(CNN) — The US is making significant strides in curbing the coronavirus pandemic as summer approaches, with average daily cases near a 14-month low and just over half of eligible Americans fully vaccinated.

About 50.1% of people ages 12 and older in the US — the cohort eligible to receive a Covid-19 vaccine in the country — were fully vaccinated as of early Tuesday, according to the Centers of Disease Control and Prevention.

But experts have warned that a coronavirus variant first identified in India and now rising to prominence in the United Kingdom — the Delta variant, or B.1.617.2 — could pose considerable danger to those who are unvaccinated, including those who were previously infected by older strains.

“We cannot let (Delta’s spread) happen in the United States,” Dr. Anthony Fauci said Tuesday in a White House Covid-19 briefing, adding it’s “such a powerful argument” to get vaccinated.

Fauci, the National Institute of Allergy and Infectious Diseases director, warned that the Delta variant “may be associated with an increased disease severity, such as hospitalization risk, compared to” the Alpha variant, B.1.1.7, which was first identified in the United Kingdom and was dominant there before Delta recently was believed to have taken over.

The Delta variant is susceptible to available two-dose vaccines from Pfizer and AstraZeneca, he said, but protection from these vaccines requires following a two-dose schedule.

“There is reduced vaccine effectiveness in the one dose,” Fauci said. “Three weeks after one dose, both vaccines, the (AstraZeneca) and the Pfizer/BioNTech, were only 33% effective against symptomatic disease from Delta.”

Lab experiments described in a recent preprint study also suggest the Moderna vaccine, as well as the Pfizer product, will offer protection against the Delta variant, although more study is needed. Johnson & Johnson says it’s gathering data on its vaccine and emerging variants.

Fauci added that variant-specific boosters may be on the horizon.

Even those who’ve already had coronavirus should get vaccinated because research shows immunity achieved through vaccination is better than immunity through previous infection, Fauci said.

Meanwhile, the country has averaged almost 14,380 new Covid-19 cases a day across the past week — the second-lowest average since March 28, 2020, according to Johns Hopkins University data. Only Friday’s average — 14,328 per day — was lower.

And the average number of new Covid-19 hospital admissions per day across a week — just more than 2,200 — is far lower than the country’s peak average of 16,500 a day on January 9, CDC Director Dr. Rochelle Walensky said Tuesday.

But health experts warn that a recent lag in vaccination rates leaves millions unprotected against Covid-19 variants that have made their way to the US from other parts of the world.

Over the last week, the US averaged more than 1.07 million Covid-19 vaccine shots administered per day — well below the peak seven-day average of 3.38 million shots per day reached on April 13, according to CDC data.

Vaccine hesitancy and accessibility issues

As the US enters what the former CDC director called the “slog-phase of the vaccination campaign,” health experts have been drawing attention to both vaccine hesitancy and accessibility issues.

In Texas, a group of Houston Methodist Hospital workers on Monday protested the health care system’s requirement that staffers be vaccinated against Covid-19, CNN affiliate KTRK reported.

Houston Methodist became the first major health care system in the US to mandate Covid-19 vaccinations on March 31, starting with managers, according to an initial announcement from Houston Methodist CEO Marc Boom.

Those who did not comply with the vaccination policy were suspended after violating the tenets of the medical profession, Boom told CNN.

“Every one of our professional tenets require us to put patients first, require us to keep our patients safe, by anything we can possibly do, so those individuals who are choosing not to get vaccinated are basically saying they are going against the tenets of our profession and they’re not putting patients first,” he said.

Vaccine maker says it is working to extend shelf life

Johnson & Johnson — maker of the only single-dose Covid-19 vaccine authorized in the US — says it is working to extend the shelf-life of its product amid reports that doses in the country may expire before they’re used.

Of the 21.4 million Johnson & Johnson doses delivered in the US, about 11 million have been administered, according to CDC data. That vaccine can be stored for up to three months at refrigerator temperatures.

The US Food and Drug Administration is looking into whether the expiration date on Johnson & Johnson vaccines can be extended and, if so, how to get the doses utilized, Fauci said Wednesday.

Johnson & Johnson is conducting stability testing “with the goal of extending the amount of time our COVID-19 vaccine can be stored before expiry,” it told CNN this week.

In Ohio, 200,000 doses of the state’s Johnson & Johnson vaccine are set to expire before the end of the month, and the state is unable to share the doses with other states or countries, Gov. Mike DeWine said this week.

In Arkansas, the retired National Guard colonel overseeing state vaccine distributionstopped ordering the Johnson & Johnson vaccine because the state has so many unused doses, he told KATV last week.

The 11 million people vaccinated with the Johnson & Johnson shot represent a small fraction of the 171.7 million people who’ve received at least one dose of a vaccine in the US.

CDC updates travel guidance as global case levels decline

The number of global Covid-19 cases and deaths this week continued a downward trend that has now lasted into its second month.

About 3 million new Covid-19 cases were reported globally during the week ending June 6, marking a 15% decline from last week. And about 73,000 new deaths were reported around the world, or an 8% decrease from the previous week, according to the World Health Organization’s weekly Covid-19 epidemiological update.

But many countries, in every region of the world, reported increases in both cases and deaths this week, the WHO update reads.

The WHO’s Africa region saw a 25% increase in new cases this week, led by South Africa, where more than 30,000 new cases were reported. Relative to the previous week, outbreaks were particularly severe in Uganda and Zambia, which saw 137% and 191% increases respectively.

Still, as the world’s total levels dropped, US health officials on Monday added 33 countries — including Iceland, Israel and Singapore — to the lowest travel risk category and recommended new guidelines for vaccinated and unvaccinated people.

The CDC uses levels 1 through 4 to determine a threat in a given country, 1 being the lowest risk and 4 being the highest, depending on the number of Covid-19 cases. At each level, the CDC advises getting vaccinated, but its guidance for unvaccinated people varies by how severe the pandemic is in each country.

The governors of Florida and Texas, meanwhile, are headed toward showdowns with some of the world’s largest cruise lines, including Norwegian and Carnival, over whether passengers boarding ships leaving their states can be required to show proof they’ve been vaccinated.

Florida Gov. Ron DeSantis and Texas Gov. Greg Abbott have staked out positions against businesses requiring so-called vaccine passports. That’s at odds with the policies cruise lines are implementing as they seek to resume operation this summer — potentially putting the jobs of cruise line workers based in their states at risk while causing confusion for people planning to board those ships.

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