WASHINGTON — Three-quarters of U.S. adults have received at least one dose of a COVID-19 vaccine as of Tuesday, according to the White House, setting a new milestone in the country’s fight against the pandemic.

But with a continued surge of cases, hospitalizations and deaths due to the delta variant of the coronavirus, President Joe Biden plans a speech Thursday to outline a “six-pronged strategy” to “get the pandemic under control,” press secretary Jen Psaki said.

The U.S. hit 70% of adults with at least one dose in early August, four weeks after Biden’s July 4 target for the achievement. Despite wide availability of free shots, hesitancy among many Americans — especially political conservatives — has left the U.S. well behind many other countries in inoculating its population.

A range of factors are now driving vaccinations, including the delta outbreak and more widespread employer mandates, which spiked after the Food and Drug Administration issued full approval for the Pfizer Inc.-BioNTech shot last month.

Psaki downplayed expectations for any fresh or expanded vaccine mandate in Biden’s speech, and said the federal government does not believe it has the authority to require vaccines broadly at the national level.

The U.S. response to the pandemic remains a patchwork and a political battleground, with some prominent Republican governors downplaying the importance of vaccines and blocking public health measures such as mask mandates.

Cases have soared through the summer in the U.S. — the country set a record for new cases on Aug. 27, exceeding the highs of last winter. Hospitalizations and deaths have also increased, including 3,911 deaths reported last Thursday, the most in one day since January.

As Biden’s administration presses to get shots into arms of the unvaccinated, it’s also preparing to launch a booster program for those who’ve already been inoculated. The administration plans to authorize third doses of Pfizer’s shot beginning Sept. 20 for anyone who had their second dose at least eight months previously.

The administration announced its booster-shot plan in August based on indications that the immunity conferred by two shots can wane over time and isn’t as effective against delta — data that some health experts outside the government say are inconclusive.

Pfizer is further than its competitors in securing regulatory approval for booster shots. Moderna Inc. submitted data to regulators last week, but the FDA has sought more information on a full-dose booster, rather than the half-dose the company proposed, according to people familiar with the matter.

Anthony Fauci, a medical adviser to Biden and longtime U.S. health official, told CNN on Sunday that it’s likely that Pfizer boosters will begin Sept. 20 with Moderna boosters following a couple weeks later. The administration has said it expects to give out approximately 100 million booster shots this fall.

It’s not yet clear what boosters will be given, or when, to those who received the Johnson & Johnson one-dose vaccine, which relies on different technology than the messenger RNA vaccines of Pfizer and Moderna.