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.

In the span of two weeks, the United States' seven-day average of daily COVID-19 cases has declined by 34% -- the steepest non-holiday-related drop in cases the country has seen since the summer, according to ABC News’ analysis of data compiled by The COVID Tracking Project, a volunteer-run effort to track the U.S. outbreak.

California appears to have seen the most drastic drop of any U.S. state, as its seven-day average of daily cases has been nearly slashed in half over the last two weeks.

Despite continued positive signs in case and hospitalization trends, daily death numbers are still incredibly high.

Another 3,943 fatalities from COVID-19 were reported on Wednesday, according to data collected by Johns Hopkins University -- that is approximately one death recorded every 22 seconds.

ABC News’ Arielle Mitropoulos contributed to this report.