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The Delta coronavirus variant, which is highly transmissible and possibly more dangerous than current predominant variants, now makes up more than 20% of all samples sequenced in the United States after the percentage doubled in a recent two-week period, Dr. Anthony Fauci said.

The country seems to be following a pattern seen in the United Kingdom, both with a variant first seen there – the B.1.1.7 Alpha – and now with the Delta variant, said the director of the National Institute of Allergy and Infectious Diseases.

“As was the case with B.1.1.7, we seem to be following the pattern with the Delta variant, with a doubling time of about two weeks,” Fauci said Tuesday at a White House Covid-19 Task Force briefing.

Some labs genetically sequence samples of the virus taken from patients to find out which variant is involved, and these test results are reported to the US Centers for Disease Control and Prevention.

Offering some good news, Fauci said the mRNA vaccines, which are made by Pfizer/BioNTech and Moderna, appear to hold up against the Delta variant.

The Pfizer/BioNTech vaccine was 88% effective in preventing Covid-19 symptoms in the Delta variant two weeks after the second dose of the vaccine, he said.

“When you look at hospitalizations, again, both the Pfizer BioNTech and the Oxford AstraZeneca are between 92 and 96% effective against hospitalizations,” Fauci said.

The challenge, though, is getting more people to take the vaccines.

Almost all Covid deaths are preventable, experts say

With vaccines widely available, almost every Covid-19 death is preventable, Dr. Rochelle Walensky of the CDC said at the White House briefing.

“Covid-19 vaccines are available for everyone ages 12 and up,” Walensky said. “They are nearly 100% effective against severe disease and death – meaning nearly every death due to Covid-19 is particularly tragic, because nearly every death, especially among adults, due to Covid-19 is at this point entirely preventable.”

Fauci said it’s “overwhelmingly” the case that those now dying of the disease are almost entirely not vaccinated.

“Every death from Covid-19 is avoidable, and it’s a tragedy when it happens,” he told CNN’s Jake Tapper.

Young people and some states putting drag on vaccination rate, Fauci said

The country may not reach President Joe Biden’s goal of getting 70% of American adults vaccinated with at least one dose by July 4 because young people aren’t rushing to be vaccinated and some states are lagging, Fauci said.

Even Vermont, where more than 81% of eligible people have started the vaccination process, is having a hard time getting young adults to get the shots.

“They’re a tough nut to crack,” Vermont Health Commissioner Dr. Mark Levine told CNN, but not because they are against being vaccinated.

“Most of them are not a truly vaccine-resistant group,” Levine said. “They don’t have that strong polarized view. They’re mostly in the category where it’s just not at the top of their list right now.”

But some state vaccination rates and efforts are lagging, and that’s also dragging down the country’s overall rate, Fauci said.

“It’s a combination of some states and regions that are below where we need them to be,” Fauci told Tapper on Tuesday. “Within that context, it’s younger people – particularly 18 to 26 – where you really want to try and get them to get vaccinated.”

In four states – Alabama, Louisiana, Mississippi and Wyoming – fewer than 50% of adults have gotten at least one dose, according to CDC data.

The issue “goes beyond the young people in some states, in some regions, ” Fauci said.

Although the country is getting close, with 65.5% of adults having received at least one dose, demand and vaccination rates have declined, leaving experts to worry if enough of the population will be vaccinated in time to curb fall and winter surges.

Fauci said he thinks Biden’s goal will be reached by mid-July.

“You set a goal. If you reach it, great. If you don’t, you keep going to try and reach it and go beyond it,” he said.

No ‘Hail Mary pass’ in sight

Although health officials are pouring efforts into convincing those still hesitant to get vaccinated against Covid-19, none of the strategies appear to be a “Hail Mary pass” to get the US to reach Biden’s vaccination goal and curb spreading variants.

“I just don’t know if there’s something out there that we’re not doing that for sure will get us over the score line,” Dr. Marcus Plescia, the chief medical officer at the Association of State and Territorial Health Officials, told CNN. “That’s the problem – we’re doing all the things that we know can be effective, but it’s just allowing us to maintain this steady state, when what we really need to do is bump the demand back up.”

Low vaccination rates are dangerous when combined with the spread of variants such as Delta, which is believed to be more transmissible and cause more serious illness. Steve Edwards, CEO of CoxHealth, a healthcare system in Springfield, Missouri, told CNN the combination is to blame for a six-fold increase in hospitalizations in his system.

“I think it is the Delta variant and there is a lot of kindling with low vaccination rates, so it’s spreading very rapidly,” Edwards said. “Almost all of our cases are unvaccinated people that, in my opinion, have put themselves in harm’s way during this pandemic.”

Fortunately, Plescia said, there are tactics underway to encourage vaccinations that have seen success.

Some states, such as Colorado, are making a final push by calling unvaccinated people directly to provide them with information and help schedule appointments. Incentives – from the big ones like million-dollar lotteries to free tickets to the zoo or food coupons – have also worked, Plescia said.

A strategy Plescia and other experts have supported recently is a transition from mass vaccination efforts to local provider’s offices – a plan that moves slowly but can be effective, he said.

“We do anticipate that this is going to be an important function of public health for quite some time,” Plescia said.

Young adults fall behind in vaccinations

Many officials have turned their vaccination attention to young adults – a population that is low in both vaccination rates and the desire to be vaccinated, US Centers for Disease Control and Prevention studies show.

“High vaccination coverage among all age groups is important for decreasing COVID-19 cases, hospitalizations, and deaths, especially among groups with lower vaccination uptake, such as young adults,” the authors of a study published Monday wrote.

If the weekly pace of vaccinations continues at the rate from the week of May 22, only 57.5% of adults under the age of 30 will have received at least one dose of Covid-19 vaccine by the end of August.

More than 71% of those between 30 and 49 years would have been vaccinated with at least one dose by the same time, as well as nearly 86% of adults 50 to 64 years old and nearly 95% of seniors 65 and older, the study found.

Only about half of adults under the age of 30 said they had been vaccinated or plan to get vaccinated, another CDC study found, less than those between 30 and 34 years (55%) and those 35 to 39 years (53%).

The authors of the study said that “a desire to protect others and resume social activities were motivators to get vaccinated, suggesting that messages emphasizing that vaccination would allow them to resume social activities and encouraging vaccination for the greater good might be effective.”

While social media is often a source of misinformation, especially when it comes to Covid-19 vaccines, it can also be used as counter the falsehoods, said Fauci, who has recently taken to TikTok to connect with young people.

“I never would have imagined in my wildest dreams I would be doing TikTok with people today and in the next couple of days,” Fauci told Tapper.

“I got on a TikTok discussion with a bunch of really dynamic young people,” he added.

CNN’s Jen Christensen, Virginia Langmaid, Deidre McPhillips, Jennifer Henderson and Lauren Mascarenhas contributed to this report.