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With trials underway, how do we know if or when a COVID-19 vaccine is ready?

There are specific markers researchers are looking at to determine if a vaccine trial is a success.

ST. PETERSBURG, Fla. — Researchers are aiming to enroll at least 30,000 volunteers in each COVID-19 vaccine trial.

While political promises swirl around how soon an effective vaccine will be available, University of South Florida virologist Dr. Michael Teng says some key metrics will determine if a trial is a success.

“You have to have a certain number of people get infected to be able to see the difference between the vaccine group and the control group,” Teng said.

That number is between 150 to 200 lab-confirmed infections, according to Teng, which are referred to as “events.”

It might not seem like many, but it’s enough to be able to compare the vaccinated group with the un-vaccinated group.

"It's significant enough to tell whether you have a 50-percent efficacy or not,” he said.

The U.S. Food and Drug Administration has set an efficacy target of 50 percent—or a 50-percent decrease in the number of symptomatic COVID-19 infections.

RELATED: 'I trust the data': Medical experts say COVID-19 vaccine approved by the FDA will be safe

Teng says that’s about in line with the flu vaccine, which is widely considered to be effective at slowing transmission of the virus or, at least, making symptoms less severe for individuals who still become infected.

“It’s extremely rare to have a 100-percent effective vaccine,” Teng said. “Even some of the best vaccines we have right now are in the 90-percent efficacy range and that’s phenomenal—like the measles vaccine.”

In these trials, some volunteers are injected with the vaccine while others are injected with a placebo. But neither the person administering the injections nor the people getting them know which is which.

So how will we know when a vaccine is working?

That’s hard to say, says Teng, because these are “event-driven trials” so volunteers need to be naturally exposed to the virus.

Due to uncertainty about where and how often infections will happen, Teng says it’s difficult to predict how long it will take for trials to complete.

But we could get our first look at early data from trials as early as next month, Teng said. That’s when an independent data safety monitoring board made up of experts tracking the data will meet to review progress.

Currently, Bio-N-Tech, Pfizer, AstraZeneca and Moderna all have vaccines in Phase 3 trials.

AstraZeneca announced late Tuesday it was pausing its trial to investigate if a patient with serious side effects is linked to their vaccine shot.

Novavax and Johnson & Johnson are currently in Phase 2 trials while Sanofi, GSK and Merck are all in Phase 1.

All nine drugmakers on Tuesday vowed they wouldn't put out a COVID-19 vaccine unless data shows it is safe.

Recent polling reveals voters are skeptical about a potential COVID-19 vaccine.

Just 21 percent of voters nationwide now say they would get a vaccine as soon as possible if one became available at no cost, down from 32 percent in late July, according to CBS News. Most would consider it but would wait to see what happens to others before getting one.

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