Typically, a vaccine must show that it works in trials involving thousands of volunteers before regulators consider permitting its use outside of research. While China and Russia are using special regulatory provisions to deploy Covid vaccines before they’ve undergone full testing, nine U.S. and European companies that are in the forefront of the vaccine effort have forsworn any shortcuts. It can take as long as a year for the Food and Drug Administration in the U.S. to decide on an approval request after vaccine trials are complete, but regulators there as well as those in the U.K. and the European Union have laid out fast-track options due to the pandemic.

To run a vaccine candidate through the necessary stages of testing requires producing tens or perhaps hundreds of thousands of doses. Making enough of a proven vaccine to supply the world in the midst of a worldwide crisis is a job of a different magnitude. The need to secure sufficient quantities of the special glass vials that hold vaccines adds to the task. A number of companies have already scaled up manufacturing capacities for their candidates. Because each vaccine has its own production process, what matters is the extent to which any vaccine that actually crosses the finish line can be mass produced. For that reason, public health specialists are hoping that more than one proves safe and effective in short order. The U.S. already has begun to stockpile doses of experimental vaccines backed by government funding, betting regulators will give a nod to one or more.

Transporting vaccines from manufacturing sites to everywhere they’re needed would be a huge undertaking. By one estimate, airlifting single-dose regimens to protect the world’s population would require the space in about 8,000 cargo planes. The scale and speed of the mobilization being planned by global health groups and governments is unprecedented, according to Gian Gandhi, a supply specialist at Unicef who is coordinating distribution efforts including the advance purchase of hundreds of millions of syringes. Complicating things, vaccines must be refrigerated when transported and stored, and some of those against Covid may need to be kept at temperatures as low as minus-112 degrees Fahrenheit (minus-80 degrees Celsius). The goal is to have 65,000 vaccine fridges in place in poorer countries by year’s end, according to Gavi, a nonprofit dedicated to increasing access to immunization in such regions.

Most of the approaches to a Covid vaccine depend on two shots, which would significantly complicate a rollout. A two-jab vaccine would more or less double the manufacturing, shipping and refrigeration challenges. It would mean twice as many needles are required for injections. And because some people don’t return for the second of a two-dose vaccine, it would make it more difficult to ensure that enough people are immunized against Covid-19 to stop its spread. Johnson & Johnson and rival Merck & Co. are betting on one-shot vaccines, which could give them a distribution edge.

Wealthy countries aren’t expected to have trouble finding money to purchase vaccines: many have already struck deals to buy those that pan out. The risk for poor countries is that rich ones will monopolize vaccine supplies, as they did in the 2009 swine flu pandemic. A program called Covax — led by Gavi, the Coalition for Epidemic Preparedness Innovations and the World Health Organization — aims to ensure that vaccines are distributed equitably around the world, with rich and poor countries receiving supplies at the same time. Its program for funding vaccines for developing countries has raised $700 million of a $2 billion target for 2020. Apart from purchasing vaccines, countries will have to fund distribution networks. U.S. officials say an estimated $6 billion in additional money is needed to help states prepare for a rollout there, for example. Still, vaccine campaign expenses are likely to pale in comparison to the cost of the pandemic: an estimated $375 billion a month globally.

Amid expectations of huge demand and limited supply, there’s concern vaccines could be diverted illegally. Novel packaging and technology including bar codes to trace doses in transit are under consideration, according to Unicef. Criminal organizations in the U.S. have already sought to smuggle fraudulent, mislabeled and unauthorized Covid products, including personal protective equipment and test kits. The stakes would be even higher with vaccines.

Vaccines are seen by health experts as the key to leading the world out of the pandemic, but not everyone will embrace them. The persistent myth that childhood vaccines pose significant risks has undercut confidence in immunization in many countries. In a few others, particular vaccine campaigns have generated distrust. The fast pace at which Covid shots are being developed has added to safety worries. In a poll in August, about a third of Americans said they wouldn’t get a Covid vaccine. One in six respondents in the U.K. said they definitely or probably wouldn’t in a June survey. A vaccine rollout can stop a virus from circulating by establishing so-called herd immunity. It’s reached when a significant percentage of the community has developed immunity by getting infected or getting a vaccine. For Covid-19, the percentage is estimated to range from 55% to 82%.

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