BA.5 in particular “has the potential to cause the numbers of infections to rise in the coming weeks,” the Biden administration said in a statement released Tuesday, encouraging those who are 50 and older or moderately or severely immunocompromised to get a second booster shot.

Those who had the original strain of Omicron, BA.1, “really don’t have a lot of good protection” against BA.4 and BA.5, Dr. Anthony Fauci, chief medical adviser to the president, said Tuesday.

“It’s very, very clear that immunity wanes, whether that’s immunity following infection or immunity following vaccine,” Fauci said at a White House COVID-19 press briefing. “We have good data now that if you were infected with BA.1, you really don’t have a lot of good protection against BA.4 and BA.5.”

U.S. COVID numbers were released Tuesday as World Health Organization officials said the BA.5 subvariant is spreading “at a very intense level” globally, comprising an increasing proportion of reported sequences.

At a Tuesday press briefing, WHO Director-General Dr. Tedros Adhanom Ghebreyesus said the virus “is running freely and countries aren’t effectively managing the disease burden,” adding that there is a “major disconnect” between public opinion of the situation and the gravity of it.

“As the virus pushes us, we must push back,” he said.

Fauci said that while Americans shouldn’t let COVID disrupt their lives, “we cannot deny that this is a reality we need to deal with,” adding that the more the virus spreads, the more apt it is to mutate.

Unlike other Omicron subvariants, BA.5 may target the lungs

BA.5 comprised 65% of cases last week, according to new data released by the U.S. Centers for Disease Control and Prevention on Tuesday. BA.4, a close relative, comprised 16.3% of cases.

BA.2.12.1, an Omicron subvariant that was dominant until BA.4 and BA.5 took over last month, came in third, at 17% of cases.

BA.5 takes some of Omicron’s worst traits—transmissibility and immune evasion—to a new level. But it also combines them with a penchant for affecting the lungs reminiscent of the Delta variant that hit the U.S last summer and fall, according to two recent studies.

In the case of Delta, COVID tended to accumulate in and affect the lungs, potentially resulting in more severe disease. Until recently, a silver lining of Omicron has been its tendency to instead accumulate in the upper respiratory tract, causing symptoms more similar to a cold or the flu.

BA.5 is different, according to a study published June 10 on medRxiv, a Yale– and British Medical Journal–affiliated website that publishes studies not yet certified by peer review. Recent reports show BA.5 shifting back to the lower respiratory tract—at least in animal models—“with a potential increase in disease severity and infection within lung tissue,” researchers from Australia’s Kirby Institute wrote. They referenced another May preprint study that found BA.5 and close relative BA.4 replicate more efficiently in the alveoli of human lungs than so-called stealth Omicron, BA.2.

“BA.5 not only gives the virus greater antibody evasion potential, but concurrently has changed [where it tends to accumulate], along with an increased transmission potential in the community,” the Kirby Institute authors write.

The scenario calls to mind the term Deltacron, which referred to a Delta-Omicron hybrid identified in the U.S. this spring that never took off. Back then, the term was used “prematurely,” Dr. Eric Topol, a professor of molecular medicine at Scripps Research and founder and director of the Scripps Research Translational Institute, wrote in a Sunday blog post on the studies.

Now, for BA.5, the term might be more appropriate, even though the subvariant isn’t a true hybrid.

“The ability to infect cells for BA.5 is more akin to Delta than the previous Omicron family of variants,” Topol wrote.

BA.4 and BA.5—dominant in many countries across the globe—were first detected in the U.S. in late March, as Fortune previously reported, but have quickly taken over the global scene.

BA.5 alone caused about 54% of COVID infections in the U.S. two weeks ago, according to the CDC.

“The Omicron subvariant BA.5 is the worst version of the virus that we’ve seen,” Topol recently wrote. “It takes immune escape, already extensive, to the next level, and, as a function of that, enhanced transmissibility,” well beyond what has been seen before.

Vaccines should continue to offer protection from severe disease and death, officials say

The jury is still out on whether current vaccines hold up against BA.5. But given that vaccines were 15% less effective against Omicron than they were against the Delta variant, even with a booster, “it would not be at all surprising to me to see further decline of protection against hospitalizations and deaths,” Topol wrote.

U.S. officials stressed Tuesday that vaccines were still expected to protect against severe disease and death, urging Americans 5 and older to get their first booster shot if they haven’t already, and adults 50 and older and those who are immunocompromised to get their second booster. The Biden administration is reportedly considering the approval of a second booster for all adults.

Getting boosted won’t prevent anyone from getting a variant-specific booster this fall or winter, Dr. Ashish Jha, the White House’s COVID response coordinator, said at the Tuesday press briefing.

“My message is simple: If you have not gotten a vaccine shot in the year 2022, if you have not gotten one this year, please go get another vaccine shot,” he said. “It could save your life.”

Individuals 50 and older with only one booster shot were four times more likely to die this spring than similar individuals who had been boosted twice, CDC Director Dr. Rochelle Walensky said Tuesday.

U.S. hospital admissions sit at about 5,100 per day, with levels doubling since early May, Walensky said. Deaths sit at about 350 per day.

While hospitalizations are trending upward and are at their highest levels since February, the increase is likely proportionate to the actual number of COVID cases, which are vastly underreported right now, Fauci said.