The head of the government's effort to develop a vaccine against Covid-19 says he expects the vaccine to have efficacy rates "in the 90%" range -- but that there might not be enough vaccine available for all Americans until the end of next year.
"I think it will be a very effective vaccine. That's my prediction," said Moncef Slaoui, chief adviser of Operation Warp Speed. "My personal opinion based on my experience and the biology of this virus, I think this vaccine is going to be highly efficacious. I wouldn't be surprised if it's in the 90% [range]."
Slaoui, an immunologist, formerly headed up the vaccine program at GlaxoSmithKline, where he led the development of five major novel vaccines.
He said there would be enough vaccine for all Americans "ideally" by the middle of 2021, but possibly not until the end of 2021.
Slaoui spoke Thursday with CNN Senior Medical Correspondent Elizabeth Cohen while touring a Covid-19 vaccine clinical trial site in Savannah, Georgia. It was his first television interview since joining Operation Warp Speed in May.
When will there be a vaccine for everyone?
In June, Dr. Anthony Fauci gave Cohen a less optimistic forecast for the efficacy of a Covid-19 vaccine.
"The best we've ever done is measles, which is 97 to 98 percent effective," said Fauci, the director of the National Institute of Allergies and Infectious Diseases. "That would be wonderful if we get there. I don't think we will. I would settle for [a] 70, 75% effective vaccine."
The US Food and Drug Administration has set an even lower standard for a Covid-19 vaccine, stating in its guidelines that the efficacy rate "should be at least 50%."
Vaccines have various effectiveness rates. According to the US Centers for Disease Control and Prevention, a full round of polio vaccines is 99% to 100% effective; full vaccination against diphtheria, tetanus and pertussis is 80-90% effective, while the flu shot is 40-60% effective.
Slaoui added that with a Covid-19 vaccine, "it's possible that we will need a booster" after initial vaccination "every year or every two years or every three years."
Slaoui said he expects "to probably have a few tens of millions of doses" of vaccine in December of this year or January of next year, and those would go to high-risk individuals because "we will not have doses for the full US population on day one."
High-risk individuals include the elderly and those with underlying health conditions such as hypertension, diabetes, and cardiovascular disease.
Slaoui said he's "optimistic" there will be vaccines for all Americans by the end of 2021, but ideally by the middle of 2021.
"That's our objective," he said.
Vaccine trials moving at record pace
Operation Warp Speed is funding eight vaccines, Slaoui said.
Five of those are already in Phase 3 clinical trials or will be starting by the end of September, he said. A Phase 3 trial is the last round of tests before a vaccine goes before regulators, and may be put on the market. The Covid-19 vaccine Phase 3 trials will each involve 30,000 study subjects.
"They are all different vaccines. They are actually going very fast," Slaoui said, noting that the virus was identified just six months ago. "I've been doing this for 30 years, and the fastest thing that I ever can remember that went from discovery to Phase Three trials was in four years."
Vaccines made by Moderna and Pfizer are in Phase 3 in the United States, and a vaccine by AstraZeneca is in Phase 3 trials in the United Kingdom, Brazil, and South Africa. Phase 3 trials for that vaccine are expected to start in August in the US.
Phase 3 trials for vaccines made by two other companies -- Johnson & Johnson and Novavax -- are scheduled to begin in September, he added.
On Friday, Sanofi and GlaxoSmithKline announced they're receiving Operation Warp Speed funding, and expect to start a Phase 3 trial by the end of the year.
Slaoui did not name the other two vaccines that will receive federal funding.
CNN's John Bonifield and Dana Vigue contributed to this report.