COVID-19 vaccine candidates will enter late-stage clinical studies by the end of the month, with others beginning in August, September and October, the U.S. government's top infectious diseases expert said on Thursday.

The news comes as Moderna Inc, which is at the forefront of the country's vaccine development efforts, reiterated earlier in the day that a late-stage trial with 30,000 volunteers would begin this month.

“We may be able to at least know whether we are dealing with a safe and effective vaccine by the early winter, late winter, (or) beginning of 2021,” Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, said in an interview to JAMA Network. Earlier on Thursday, Dr. Francis Collins, the director of the U.S. National Institutes of Health, said the Trump administration's vaccine-acceleration program could generate a safe and effective COVID-19 vaccine by year-end.