The flu situation in the United States right now is, in a word, bad. Infections have skyrocketed in recent weeks, filling hospitals nearly to capacity; viral levels are “high” or “very high” in most of the country. In late December, New York reported the most flu cases the state had ever recorded in a single week. My own 18-month-old brought home influenza six days before Christmas: He spiked a fever above 103 degrees for days, refusing foods and most fluids; I spent the holiday syringing electrolyte water into his mouth, while battling my own fever and chills. This year’s serving of flu already seems set to be more severe than average, Seema Lakdawala, a flu virologist at Emory University, told me. This season could be a reprise of last winter’s, the most severe on record since the start of the coronavirus pandemic—or, perhaps, worse. At the same time, what the U.S. is experiencing right now “fits within the general spectrum of what we would expect,” Taison Bell, an infectious-disease and critical-care physician at the University of Virginia Health System, told me. This is simply how the flu behaves: The virus is responsible for one of the roughest respiratory illnesses that Americans regularly suffer, routinely causing hundreds of thousands of people to be hospitalized annually in the U.S., tens of thousands of whom die. (So far this season, the flu has killed more than 5,000 people, including at least nine children.) Influenza is capable of even worse—sparking global pandemics, for instance, including some of the deadliest in history. These current tolls, however, are well within the bounds of just how awful the “seasonal” flu can be. “It’s another flu year, and it sucks,” Bell said. Although flu is a ubiquitous winter illness, it is also one of the least understood. Scientists have been puzzling over the virus for decades, but many aspects of its rapid evolution and transmission patterns, as well as the ways in which our bodies defend against it, remain frustratingly mysterious. Flu seasons, as a rule, differ drastically from one another, and “we don’t have a great understanding of why one ends up being more severe than another,” Samuel Scarpino, an infectious-disease-modeling researcher at Northeastern University, told me. Experts’ flu-dar has also been especially out of whack in recent years, since the arrival of COVID-19 disrupted typical flu-transmission patterns. (An entire lineage of flu, for instance, may have been driven to extinction by pandemic-mitigation measures.) The virus is still finding its new norm. Even so, a few things about this season’s ongoing torment are clear. Much of the blame rests on the season’s dominant flu variant—subclade K, which belongs to the H3N2 group of influenza. As flus go, H3N2s tend to be more likely to hospitalize and kill people; most of the worst flu seasons of the past decade in the U.S. have been driven by H3N2 surges. Subclade K doesn’t seem to be an unusually virulent variant, which is to say it’s probably no more likely to cause severe disease than a typical version of H3N2. But it does seem to be better at dodging our immune defenses, making the net effect similar, because it can lead to more people getting sicker than they otherwise would. That’s not a trivial effect for a disease that, even in mild cases, can cause days of high fevers and chills, followed by potentially weeks of that delightful run-over-by-a-truck feeling. At UVA Health, Bell has seen a major uptick in people testing positive for the virus in recent weeks. Like others, his hospital is close to full, straining its capacity to treat other illnesses, he said. In Michigan, too, where Molly O’Shea cares for children at multiple pediatric practices, “we are seeing a ton of influenza, just a ton,” she told me. “Our schedule is overflowing.” Several of her school-age patients have wound up in the hospital, despite being previously healthy; a few have ended up with serious complications such as pneumonia and brain inflammation. The worst cases, she said, have been among the children who didn’t get their annual flu shot. Flu vaccines are not among the most impressive immunizations in our roster. Although they’re generally pretty effective at protecting against severe disease, hospitalization, and death, they don’t reliably stave off infection or transmission. And they’re frequently bamboozled by the virus itself, which shape-shifts so frequently throughout the year, as it ping-pongs from hemisphere to hemisphere, that by the time flu vaccines roll out to the public, they’re often at least a little out of sync with what’s currently circulating. That’s another aggravating factor this year. Researchers first detected subclade K in June, months after experts selected the strains that would go into the fall flu-vaccine formulation. Recent data suggest that vaccination may still elicit some immune defenses that recognize subclade K, and preliminary estimates from the United Kingdom suggest that this year’s formulations may be especially effective at preventing severe disease in children, who, along with the elderly, are highly vulnerable to the flu. (For all the misery my family endured, none of us ended up in the hospital—which suggests that our vaccinations did their job.) Children also tend to be the biggest drivers of flu’s spread. “They are the source, many times, of explosions of transmission,” Lakdawala told me. In the U.K., for instance, which experienced an unusually early start to the flu season, school-age kids appear to have driven much of the epidemic, Scarpino pointed out. In the U.S., too, case rates among children have been particularly high. Although the vaccine primarily limits severe disease, it can also affect how quickly the virus travels through a community. And yet only about half of American kids get the vaccine each year, despite long-standing universal recommendations for annual immunization. “It’s a vaccine that parents have never really treated as a vaccine that every child should get,” O’Shea said. Those choices might be influenced by the ways many people underestimate the flu—a term often used to describe any cold-weather ailment that comes with a runny nose, cough, or even gastrointestinal upset. In reality, flu has long clocked as one of the U.S.’s top 10 or top 15 causes of death—a scourge that, through its impact on the health-care system, the workforce, and the economy at large, costs the country billions of dollars each year. Against such a substantial threat, we should be using “everything in our toolbox to protect ourselves,” Lakdawala said. Yet the Trump administration is actively impeding the process of flu vaccination. Health and Human Services Secretary Robert F. Kennedy Jr. has also said that it may be “a better thing” if fewer people are immunized against the flu—and insisted, incorrectly, that “there is no scientific evidence that the flu vaccine prevents serious illness, hospitalizations, or death in children.” The federal government recommended annual flu vaccines for all children until earlier this month, when HHS pushed through changes that demoted multiple immunizations from its recommended schedule. HHS now says that families should consult with their health-care provider before taking the shot. Such a recommendation suggests that the vaccines’ overall benefits are ambiguous enough to require discussion—and puts an additional burden on both patients and health-care providers, who can administer what was once a routine vaccine only after a conversation that must then be documented. The nation’s leaders have also compromised one of the country’s best chances to develop more effective, better-matched flu vaccines in the future, by defunding research into mRNA vaccines. The current flu-vaccine manufacturing process takes so long that the included strains for the Northern Hemisphere must be selected by February or so—which provides plenty of time for the virus to evolve before the autumn rollout begins, as happened this year. “We pretty regularly have a bad match for the flu,” Scarpino said. mRNA vaccines promised the possibility of faster development, allowing researchers to stay more closely on the flu’s heels and switch out viral ingredients in as little as two or three months. That degree of flexibility also would have sped the response to the next flu pandemic. In an email, Andrew Nixon, HHS’s deputy assistant secretary for media relations, disputed the characterization that the department’s new policies impede flu vaccination, writing, “Providers continue to offer flu vaccines, and insurance coverage remains unchanged. The recommendation supports shared clinical decision-making between patients and clinicians and does not prevent timely vaccination. People can continue to receive flu vaccines if they choose to do so.” For the current season, much of the U.S.’s fate may already be sealed: Less than half of Americans have gotten a flu vaccine this season, while the virus continues to spread. “If you find yourself in a place where there are people sick with flu, you’re probably gonna get sick,” Scarpino said. That logic likely holds true for his own family, in Massachusetts, where flu activity has been high for weeks. They’ve so far made it through unscathed, but Scarpino said, “I feel like it’s a matter of time.”