Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, testifies at a House subcommittee hearing in Washington, D.C., the United States, on July 31, 2020. U.S. top infectious disease expert Anthony Fauci said on Friday he is "cautiously optimistic" that the United States would have a safe and effective COVID-19 vaccine this late fall or early winter. (Kevin Dietsch/Pool via Xinhua)

US top infectious disease expert Anthony Fauci said on Friday he is "cautiously optimistic" that the United States would have a safe and effective COVID-19 vaccine this late fall or early winter.

"We hope that by the time we get into late fall and early winter, we will have in fact a vaccine that we can say that would be safe and effective. One can never guarantee the safety or effectiveness unless you do the trial, but we are cautiously optimistic this will be successful," said Fauci, director of the US National Institute of Allergy and Infectious Diseases, at a House subcommittee hearing.

An experimental COVID-19 vaccine being developed by Fauci's agency and American biotechnology company Moderna, known as mRNA-1273, started phase 3 clinical trial on Monday to evaluate if it can prevent COVID-19 in adults.

The trial, which will be conducted at US clinical research sites, is expected to enroll approximately 30,000 adult volunteers who do not have COVID-19.

According to Fauci, in the phase 1 study of the vaccine, it clearly showed that individuals who were vaccinated mounted a neutralizing antibody response that was at least comparable and in many respects better than in convalescent serum from individuals who had recovered from COVID-19.

Fauci said a COVID-19 vaccine may not be available to all Americans immediately, but in phases.

He reassured lawmakers that all safety precautions will be taken by the US Food and Drug Administration before the vaccine is made available to the public, encouraging all Americans to take the vaccine.