US President Joe Biden's administration on Monday announced a raft of new actions to ramp up the national immunization campaign and ensure that 90 percent of adults will be eligible for vaccination against Covid by April 19.

The move came as the nation's top public health official pleaded with people to "hold on a little longer" with measures to stop the coronavirus, admitting she felt a sense of "impending doom" as data showed the country tipping into another wave.

According to a White House statement, Biden will say his administration is increasing the number of pharmacies in the federal pharmacy vaccination program from 17,000 to nearly 40,000 while creating a dozen more mass vaccination sites by April 19.

"He will also announce a new effort to fund community organizations to provide transportation and assistance for the nation's most at-risk seniors and people with disabilities to access vaccines," the statement added.

The overall goal is for 90 percent of adults to have a vaccination site within five miles of where they live.

The US has now administered 143 million shots and fully vaccinated 16 percent of the population, including almost 50 percent of over 65s.

But fears are growing that's still not fast enough to avert a fourth surge unless accompanied by a return to tougher curbs.

Speaking to reporters Monday, Centers for Disease Control and Prevention (CDC) director Rochelle Walensky told reporters she wanted to speak off the cuff to convey the gravity of the situation.

"I'm going to reflect on the recurring feeling I have of impending doom," she said.

"We have so much to look forward to, so much promise and potential of where we are, and so much reason for hope. But right now, I'm scared."

The rate of infection in the United States had been plateauing for several weeks, but is now once more on the rise, with the most recent data showing the seven-day average at close to 60,000 new cases.

That represents an increase of 10 percent over the prior seven-day period, while hospitalizations are up to 4,800 per day from 4,600 comparing the same timeframes.

Deaths have risen three percent to around 1,000 per day.

Walensky recalled her experiences as a frontline physician at Massachusetts General Hospital during the earlier part of the pandemic.

"I know what it's like to stand in that patient room, gowned, gloved, masked, shielded, and to be the last person to touch someone else's loved one because their loved one couldn't be there," she said.

She added that while having "three historic, scientific breakthrough" vaccines was a cause for hope, not enough of the population had yet been covered to avoid a fourth surge unless people held fast to mitigation measures.

"Please hold on for a little while longer," she said, calling on elected leaders and people who are influential in their communities to drive home the message.

"I so badly want to be done. I know you all so badly want to be done. We are just almost there, but not quite yet."

More real-world vaccine data

In more positive news, a real-world study by the CDC showed that the Pfizer and Moderna Covid vaccines, both based on new messenger RNA technology, were 90 percent effective in preventing coronavirus infection.

The results came from 4,000 health workers vaccinated between December 2020 and March 2021, and also showed partial vaccination with one dose resulted in 80 percent protection against infection two weeks after the first shot.

One of the big strengths of the study was that participants self-collected nasal swab tests each week for lab testing, regardless of whether they developed symptoms or not.

This adds to a growing body of evidence that the vaccines halt not just symptomatic disease but infection itself, making them an important tool in stemming the spread of the virus.

The participants included doctors, nurses, first responders and other health care workers from Arizona, Florida, Minnesota, Oregon, Texas and Utah.

The study's authors said they could not make product-specific estimates because of the limited number of infections.

The study is ongoing, and scientists will look to sequence the virus in cases where it was able to infect people despite vaccination, to better understand why this happens in some cases.