An approved coronavirus vaccine could end up being effective only 50-60 percent of the time, meaning public health measures will still be needed to keep the pandemic under control, Dr. Anthony Fauci, the top U.S. infectious diseases expert, said on Friday. 

"We don't know yet what the efficacy might be. We don't know if it will be 50 percent or 60 percent. I'd like it to be 75 percent or more," Fauci said in a webinar hosted by Brown University. "But the chances of it being 98 percent effective is not great, which means you must never abandon the public health approach."

On the same day, U.S. Food and Drug Administration Commissioner Stephen Hahn said the country will need to have independent experts review COVID-19 vaccine candidates before approval. 

He said COVID-19 vaccine candidates will be reviewed according to established legal and regulatory standards for medical products, including by an outside advisory committee. 

"Given the widespread potential use of a COVID-19 vaccine, transparent discussion at FDA's Vaccines and Related Biological Products Advisory Committee will be needed," Hahn and colleagues wrote in the Journal of the American Medical Association. 

The U.S. government's Operation Warp Speed aims to expedite development of a vaccine and therapies to treat the novel coronavirus, and the emphasis on speed has provoked public anxiety about the safety and effectiveness of these vaccines, Hahn and the other officials wrote. 

The novel coronavirus has infected nearly five million people in the United States and killed more than 160,000. 

Lockdown measures imposed to keep the virus from spreading have devastated the economy, which suffered its biggest blow since the Great Depression in the second quarter, with gross domestic product dropping at its steepest pace in at least 73 years. 

As infections have spiked around the country after states started to open up, public health experts, including Fauci, have stressed the importance of steps that each American can take, including social distancing, washing hands and wearing masks. 

Fauci said on Friday that studies of Moderna's COVID-19 vaccine could produce definitive data in November or December of this year. 

He said earlier this week that he expects tens of millions of COVID-19 vaccine doses to be available by early 2021, and a billion doses by the end of that year. He also said political pressure will not determine when a coronavirus vaccine is approved. 

U.S. President Donald Trump has put forward a more optimistic forecast, saying on Thursday the country could have a coronavirus vaccine before the November 3 election.