Every American 16 years and older is eligible for a COVID-19 vaccine as of Monday.

"Folks, I have good news," President Joe Biden said in a White House video posted Monday on Twitter. "Everybody is eligible, as of today, to get the vaccine. We have enough of it; you need to be protected, and you need, in turn, to protect your neighbors and your family. So please, get the vaccine."

New Jersey, Massachusetts, Oregon, Rhode Island, Vermont and Hawaii were the last states to expand eligibility, opening vaccinations to all adults on Monday.

The US is administering an average of 3.2 million doses a day, up from roughly 2.5 million a month before. More than half of all US adults, or 131 million people, have now gotten at least one dose of a COVID-19 vaccine, according to data from the Centers for Disease Control and Prevention (CDC).

About 84.3 million, or 33 percent of Americans, have been fully vaccinated. Of those age 65 and older, 81 percent have received one dose or more, and about two-thirds are fully vaccinated.

Ninety percent of Americans now live within five miles of a vaccination site, Biden administration officials said. Nearly 40,000 pharmacies are now equipped to vaccinate through a direct-to-pharmacy federal vaccine program, up from a few thousand when the program launched two months ago, CNN reported.

Health officials have confirmed fewer than 6,000 reports of breakthrough COVID-19 cases in fully vaccinated Americans, CDC Director Rochelle Walensky said during a White House briefing on Monday. Breakthrough cases occur when someone contracts the virus more than 14 days after their second shot, she said.

That represents 0.007 percent of 84 million Americans with full protection against the virus. Despite the breakthrough infections, none of the patients has died or gotten severely ill. It shows "these vaccines are working", Walensky said.

Meanwhile, the CDC is looking at more cases of severe side effects possibly linked to Johnson & Johnson's COVID-19 vaccine after it was paused nationwide.

"There's been a handful of cases, not an overwhelming number of cases. We are working through and adjudicating them and verifying whether they do in fact reflect a true case," Walensky said.

"Right now, we are encouraged that it hasn't been an overwhelming number of cases but we are looking and seeing what has come in," Walensky said at the news briefing.

The J&J vaccine pause is likely to be lifted by Friday, although some restrictions may be required, Dr Anthony Fauci, White House chief medical adviser, said Sunday.

"By Friday, we should have an answer as to where we're going with it," Fauci said on ABC News. "I would think that we're not going to go beyond Friday in the extension of this pause."

Fauci said he doubted that the Food and Drug Administration (FDA) and the CDC would "just cancel" the J&J vaccine and continue allowing only the two-dose vaccines developed by Pfizer and Moderna, he told NBC's Meet the Press.

"Everything is on the table," Fauci said. "My estimate is that we will continue to use it in some form. I doubt very seriously if they just cancel it. I don't think that's going to happen. I do think that there will likely be some sort of warning or restriction or risk assessment."

Last week, health officials called for a nationwide pause after six women ranging in age from 18 to 48 developed a blood clotting disorder within about one to three weeks after receiving the J&J vaccine.

Emergent BioSolutions has shut down new vaccine manufacturing for the J&J shot at its Baltimore plant after an inspection by the FDA last week, the company said Monday.

The latest development could create further delays for J&J's vaccine manufacturing. The New Brunswick, New Jersey-based company enlisted Emergent early in the pandemic to help produce millions of doses of its vaccine, but the company mixed up ingredients, ruining 15 million doses.

Agencies contributed to this story.