A coronavirus vaccine that is safe and at least 50% effective would be a “game changer” in battling the pandemic, according to a doctor who is serving as an investigator for Moderna‘s clinical trials.

“Developing vaccines against respiratory virus is incredibly difficult. If you think about the flu vaccine that we use every year, it’s only about 40% to 60% effective,”  Dr. Carlos del Rio of Emory University said Tuesday on CNBC’s “Squawk Box.”

The Food and Drug Administration indicated last month it would authorize a coronavirus vaccine as long as it is safe and at least 50% effective. Dr. Stephen Hahn, the agency’s commissioner, said at the time it would be “unrealistic” to expect a vaccine to be 100% effective. 

A vaccine that is 50% effective means it would reduce the average person’s risk of coronavirus infection by 50%.

“A 50% efficacy would be transformative. It’s much better than zero, which is what we have right now,” del Rio told CNBC. ”So I think it’d really be a game changer if we get a vaccine with 50% or greater efficacy.”

Dr. Anthony Fauci said, the nation’s leading infectious disease expert, said earlier this month that scientists hope a vaccine to prevent the coronavirus would be at least 75% effective. But the White House health advisor said 50% to 60% efficacy also would be acceptable.  

“You’ve got to think of the vaccine as a tool to be able to get the pandemic to no longer be a pandemic, but to be something that’s well controlled,” Fauci said Aug. 7 during a question-and-answer session with Brown University’s School of Public Health. 

Del Rio said widespread inoculation of a vaccine, even one with 50% efficacy, would help the U.S. achieve so-called herd immunity — the point where enough people have antibodies to fight off the virus and therefore significantly curtail its spread in a population. 

“Between the people that have been infected plus the people that you’ve vaccinated, at 50% you would get there,” said del Rio, a professor in the infectious disease division of Atlanta-based Emory’s medical school. He also is co-director of the Emory Center for AIDS Research. 

It is possible people who receive a coronavirus vaccine — should it receive regulatory approval — may still get infected, del Rio noted. However, being vaccinated could reduce the risk of severe illness from Covid-19, he said. 

“One of the theories is that you may still get infected but you will not develop complications. You will not end up in the hospital. You will not end up in the ICU,” said del Rio. “And that I think also would be quite transformative because you will definitely decrease the morbidity and mortality of the disease.” 

Del Rio is an investigator for Moderna’s potential coronavirus vaccine. The Massachusetts-based biotech firm began its late-stage human trials last month. The National Institutes of Health has been working alongside Moderna, which was the first company to begin human trials in the U.S. in March. 

Moderna has received commitments worth up to nearly $2.5 billion from the U.S. government to aid its vaccine development. Through Operation Warp Speed, the government is providing billions of dollars to various companies that are working on a vaccine. 

As part of those deals, companies are ramping up manufacturing capacity to expedite the deployment of a vaccine, should it prove safe and effective. Del Rio said that is the right strategy. 

“So if everything goes well, you may be talking about March or April of next year where you can say, ‘OK, we’ve got a vaccine and we’ve got plenty of doses to start giving out to people,\'” said del Rio.