Joe Biden has warned that the Delta variant of the coronavirus now makes up half of cases in many areas of the US and pledged to deploy federal “surge response teams” to help local officials stop the spread.

The president spoke on Tuesday after narrowly missing his self-imposed target of 70% of American adults receiving at least one dose of the Covid-19 vaccine by Independence Day on 4 July.

Although he noted that coronavirus cases and deaths are down 90% since January, Biden urged younger adults in particular to get vaccinated as the Delta variant, already raging across the world, threatens to become the dominant one in America.

“Our fight against this virus is not over,” he said in public remarks after a briefing by the White House Covid-19 response team. “Right now, as I speak to you, millions of Americans are still unvaccinated and unprotected and because of that, their communities are at risk. Their friends are at risk, the people that they care about are at risk.”

He continued: “This is an even bigger concern because of the Delta variant. In today’s briefing, we discussed how the Delta variant is already responsible for half of all cases in many parts of this country. It’s more easily transmissible, potentially more dangerous.”

Biden pleaded: “It seems to me it should cause everybody to think twice and it should cause reconsideration, especially in young people who may have thought that they didn’t have to be vaccinated, they didn’t have to worry about it, they didn’t have to do anything up to now.”

He described getting vaccinated as a “patriotic” thing to do and noted that the vaccines are highly effective against the Delta variant, with studies showing that virtually every hospitalisation and death from the coronavirus since early May has been among the unvaccinated. He suggested that the Delta variant will hit them hard.

“We’re stepping up our preparations to respond to the outbreaks we’re going to see among the unvaccinated,” Biden said. “For that, we’re mobilising what I’m calling Covid-19 surge response teams.”

The teams are made up of experts from the Federal Emergency Management Agency, Centers for Disease Control and Prevention and other government groups. “They’re going to help states that have particular problems prevent, detect and respond to the spread of the Delta variant among unvaccinated people in communities with low vaccination rates. And some states have very low vaccination rates.”

Outbreaks are “going to happen” in such states, he said. Arkansas, Florida, Missouri and Nevada have all seen significant rises in case numbers in recent weeks.

The rate of vaccinations has slowed dramatically since its April peak, with states that voted for Donald Trump in last year’s presidential election lagging behind. Biden outlined an approach to reaching more people from community to community, neighbourhood to neighbourhood and door to door, “literally knocking on doors”.

He said the White House is partnering with 42,000 local pharmacies, as well as family doctors and pediatricians, to persuade people including 12- to 18-year-olds to get their shot. Mobile clinics will make the vaccine available at summer festivals, sports events and places of worship.

The White House expects 160 million Americans to be fully vaccinated by the end of the week. In a rare moment of bipartisan accord, Mitch McConnell, the Republican minority leader in the Senate, said on Tuesday: “We’re in the red zone on vaccines but we’re not quite in the end zone yet and I want to say again there’s no good reason not to get vaccinated. We need to finish the job.”

But a recent Washington Post-ABC poll found that three in 10 adults say they have not been vaccinated and probably or definitely will not be. The poll showed that 86% of Democrats have received at least one shot compared to 45% of Republicans.

Jim Justice, the Republican governor of West Virginia, where just 52% of adults have got at least one dose, told ABC’s This Week programme on Sunday: “The red states probably have a lot of people that are very, very conservative in their thinking and they think, ‘Well, I don’t have to do that.’ But they’re not thinking right.”

More than 600,000 Americans have died since the pandemic began and hundreds continue to die each day.