The world's most promising coronavirus vaccine candidates need to be kept in cold storage to be safe and effective.

Health officials have noted progress in equipping developing countries with the machinery needed to keep the vaccine cold. Yet nearly 3 billion people live in places where temperature-controlled storage is lacking for a vaccination campaign to bring the virus under control.

The result: Poor people who were among the hardest hit by COVID-19 are also likely to be the last to recover from it.

Maintaining the "cold chain" for coronavirus vaccines will not be easy even in the richest countries. To stay effective, some vaccines must be kept in temperatures of around minus 70 degrees Celsius. Investment in cooling technology is lower this year because of the COVID-19 health crisis. So is spending for transportation and other infrastructure.

Experts warn that large parts of the world lack the refrigeration equipment necessary to administer an effective vaccination program. This includes most of Central Asia, much of India and Southeast Asia, parts of Latin America, and all but a very small part of Africa.

Broken cold chain

The cold chain breaks down at Gampela, a small medical center in the West African nation of Burkina Faso. The center serves a population of 11,000. And it has gone nearly a year without a working refrigerator.

After its refrigerator broke last year, the center could no longer keep vaccines against diseases such as tetanus, yellow fever, and tuberculosis. Nurse Julienne Zoungrana said workers used motorbikes to get vaccines from a hospital in the capital, Ouagadougou. They made the 40-minute round-trip on a narrow road.

Adama Tapsoba is a mother of two small children. She often walks four hours under the hot sun to get vaccinations for her baby and waits hours more to see a doctor. Recently, her 5-month-old son had missed a scheduled vaccine shot because Tapsoba's daughter was sick and she could only bring one child on foot.

"It will be hard to get a (COVID-19) vaccine," Tapsoba said, "People will have to wait at the hospital, and they might leave without getting it."

To maintain the cold chain in developing nations, international organizations have added tens of thousands of solar-powered vaccine refrigerators. From the time vaccines are made until they are given to patients, the cold chain also requires mobile refrigeration, dependable electricity, good roads and careful planning.

For poor countries like Burkina Faso, the best chance of receiving a coronavirus vaccine is through the Covax initiative. It is a project of the World Health Organization (WHO) and the Gavi vaccine alliance.

The goal of Covax is to place orders for several promising vaccine candidates and to provide the safest, most successful ones to all nations.

The United Nations' children's agency (UNICEF) began preparing for the distribution of COVID-19 vaccines months ago in Copenhagen. There UNICEF crews are busy at work in the world's largest supply center for humanitarian aid. They are trying to predict shortages by learning from the past, like when protective equipment disappeared from airports or was stolen.

The WHO says 42 coronavirus vaccine candidates are currently being tested in human volunteers. The vaccines most likely to be offered by the Covax initiative must be stored at 2 to 8 degrees Celsius.

The American drug company Pfizer is testing one of the most promising vaccine candidates. It requires storage at temperatures of minus 70 degrees Celsius. The company has designed a special carrying case for its vaccine. Pfizer has signed deals to supply the vaccine to the United States, Europe and Japan. It has also expressed an interest in Covax.

Medical freezers that go down to minus 70 degrees Celsius are rare even in U.S. and European hospitals. Many experts believe some West African countries may be best positioned. Those areas suffered through the Ebola health crisis from 2014 to 2016. Like the coronavirus vaccine, the Ebola vaccine requires very cold storage.

Since 2017, Gavi and UNICEF have worked to supply much of Africa and Asia with freezers for storing vaccines. UNICEF is now offering governments a list of what they will need to maintain a vaccine supply chain and they are asking them to develop a plan.

"The governments are in charge of what needs to happen in the end," said Benjamin Schreiber, who is among the directors of the UN agency's vaccination program.

Transporting vaccines

Vaccines do not last long. Container ships are not equipped to refrigerate drugs for long periods. Shipping vaccines by air costs a lot more.

The WHO estimates that as much as half of vaccines are lost internationally because of waste, theft or heat during shipping.

The German shipping company DHL estimated that 15,000 flights would be needed to send COVID-19 vaccines around the world. That would stretch the availability of aircraft and supplies of cooling materials such as dry ice to protect the vaccines.

"We need to find a bridge" for every gap in the cold chain, Katja Busch of DHL said. "We're talking about investments ... as a society, this is something we have to do."

Gavi and UNICEF have experimented with sending vaccines by drone aircraft. Indian officials have also suggested the idea of setting aside part of the country's large food storage system for coronavirus vaccines.

In countries such as India and Burkina Faso, a lack of public transportation presents another barrier to protecting citizens before vaccines go bad. When parts of Venezuela lost power for a week last year, the largest children's hospital in the country had to throw away thousands of shots of vaccines for diseases like diphtheria.

Back in Burkina Faso, a solar-powered freezer finally arrived days after reporters from The Associated Press visited the health center near the capital. Health workers are waiting to be sure the freezer works before storing it with vaccines.

Nationwide, Burkina Faso needs another 1,000 medical freezers. Health officials said less than 40 percent of the health centers that provide vaccines have working freezers.

If Burkina Faso were given 1 million shots of coronavirus vaccine today, the country would not be able to administer the vaccine program.

Jean-Claude Mubalama is UNICEF's head of health and nutrition for the African nation. He said, "If we had to vaccinate against the coronavirus now, at this moment, it would be impossible."

I'm Dorothy Gundy.

And I'm Jonathan Evans.

The Associated Press reported this story. Hai Do adapted the story for Learning English. George Grow was the editor.

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maintain - v. to provide support, to keep something in good condition

chain - n. a series of things that are connected to each other

nurse - n. a person who is trained to care for sick or injured people

scheduled - adj. planned at a certain time

solar - adj. of or relating to the sun

distribution - n. the act of delivering something

gap - n. space where something is missing