CNN  — 

While coronavirus keeps spreading and killing with impunity, the world waits for a vaccine that could quash the pandemic.

But details and timelines keep shifting. Here’s the latest on where we stand in the race for a vaccine:

When will a Covid-19 vaccine be available to the public?

No one’s sure yet, but the target is sometime in early 2021.

Vaccines in development around the world are in various stages of testing. Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said he’s confident one of the vaccine candidates will be proven safe and effective by the first quarter of 2021.

But it’s not clear which candidate shows the greatest promise yet.

In the meantime, the US government is helping companies such as Moderna ramp up development of their candidate vaccines so that if they’re proven to work safely, they can be rolled out quickly.

“By the beginning of 2021, we hope to have a couple of hundred million doses,” Fauci said.

Dr. Francis Collins, director of the National Institutes of Health, gave a similar forecast: “If all goes well, maybe as many as 100 million doses by early 2021” would be possible, Collins said.

But many doctors say getting an effective vaccine out by January is a highly ambitious goal.

“Everything will have to go incredibly perfectly if that’s going to happen,” said Dr. Larry Corey, an expert in virology, immunology and vaccine development.

Why does it take so long to develop a vaccine?

Vaccines have to go through multi-phase trials to make sure they’re effective and safe.

Typically, a vaccine takes eight to 10 years to develop, said Dr. Emily Erbelding, an infectious disease expert at the NIAID.