Coronavirus updates: Herd immunity by fall 'ambitious,' says surgeon general nominee

Dr. Anthony Fauci, chief medical adviser to U.S. President Joe Biden, said his “best-case scenario” is getting 85% of Americans vaccinated by the end of the summer.

"If we get 70% to 85% of the country vaccinated, let’s say by the middle of the summer, I believe by the time we get to the fall, we will be approaching a degree of normality," Fauci, who is also the director of the National Institute of Allergy and Infectious Diseases, told reporters at Thursday's White House press briefing.

"If we get 75% to 80% vaccinated, we could have a degree of herd immunity to get us back to normal," Fauci added. "The concern I have is people who have vaccine hesitancy who don't want to get vaccinated. We need to do a lot of good outreach."

Fauci said he believes Biden’s goal of 100 million vaccinations in his first 100 days as president "is quite a reasonable goal."

U.S. President Joe Biden on Thursday signed executive actions related to the coronavirus pandemic including: establishing a testing board to direct help where it’s needed most; mandating masks in airports and on certain modes of transportation; and requiring travelers coming to the U.S. from other countries to test negative before arriving.

Biden said he’s also advocating for stricter workplace standards and is directing both the U.S. Department of Education and the U.S. Department of Health and Human Services to provide guidance to schools on how they can reopen safely.

The president said his plan also begins with an "aggressive" vaccination campaign.

"Rollout has been a dismal failure thus far," Biden said. "I understand why many governors, mayors, county officials, tribal leaders feel like they're left on their own without a clear national plan to get them through the crisis."

"We'll move heaven and Earth to get more people vaccinated for free and create more places for them to get vaccinated ... and to increase vaccine supply and get it out the door as fast as possible," he said.

In England, those who attend house gatherings with more than 15 people -- in violation of lockdown rules -- will face hefty fines beginning next week, British Interior Minister Priti Patel announced Thursday.

First-time offenders face a fine of 800 pounds (around $1,097). The fine will double for each repeat offense to a maximum of 6,400 pounds (around $8,779).

More police officers are on dedicated patrols "taking targeted action against those small few who are letting everybody down,” said Martin Hewitt, chair of the U.K. National Police Chiefs' Council.

When asked by reporters Thursday whether England's lockdown is likely to end by the spring or summer, British Prime Minister Boris Johnson said it is "too early to say."ABC News’ Christine Theodorou contributed to this report.

Dr. Anthony Fauci, chief medical adviser to U.S. President Joe Biden, said received his second dose of the COVID-19 vaccine on Tuesday.

Speaking to reporters at the White House on Thursday afternoon, Fauci said he "felt under the weather for about a day" but now feels fine.

ABC News’ Ben Gittleson contributed to this report.