Dr. David Kessler, a former head of the Food and Drug Administration who is now overseeing the U.S. effort to accelerate the development, manufacture and distribution of COVID-19 vaccines, pushed back the timeline in which most Americans can expect to get vaccinated during an interview on SiriusXM’s "Doctor Radio Reports."

"We're not going to have everyone vaccinated in April, in May, right? Just not gonna happen," Kessler told the radio show's host, Dr. Marc Siegel. "We got to get over 65, essential workers. I think this is going to take us into the fall. We got to get there before next winter, and one of the things I care about very honestly, is we can't do this again.”

The U.S. outbreak may end sooner if Johnson & Johnson's vaccine is more than 80% effective, according to Kessler. But even if that vaccine is highly effective, he added, it will still take months to get priority groups, like older adults and essential workers, vaccinated.

"The bottom line is I wish I could tell you there's plenty of vaccine and we can fill all these endless amounts of appointments. We can't. It's going to take us months to have enough supply," Kessler said.

-ABC News' Stephanie Ebbs contributed to this report.

Massachusetts lifted its stay-at-home order Monday, which had asked residents not to go out between the hours of 10 p.m. and 5 a.m. and required businesses to close at 9:30 p.m.

"Vaccines are reaching residents, positive case rates and hospitalizations have stabilized; those trends are moving in the right direction," Gov. Charlie Baker said during a Jan. 21 press conference announcing the loosened restrictions. "As a result, we believe it's OK and it's time to start a gradual easing of some of the restrictions we put in place in the fall," Baker said.

Restaurants and other businesses will still be capped at 25% occupancy until at least Feb. 8, according to Baker. Gatherings are still limited to 10 people indoors and 25 people outdoors.

-ABC News' Brian Hartman contributed to this report.

The California Department of Public Health lifted a stay-at-home order for all regions statewide on Monday.

Four-week intensive care unit capacity projections for the three regions that had still been under the order -- San Joaquin Valley, Bay Area and Southern California -- are now above 15%, the threshold that allows regions to exit the order. The Sacramento region exited the order on Jan. 12 and the Northern California region never entered the order, according to a press release from the California Department of Public Health.

The move allows all counties statewide to return to the rules and framework of California's "Blueprint for a Safer Economy" and color-coded tiers that indicate which activities and businesses are open based on local COVID-19 infection rates and test positivity. The majority of the counties are in the strictest -- or purple -- tier. Individual counties may choose to impose stricter rules, according to the press release.

"Californians heard the urgent message to stay home as much as possible and accepted that challenge to slow the surge and save lives," Dr. Tomas Aragon, director of the California Department of Public Health, said in a statement. "Together, we changed our activities knowing our short-term sacrifices would lead to longer-term gains. COVID-19 is still here and still deadly, so our work is not over, but it’s important to recognize our collective actions saved lives and we are turning a critical corner."

California, home to nearly 40 million people, has the highest tally of diagnosed COVID-19 cases of any U.S. state, with more than 3.1 million, according to a real-time count kept by Johns Hopkins University.

Laboratory experiments that tested Moderna's COVID-19 vaccine against new variants of the novel coronavirus show little to no impact on its efficacy, according to a press release from the company.

The vaccine produced antibodies that neutralized all key emerging variants tested, including B117 and B1351, which were first identified in the United Kingdom and in South Africa, respectively. The study showed "no significant impact" on neutralizing titers against the B117 variant. Meanwhile, a six-fold reduction in neutralizing titers was observed with the B1351 variant but remain above levels that are expected to be protective, according to the press release.

Although the two-dose regimen of the vaccine is expected to be protective against emerging strains to date, Moderna announced Monday that it will test two additional strategies: one to explore whether adding an extra dose could offer more protection, and another to study a version of their original vaccine that's more tailored to the South Africa variant.

"As we seek to defeat the COVID-19 virus, which has created a worldwide pandemic, we believe it is imperative to be proactive as the virus evolves. We are encouraged by these new data, which reinforce our confidence that the Moderna COVID-19 Vaccine should be protective against these newly detected variants," Moderna CEO Stephane Bancel said in a statement. "Out of an abundance of caution and leveraging the flexibility of our mRNA platform, we are advancing an emerging variant booster candidate against the variant first identified in the Republic of South Africa into the clinic to determine if it will be more effective to boost titers against this and potentially future variants."

ABC News' Sony Salzman contributed to this report.

Dr. Anthony Fauci, the nation's leading infectious disease expert, opened up about his experience working with former U.S. President Donald Trump in an interview with The New York Times that was published Sunday.

When COVID-19 began to rapidly spread in the northeastern part of the country last year, particularly in New York City, Fauci said Trump had "almost a reflex response" to try to "minimize" the situation.

"I would try to express the gravity of the situation, and the response of the president was always leaning toward, 'Well, it's not that bad, right?' And I would say, 'Yes, it is that bad,'" Fauci, director of the National Institute of Allergy and Infectious Diseases, told the newspaper. "It was almost a reflex response, trying to coax you to minimize it. Not saying, 'I want you to minimize it,' but, 'Oh, really, was it that bad?'"

Fauci, who was a key member of the Trump administration's coronavirus task force, said another thing that made him "really concerned" was the former president taking input from non-experts on unproven methods to treat COVID-19, like hydroxychloroquine.

"It was clear that he was getting input from people who were calling him up, I don’t know who, people he knew from business, saying, 'Hey, I heard about this drug, isn't it great?' or, 'Boy, this convalescent plasma is really phenomenal,'" Fauci told the newspaper. "And I would try to, you know, calmly explain that you find out if something works by doing an appropriate clinical trial; you get the information, you give it a peer review. And he’d say, 'Oh, no, no, no, no, no, no, no, this stuff really works.'"

"He would take just as seriously their opinion -- based on no data, just anecdote -- that something might really be important," Fauci added. "That’s when my anxiety started to escalate."

When the leadership of the White House coronavirus task force changed hands last February, with U.S. Vice President Mike Pence coordinating the government's response and Trump at the podium taking questions from reporters during the press briefings, Fauci said it went from "the standard kind of scientifically based, public-health-based meetings" to "the anecdotally driven situations, the minimization, the president surrounding himself with people saying things that didn’t make any scientific sense."

"Then I started getting anxious that this was not going in the right direction," he told the newspaper. "We would say things like: 'This is an outbreak. Infectious diseases run their own course unless one does something to intervene.' And then he would get up and start talking about, 'It’s going to go away, it’s magical, it’s going to disappear.'"

That's when Fauci said it became clear to him that he needed to speak up, even if it meant contradicting the president.

"He would say something that clearly was not correct, and then a reporter would say, 'Well, let’s hear from Dr. Fauci.' I would have to get up and say, 'No, I’m sorry, I do not think that is the case,' he told the newspaper. "It isn’t like I took any pleasure in contradicting the president of the United States. I have a great deal of respect for the office. But I made a decision that I just had to. Otherwise I would be compromising my own integrity, and be giving a false message to the world. If I didn’t speak up, it would be almost tacit approval that what he was saying was OK."

This upset Trump's "inner circle," Fauci said.

"That’s when we started getting into things I felt were unfortunate and somewhat nefarious -- namely, allowing Peter Navarro to write an editorial in USA Today saying I’m wrong on most of the things I say," he told the newspaper. "Or to have the White House press office send out a detailed list of things I said that turned out to be not true -- all of which were nonsense because they were all true. The very press office that was making decisions as to whether I can go on a TV show or talk to you."

Fauci said there were a couple times where Trump even called him personally to say, "Hey, why aren’t you more positive? You’ve got to take a positive attitude. Why are you so negativistic? Be more positive."

Fauci said he and his family have received death threats, beginning last March, and that his wife once suggested he consider quitting.

"But I felt that if I stepped down, that would leave a void. Someone’s got to not be afraid to speak out the truth," he told the newspaper. "Even if I wasn’t very effective in changing everybody's minds, the idea that they knew that nonsense could not be spouted without my pushing back on it, I felt was important. I think in the big picture, I felt it would be better for the country and better for the cause for me to stay, as opposed to walk away."