(CNN) Millions of Americans still need to get vaccinated to slow or stop the spread of Covid-19 and getting the pandemic under control could take "many, many" more vaccine mandates, Dr. Anthony Fauci said.
Fauci, the director of the National Institute of Allergy and Infectious Diseases, said if more people aren't persuaded to get vaccinated by messaging from health officials and "trusted political messengers," additional mandates from schools and businesses may be necessary.
"I believe that's going to turn this around because I don't think people are going to want to not go to work or not go to college ... They're going to do it," Fauci told CNN's Jen Christensen during an interview at the NLGJA, the Association of LGBTQ Journalists, convention Sunday. "You'd like to have them do it on a totally voluntary basis, but if that doesn't work, you've got to go to the alternatives."
The combination of the highly contagious Delta variant and the vaccine holdouts has put the United States in a "very difficult period" of the Covid-19 pandemic, Fauci said.
Of the eligible population in the US, which is currently limited to people 12 and older, 63% are fully vaccinated, according to data from the Centers for Disease Control and Prevention. Health experts and officials are aiming for the vast majority of the population to be inoculated to control the spread.
Last week, President Joe Biden announced vaccine requirements that include a mandate for businesses with more than 100 employees to require vaccination or regular testing for employees.
Businesses that want employees to return to work and stay at work will benefit from vaccine requirements, US Surgeon General Dr. Vivek Murthy said. The mandate will benefit employees as well, he added.
"I believe that will not only improve public health, but it will give people some more peace of mind," Murthy told CNN Sunday.
As the debate over mandates continues, some hospitals are feeling the impact of lagging vaccination rates.
Colorado Gov. Jared Polis sounded the alarm Friday, saying, "We actually have the lowest ICU available rate that we've had since the start of this crisis, in part due to the unvaccinated with Covid and just other types of trauma that goes up seasonally this time of year."
Polis said some hospitals in his state "reaching very close to their capacity limits. And that wouldn't be happening if people were vaccinated."
An international group of vaccine scientists, including some from the US Food and Drug Administration and the World Health Organization, say the current evidence on Covid-19 vaccines does not appear to support a need for booster shots in the general public.
The scientists, who authored a paper about this, include two senior FDA vaccine leaders, Dr. Philip Krause and Marion Gruber, who will be stepping down in October and November, the FDA announced late last month. No further details were released about their retirements, although they sparked questions about whether the departures would affect the agency's work.
The paper's authors write that vaccine efficacy remains high against severe disease, including for the highly transmissible Delta variant, although less so for symptomatic disease.
"Current evidence does not, therefore, appear to show a need for boosting in the general population, in which efficacy against severe disease remains high," the scientists write in the new paper, published Monday in the medical journal The Lancet.
The FDA and other public health agencies around the world continue to examine evidence on Covid-19 vaccine efficacy and the role booster doses of the vaccine might play in improving immunity against the disease.
Other data on the subject will soon be coming. The New England Journal of Medicine will publish Israeli data showing that a booster shot of Pfizer's vaccine dramatically decreased severe Covid-19, according to Dr. Nachman Ash, director general of the Israeli Ministry of Health.
Israel began its booster program on August 1; booster shots have been given to 2.8 million people there so far. To date, much of the country's data on the efficacy of booster shots has not been reviewed by outside experts and published in a medical journal.
US federal health officials have announced plans to offer booster doses this fall, starting September 20, subject to authorization from the FDA and consent from the CDC.
The FDA's Vaccines and Related Biological Products Advisory Committee is meeting on Friday to discuss the application by vaccine makers Pfizer and BioNTech to administer their Covid-19 vaccine as a third dose, or "booster" shot, to people ages 16 and older.
Parents concerned about protecting their young children from the virus could have access to vaccinations for them by for Halloween, said the former commissioner of the FDA.
Dr. Scott Gottlieb, who is a board member at Pfizer, told CBS' "Face the Nation" Sunday the company is expected to have data on vaccinations for children ages 5-11 ready for the FDA by the end of September.
"The FDA says it will be a matter of weeks, not months, to make a determination if they're going to authorize vaccines for kids between 5 to 11. I interpret that to be perhaps four weeks, maybe six weeks," said Gottlieb.
However, CDC Director Dr. Rochelle Walensky was more guarded about when kids can be vaccinated. She said Monday that the CDC is working urgently on a Covid-19 vaccine for younger children, with the hope that they will be vaccinated by the end of the year.
"We're waiting for the companies to submit the data to the FDA. We're anticipating that will happen in the fall," she told NBC's "Today" show.
"We will look at that data from the FDA, from the CDC, with the urgency that we all feel for getting our kids vaccinated and we're hoping by the end of the year."
In the meantime, the FDA cautioned parents not to race to vaccinate their children before approval from the agency.
"Children are not small adults -- and issues that may be addressed in pediatric vaccine trials can include whether there is a need for different doses or different strength formulations of vaccines already used for adults," the FDA said in a statement Friday.
Until it is safe to vaccinate that age group, Dr. James Versalovic, pathologist-in-chief at Texas Children's Hospital, told CBS that prevention is crucial.
"In addition to prevention ... we need to continue to emphasize to all parents and families the importance of timely diagnosing through testing," Versalovic said. "And then triage the care appropriately. Decide whether that child needs hospital-based care. We know how to treat children at this point in the pandemic."
New York City public school system reopened Monday morning with 100% of its students back in classrooms.
"We've been working for 18 months to get ready for this day," New York Education Chancellor Meisha Ross Porter told CNN in an interview Friday.
All students and teachers returning to school on Monday will be required to wear masks, and the city previously announced a vaccine mandate for all public school employees with no testing opt-out. On Friday, the United Federation of Teachers said in a statement that an independent arbitrator decided teachers who have documented or religious exemptions must be offered a non-classroom assignment.
Department of Education officials said Monday that 74% of teachers and 66% of students 12-17 years old have been vaccinated in New York City.
US Education Secretary Miguel Cardona appeared virtually from a Bronx school and praised NYC officials for the reopening plan.
"To all the families that are watching -- they've worked so hard, they've prepared, they are doing everything to make sure your children and staff are safe," Cardona said.
Also on Monday, NYC began enforcing its vaccination passport rules, which makes vaccinations required for indoor dining, gyms, and other venues.
"This is the day we have been waiting for," de Blasio said Monday. History will remember it, he said, as "a day that was a game changer, a difference maker, a turn around day. This is the day NYC's coming back in full force."