As vaccine makers seek authorization for a fourth dose of their Covid vaccines in America, existing delays with vaccination and a lack of federal funding could slow the next booster rollout across the country, experts say.

“We’re way behind the eight-ball,” said Eric Topol, founder and director of the Scripps Research Translational Institute. The rollout of the first round of boosters, authorized in the US last fall, “just fell off the cliff”, with many Americans still not realizing they are eligible or that the booster is recommended.

With a potential second booster on the horizon for vulnerable groups, the Biden administration is still struggling to drum up American public interest in additional shots – and funding from Congress to pay for Covid initiatives.

“We’re out of money pretty much for the pandemic spending, which is terrifying because we don’t know what’s coming around the corner,” said Katrine Wallace, an epidemiologist at the University of Illinois Chicago.

Pfizer-BioNtech asked the US Food and Drug Administration (FDA) on 15 March for another round of boosters for those 65 and up, while Moderna went a step further and asked on 17 March for more boosters among those 18 and up to give the FDA “flexibility” in considering who would benefit from additional shots, including vulnerable younger people, the company said.

The doses in question would be the original formulation of the vaccines. Omicron-specific vaccines are still in the trial phase, but scientists believe updating the vaccines as the virus evolves could broaden immune responses to future variants.

A $15bn funding package for testing, treatment, vaccines and more was cut unexpectedly from an omnibus spending bill in Congress on 9 March.

Health officials spoke to Democratic senators about the urgent need for Covid funding in a meeting on Wednesday, Politico reports, but the plan may meet with opposition: Republicans, who were not at the meeting, say the White House’s $22.5bn request must be accompanied with equal cuts to government spending elsewhere.

There is enough funding to give fourth doses of the vaccines to immunocompromised people, who already qualify for the shots, and for those over 65, if the shot is authorized for them in coming weeks, the coronavirus response coordinator, Jeff Zients, said at a White House briefing on Wednesday.

But wider booster campaigns would not have funding under the current budget shortfalls, and first- and second-shot campaigns could also be affected in the longer term.

The funding collapse may also affect future research on updated vaccines and treatments. “Maybe we will see a new variant that’s escaped all of these, and we need a new vaccine,” Wallace said. Without funding to create and then distribute the updated vaccines, “that is going to be an issue”.

Three doses were on average 94% effective against needing mechanical ventilation or dying during the Omicron surge, according to research from the US Centers for Disease Control and Prevention (CDC) published on Friday.

That kind of effectiveness is “a miracle”, Topol said. “That’s incredible, but that’s never been conveyed to the public – it’s amazing to me.”

“If we had a chemo that would do that for cancer – increase the odds of survival by that much – everybody with cancer would want to get it,” Wallace said.

Yet only about 29% of the US population has been boosted. Less than half (44%) of all Americans who received their initial shots continued in the series, although that figure is higher (67%) in those aged 65 and older.

“There was a huge push to get people fully vaccinated, which was the two doses, but not as big of a push for the booster,” Wallace said. “A lot of people just don’t understand that the booster is now available to everybody.”

When the boosters were first rolled out, they were limited to certain populations, including older and immune-compromised Americans as well as health workers, before they were opened to all adults and eventually to children 12 and up.

“There was mass confusion, and that’s why the uptake is so poor,” Topol said.

Some populations – including older Americans and health workers – received their first boosters last fall, raising concerns about waning efficacy among those who are most at-risk of getting or becoming very sick from the virus.

The effectiveness of the third dose at preventing hospitalization wanes to 78% four months after the booster, according to another recent CDC report.

In a recent study from Israel, a fourth mRNA dose increased antibody levels and protected against infection slightly better.

The members of the FDA’s independent advisory committee will meet on 6 April to discuss booster authorization policies moving forward, especially in the light of new and emerging variants.

No vote is scheduled for the advisers’ meeting, which will focus on a framework for boosters rather than specific applications for authorization.

The CDC recommends that everyone 12 and up who received two doses of an mRNA vaccine or one dose of the Johnson & Johnson vaccine should also receive a booster dose two or five months later, depending on the vaccine.

Immunocompromised people – including those undergoing treatment for cancer, organ transplant recipients, people living with HIV, and patients regularly taking immune-suppressing drugs like corticosteroids – already qualify for a fourth dose, because they may not mount a strong or lasting response to the initial three shots. Roughly 2.7% of Americans, or about 9 million people, are immunocompromised.

Officials also need to step up efforts to vaccinate those who aren’t fully vaccinated, representing about one-third of the US population. “We need to somehow try to make inroads in that group, because it’s big,” Wallace said.

And vaccinating the rest of the world is key for ending surges of the virus and the emergence of new variants globally.

These three groups – the immunocompromised, those over 65, and those who haven’t been vaccinated – should take priority before others receive fourth doses, Wallace said.

The US is likely to see another wave of Covid and vaccines can take weeks to become fully effective, making vaccination campaigns urgent now, experts said.

“It is good that there’s a lull in circulating virus – that’s wonderful,” Topol said. “This is the time to get protected for the next wave, of which there will be one or two or more – but that hasn’t been conveyed.

“People have been lulled into a zone of complacency, which is unfortunate,” Topol said. “It’s understandable after all the fatigue, and everyone is so sick of this, but it’s not what’s in the cards, and we need to prepare, defend and protect people.”