A guide to the dozens of COVID-19 vaccines that are in development.

More than 150 coronavirus vaccines are in development across the world—and hopes are high to bring one to market in record time to ease the global crisis. Several efforts are underway to help make that possible, including the U.S. government’s Operation Warp Speed initiative, which has pledged $10 billion and aims to develop and deliver 300 million doses of a safe, effective coronavirus vaccine by January 2021. The World Health Organization is also coordinating global efforts to develop a vaccine, with an eye toward delivering two billion doses by the end of 2021.

The candidates, like all vaccines, essentially aim to instruct the immune system to mount a defense, which is sometimes stronger than what would be provided through natural infection and comes with fewer health consequences.

To do so, some vaccines use the whole coronavirus, but in a killed or weakened state. Others use only part of the virus—whether a protein or a fragment. Some transfer the coronavirus proteins into a different virus that is unlikely to cause disease or even incapable of it. Finally, some vaccines under development rely on deploying pieces of the coronavirus’s genetic material, so our cells can temporarily make the coronavirus proteins needed to stimulate our immune systems. (Here’s what vaccines are and how they work.)

It can typically take 10 to 15 years to bring a vaccine to market; the fastest-ever—the vaccine for mumps—required four years in the 1960s. Vaccines go through a multi-stage clinical trial process, which starts by checking their safety and whether they trigger an immune response in a small group of healthy humans. The second phase widens the testing pool to include groups of people who may have the disease or be more likely to catch it, to gauge the vaccine’s effectiveness. The third phase expands the pool up to the thousands to make sure the vaccine is safe and effective among a wider array of people, given that immune response can vary by age, ethnicity, or by underlying health conditions. It then goes to regulatory agencies for approval—which can be a lengthy process itself.

Even after a vaccine is approved, it faces potential roadblocks when it comes to manufacturing and distribution, from scaling up the production to meet demands to deciding which populations should get it first—and at what cost. Many vaccines also stay in what’s called phase four, a perpetual stage of regular study. (Here's how we'll know when a COVID-19 vaccine is ready.) But vaccine developers are attempting to compress that process for SARS-CoV-2 by running clinical trial phases simultaneously, and the U.S. Food and Drug Administration has promised to fast-track the approval process.

Though it’s too soon to say which candidates will ultimately be successful, here’s a look at the vaccine prospects that have made it to phase three and beyond—including a quick primer on how they work and where they stand. ...