic are also likely to be the last to recover from it.
The vaccine cold chain hurdle is just the latest disparity of the pandemic weighted against the poor, who more often live and work in crowded conditions that allow the virus to spread, have little access to medical oxygen that is vital to COVID-19 treatment, and whose health systems lack labs, supplies or technicians to carry out large-scale testing.
Maintaining the cold chain for coronavirus vaccines won't be easy even in the richest of countries, especially when it comes to those that require ultracold temperatures of around minus 70 degrees Celsius (minus 94 F).
Investment in infrastructure and cooling technology lags behind the high-speed leap that vaccine development has taken this year due to the virus.
With the pandemic now in its eighth month, logistics experts warn that vast parts of the world lack the refrigeration to administer an effective vaccination program.
This includes most of Central Asia, much of India and southeast Asia, Latin America except for the largest countries, and all but a tiny corner of Africa.
The medical clinic outside Burkina Faso's capital, a dirt-streaked building that serves a population of 11,000, is a microcosm of the obstacles.
After its refrigerator broke last fall, the clinic could no longer keep vaccines against tetanus, yellow fever, tuberculosis and other common diseases on site, nurse Julienne Zoungrana said.
Staff instead used motorbikes to fetch vials in insulated carriers from a hospital in Ouagadougou, making a 40-minute round-trip drive on a narrow road that varies between dirt, gravel and pavement.
A mother of two who visits the Gampela clinic says she thinks a coronavirus inoculation programme will be challenging in her part of the world.
Adama Tapsoba, 24, walks four hours under scorching sun to get her baby his routine immunizations and often waits hours more to see a doctor.
A week earlier, her 5-month-old son had missed a scheduled shot because Tapsoba's daughter was sick and she could only bring one child on foot.
"It will be hard to get a (COIVD-19) vaccine," Tapsoba said, bouncing her 5-month-old son on her lap outside the clinic.
"People will have to wait at the hospital, and they might leave without getting it."
To uphold the cold chain in developing nations, international organisations have overseen the installation of tens of thousands of solar-powered vaccine refrigerators.
Keeping vaccines at stable temperatures from the time they are made until they are given to patients also requires mobile refrigeration, reliable electricity, sound roads and, above all, advance planning.
For poor countries like Burkina Faso, the best chance of receiving a coronavirus vaccine is through the Covax initiative, led by the World Health Organization and the Gavi vaccine alliance.
The goal of Covax is to place orders for multiple promising vaccine candidates and to allocate the successful ones equitably.
The United Nations' children's agency, UNICEF, began laying the global distribution groundwork months ago, in Copenhagen.
At the world's largest humanitarian aid warehouse, logistics staff are trying to foresee shortages by learning from the past, especially the spring chaos surrounding global shortages of masks and other protective gear that were commandeered off airport tarmacs or stolen and traded on the black market.
Currently, 42 coronavirus vaccine candidates are in clinical trials and another 151 are in pre-clinical evaluation, according to WHO.
The ones most likely to end up in the Covax mix must be stored at 2 to 8 degrees Celsius (25-46 F).
A Pfizer candidate is among the ones in advanced testing requiring storage at ultracold temperatures.
The company, which has designed a special carrying case for its vaccine, has expressed interest in Covax and signed contracts with the United States, Europe and Japan.
Medical freezers that go down to minus 70 degrees Celsius are rare even in U.S. and European hospitals.
Many experts believe the West African countries that suffered through a 2014-16 Ebola outbreak may be the best positioned, because a vaccine against that virus also requires ultracold storage.
For more than two-thirds of the world, however, the advanced technology is nowhere on the horizon, according to a study by German logistics company DHL.
Meanwhile, billions of people are in countries that don't have the necessary infrastructure to maintain the cold chain for either existing vaccines or more conventional coronavirus candidates, the study said.
Opportunities for vaccines to be lost expand the farther a vaccine travels.
DHL estimated that 15,000 cargo flights would be required to vaccinate the entire planet against COVID-19, stretching global capacity for aircraft and potentially supplies of materials such as dry ice.