When could a COVID-19 vaccine be ready?

Top health officials say the end of this year would be a best case scenario. But scientists have never created a vaccine so quickly, and there’s no guarantee any under development will ultimately work.

Worldwide, testing recently started or is about to start for about a dozen potential vaccines. The most promising vaccine candidates are expected to move into larger tests this summer.

How quickly those studies can determine whether the vaccines are safe and effective depends in part on how widely the coronavirus is still spreading. The studies will need to enroll 20,000 people or more for each vaccine candidate, with half of them getting the real vaccine and the rest getting a dummy shot. Then it’s a matter of waiting to see how many in each group become infected with the virus.

Answers will come faster if volunteers are recruited in places where outbreaks are worsening, a trend that can be hard to predict.

One way to speed up distribution is to start manufacturing doses before test results are in. But it’s a gamble that could mean throwing away tons of vaccines that fail.