The 1918 influenza pandemic, the deadliest pandemic in modern history, is typically associated with a single year, but it actually lasted for more than two. The virus that caused it is thought to have emerged in the United States in January of that year, and it claimed its tens of millions of victims quickly. By the summer of 1919, the worst was over. But in some parts of the world, the pandemic dragged on into the spring of 1920, cresting in a fourth wave that killed more people in New York City than the first did. The United States is now in a fourth wave of another pandemic, and no one is quite sure when this wave will end or whether more will follow. Unlike in 1919 or 1920, state-of-the-art vaccines are widely available in the United States, but hospitalizations keep rising because too many Americans have avoided inoculation. Adding to the sense of unease, the Biden administration this week announced that most Americans should eventually get a third vaccine dose. How has the Delta variant altered the path of the pandemic, and where will it go next? Here’s what people are saying. Has Delta changed the vaccine calculus? Just a month ago, the Biden administration was striking a fairly optimistic tone about the country’s trajectory — at least for the vaccinated. “The good news is that if you are fully vaccinated, you are protected against severe Covid, hospitalization and death and are even protected against the known variants — including the Delta variant — circulating in the country,” Dr. Rochelle Walensky, the Centers for Disease Control and Prevention director, said in July. Why the whiplash? Federal health officials’ concern began with data from Israel that suggested that the Pfizer vaccine’s protection against severe disease had fallen significantly for people over 60 who got their second shot in January or February. This week the CDC released three studies suggesting that the vaccines’ protection against infection had declined. “We are concerned that this pattern of decline we are seeing will continue in the months ahead, which could lead to reduced protection against severe disease, hospitalization and death,” Dr. Vivek Murthy, the surgeon general, said at a White House press briefing. The cause of the decline remains uncertain: It could result from waning vaccine immunity, but it could also result from a lapse in precautions like mask wearing or the rise of the highly contagious Delta variant — or a combination of all three. Yet some public health experts strongly disagree with the Biden administration’s decision to recommend third shots, given that billions of people around the world have yet to receive their first. While breakthrough infections are on the rise in some vaccine-rich countries, the CDC studies and other Israeli data indicate that the vaccines remain highly protective against hospitalization across all age groups. “These data support giving additional doses of vaccine to highly immunocompromised persons and nursing home residents, not to the general public,” Dr. Celine Gounder, an infectious-disease specialist at Bellevue Hospital Center and a former pandemic adviser to the Biden administration, told The Times. “Feeling sick like a dog and laid up in bed, but not in the hospital with severe Covid, is not a good enough reason” for universal booster shots, she added. “We’ll be better protected by vaccinating the unvaccinated here and around the world.” The Biden administration denies any trade-off between a domestic booster program and the global vaccination effort. But many experts say the trade-off is plain to see. “Anyone who thinks that vaccinating Americans with a third dose is not going to come at the expense of getting the vaccine to other places in the world — if that’s what you think, you’re just kidding yourself,” Scott Hensley, a vaccine researcher at the University of Pennsylvania’s Penn Institute for Immunology, told Stat. The Delta variant hasn’t changed the fact that vaccines remain the best way for individuals to protect themselves, Ed Yong wrote in The Atlantic. Adults might need boosters eventually, but he estimated immunity won’t wane substantially for at least two years. But the Delta variant’s higher transmissibility does mean that the coronavirus will be a permanent part of our lives; the goal of eradicating it, always unlikely, is now a fantasy. “Most people will meet the virus eventually,” he wrote. “We want to ensure that as many people as possible do so with two doses of vaccine in them, and that everyone else does so over as much time as possible.” Will Delta be the last major variant? There are three ways the coronavirus could continue to evolve, as Dhruv Khullar, a physician and an assistant professor at Weill Cornell Medical College, explained in The New Yorker: - In the first scenario, the virus simply fails to find a way to escape vaccine-generated immunity. Some scientists think that’s a plausible outcome: Many viruses — measles, mumps, polio and smallpox among them — have never beaten the vaccines created for them. - In the second scenario, the virus manages to partly evade the vaccines’ protections, but at a price (a decline in infectiousness or lethality, for example). This scenario occurred with HIV in the 1990s, when the virus developed a mutation that increased its resistance to an antiviral drug but lowered its rate of replication inside the body. - In the third scenario, the most concerning, the virus manages to find a way around vaccine-generated immunity while maintaining — or even gaining — transmissibility or lethality. Many scientists say that there are biological and epidemiological constraints on how fearsome the coronavirus can become: If it grows too transmissible or too lethal, “it’ll burn itself out,” Nash Rochman, who studies computational genomics at the National Institutes of Health, told Vox. But no one knows for certain how much evolutionary runway the virus has left. “There are certainly limits,” Kristian Andersen, an infectious-disease researcher at the Scripps Research Institute, told Khullar. “We just have no idea where they are.” But even if another concerning variant emerges, it won’t necessarily spell disaster. As John M. Barry, a historian and the author of “The Great Influenza,” noted in The Washington Post, all five flu pandemics we have detailed information about developed more virulent variants before petering out. And if the coronavirus circumvents the vaccines, it will happen gradually, he said, leaving time for vaccine producers to make the necessary adjustments. “Whether we need booster shots with existing vaccines or not, we will eventually require updated vaccines targeted against the latest variant, just as we do each year for influenza,” he wrote. “Even if Delta is the worst variant we see, the virus will continue to mutate. As with influenza, the goal is to develop a vaccine that protects against all variants.” If the coronavirus isn’t going away, how do we live with it? Eventually, experts say, when everyone has been vaccinated or infected, the coronavirus will become endemic: Outbreaks will be rarer and smaller, and hospitalizations and deaths will decline. “There will be a time in the future when life is like it was two years ago: You run up to someone, give them a hug, get an infection, go through half a box of tissues and move on with your life,” Jennie Lavine, an infectious-disease researcher at Emory University, told Yong. “That’s where we’re headed, but we’re not there yet.” How long it will take to get there remains an open question. In the United States, it’s possible that the Delta variant will soon run out of unvaccinated people to infect, as some speculate has occurred in Britain. But however long it takes to reach endemicity, the road may be rocky, The Atlantic’s Sarah Zhang wrote — especially psychologically. “People are having a hard time understanding one another’s risk tolerance,” Julie Downs, a psychologist who studies health decisions at Carnegie Mellon University, told her. “We need to prepare people that it’s not going to come down to zero. It’s going to come down to some level we find acceptable.” We will also need to better understand and treat long Covid, Adam Gaffney and Zackary Berger wrote in The Times. Seventeen months into the pandemic, the link between the coronavirus and the symptoms most people associate with long Covid — brain fog, severe fatigue, chronic pain, shortness of breath — is still complicated and not entirely clear. One peer-reviewed study of people who reported long Covid symptoms found that most of those who were tested for antibodies had negative results, suggesting the syndrome may have causes unrelated to the virus. “No matter what the underlying cause and whether there is evidence of prior infection, long Covid, even among those with little or no evidence of previous infection, brings significant suffering, including several reported deaths by suicide,” they wrote. “Action, not just acknowledgment, is needed.” The New York Times