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Tag: versions of the virus

  • How Worried Should We Be About XBB.1.5?

    How Worried Should We Be About XBB.1.5?

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    After months and months of SARS-CoV-2 subvariant soup, one ingredient has emerged in the United States with a flavor pungent enough to overwhelm the rest: XBB.1.5, an Omicron offshoot that now accounts for an estimated 75 percent of cases in the Northeast. A crafty dodger of antibodies that is able to grip extra tightly onto the surface of our cells, XBB.1.5 is now officially the country’s fastest-spreading coronavirus subvariant. In the last week of December alone, it zoomed from 20 percent of estimated infections nationwide to 40 percent; soon, it’s expected to be all that’s left, or at least very close. “That’s the big thing everybody looks for—how quickly it takes over from existing variants,” says Shaun Truelove, an infectious-disease modeler at Johns Hopkins University. “And that’s a really quick rise.”

    All of this raises familiar worries: more illness, more long COVID, more hospitalizations, more health-care system strain. With holiday cheer and chilly temperatures crowding people indoors, and the uptake of bivalent vaccines at an abysmal low, a winter wave was already brewing in the U.S. The impending dominance of an especially speedy, immune-evasive variant, Truelove told me, could ratchet up that swell.

    But the American public has heard that warning many, many, many times before—and by and large, the situation has not changed. The world has come a long way since early 2020, when it lacked vaccines and drugs to combat the coronavirus; now, with immunity from shots and past infections slathered across the planet—porous and uneven though that layer may be—the population is no longer nearly so vulnerable to COVID’s worst effects. Nor is XBB.1.5 a doomsday-caliber threat. So far, no evidence suggests that the subvariant is inherently more severe than its predecessors. When its close sibling, XBB, swamped Singapore a few months ago, pushing case counts up, hospitalizations didn’t undergo a disproportionately massive spike (though XBB.1.5 is more transmissible, and the U.S. is less well vaccinated). Compared with the original Omicron surge that pummeled the nation this time last year, “I think there’s less to be worried about,” especially for people who are up to date on their vaccines, says Mehul Suthar, a viral immunologist at Emory University who’s been studying how the immune system reacts to new variants. “My previous exposures are probably going to help against any XBB infection I have.”

    SARS-CoV-2’s evolution is still worth tracking closely through genomic surveillance—which is only getting harder as testing efforts continue to be pared back. But “variants mean something a little different now for most of the world than they did earlier in the pandemic,” says Emma Hodcroft, a molecular epidemiologist at the University of Bern, in Switzerland, who’s been tracking the proportions of SARS-Cov-2 variants around the world. Versions of the virus that can elude a subset of our immune defenses are, after all, going to keep on coming, for as long as SARS-CoV-2 is with us—likely forever, as my colleague Sarah Zhang has written. It’s the classic host-pathogen arms race: Viruses infect us; our bodies, hoping to avoid a similarly severe reinfection, build up defenses, goading the invader into modifying its features so it can infiltrate us anew.

    Read: How long can the coronavirus keep infecting us?

    But the virus is not evolving toward the point where it’s unstoppable; it’s only switching up its fencing stance to sidestep our latest parries as we do the same for it. A version of the virus that succeeds in one place may flop in another, depending on the context: local vaccination and infection histories, for instance, or how many elderly and immunocompromised individuals are around, and the degree to which everyone avoids trading public air. With the world’s immune landscape now so uneven, “it’s getting harder for the virus to do that synchronized wave that Omicron did this time last year,” says Verity Hill, an evolutionary virologist at Yale. It will keep trying to creep around our defenses, says Pavitra Roychoudhury, who’s monitoring SARS-CoV-2 variants at the University of Washington, but “I don’t think we need to have alarm-bell emojis for every variant that comes out.”

    Read: The coronavirus’s next move

    Some particularly worrying variants and subvariants will continue to arise, with telltale signs, Roychoudhury told me: a steep increase in wastewater surveillance, followed by a catastrophic climb in hospitalizations; a superfast takeover that kicks other coronavirus strains off the stage in a matter of days or weeks. Omens such as these hint at a variant that’s probably so good at circumventing existing immune defenses that it will easily sicken just about everyone again—and cause enough illness overall that a large number of cases turn severe. Also possible is a future variant that is inherently more virulent, adding risk to every new case. In extreme versions of these scenarios, tests, treatments, and masks might need to come back into mass use; researchers may need to concoct a new vaccine recipe  at an accelerated pace. But that’s a threshold that most variations of SARS-CoV-2 will not clear—including, it seems so far, XBB.1.5. Right now, Hodcroft told me, “it’s hard to imagine that anything we’ve been seeing in the last few months would really cause a rush to do a vaccine update,” or anything else similarly extreme. “We don’t make a new flu vaccine every time we see a new variant, and we see those all through the year.” Our current crop of BA.5-focused shots is not a great match for XBB.1.5, as Suthar and his colleagues have found, at least on the antibody front. But antibodies aren’t the only defenses at play—and Suthar told me it’s still far better to have the new vaccine than not.

    In the U.S., wastewater counts and hospitalizations are ticking upward, and XBB.1.5 is quickly elbowing out its peers. But the estimated infection rise doesn’t seem nearly as steep as the ascension of the original Omicron variant, BA.1 (though our tracking is now poorer). XBB.1.5 also isn’t dominating equally in different parts of the country—and Truelove points out that it doesn’t yet seem tightly linked to hospitalizations in the places where it’s gained traction so far. As tempting as it may be to blame any rise in cases and hospitalizations on the latest subvariant, our own behaviors are at least as important. Drop-offs in vaccine uptake or big jumps in mitigation-free mingling can drive spikes in illness on their own. “We were expecting a wave already, this time of year,” Hill told me. Travel is up, masking is down. And just 15 percent of Americans over the age of 5 have received a bivalent shot.

    The pace at which new SARS-CoV-2 variants and subvariants take over could eventually slow, but the experts I spoke with weren’t sure this would happen. Immunity across the globe remains patchy; only a subset of countries have access to updated bivalent vaccines, while some countries are still struggling to get first doses into millions of arms. And with nearly all COVID-dampening mitigations “pretty much gone” on a global scale, Hodcroft told me, it’s gotten awfully easy for the coronavirus to keep experimenting with new ways to stump our immune defenses. XBB.1.5 is both the product and the catalyst of unfettered spread—and should that continue, the virus will take advantage again.

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    Katherine J. Wu

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  • China’s COVID Wave Is Coming

    China’s COVID Wave Is Coming

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    In China, a dam seems on the verge of breaking. Following a wave of protests, the government has begun to relax some of its most stringent zero-COVID protocols, and regional authorities have trimmed back a slew of requirements for mass testing, quarantine, and isolation. The rollbacks are coming as a relief for the many Chinese residents who have been clamoring for change. But they’re also swiftly tilting the nation toward a future that’s felt inevitable for nearly three years: a flood of infections—accompanied, perhaps, by an uncharted morass of disease and death. A rise in new cases has already begun to manifest in urban centers such as Chongqing, Beijing, and Guangzhou. Now experts are waiting to see just how serious China’s outbreak will be, and whether the country can cleanly extricate itself from the epidemic ahead.

    For now, the forecast “is full of ifs and buts and maybes,” says Salim Abdool Karim, an epidemiologist at the Centre for the AIDS Programme of Research in South Africa. Perhaps the worst can be averted if the government does more to vaccinate the vulnerable and prep hospitals for a protracted influx of COVID patients; and if the community at large reinvests in a subset of mitigation measures as cases rise. “There is still the possibility that they may muddle through it without a mass die-off,” says Yanzhong Huang, a senior fellow for global health at the Council on Foreign Relations. “But even the most smooth and orderly transition,” he told me, “will not prevent a surge of cases.”

    China represents, in many ways, SARS-CoV-2’s final frontier. With its under-vaccinated residents and sparse infection history, the nation harbors “a more susceptible population than really any other large population I can think of,” says Sarah Cobey, an computational epidemiologist at the University of Chicago. Soon, SARS-CoV-2 will infiltrate that group of hosts so thoroughly that it will be nearly impossible to purge again. “Eventually, just like everyone else on Earth, everyone in China should expect to be infected,” says Michael Worobey, an evolutionary virologist at the University of Arizona.

    Read: How many COVID deaths will Chinese protesters accept?

    Whatever happens, though, China’s coming wave won’t recapitulate the one that swept most of the world in early 2020. Though it’s hard to say which versions of the virus are circulating in the country, a smattering of reports confirm the likeliest scenario: BF.7 and other Omicron subvariants predominate. Several of these versions of the virus seem to be a bit less likely than their predecessors to trigger severe disease. That, combined with the relatively high proportion of residents—roughly 95 percent—who have received at least one dose of a COVID vaccine, might keep many people from falling dangerously ill. The latest figures out of China’s CDC marked some 90 percent of the country’s cases as asymptomatic. “That’s an enormous fraction” compared with what’s been documented elsewhere, says Ben Cowling, an epidemiologist at the University of Hong Kong.

    That percentage, however, is undoubtedly increased by the country’s ultra-rigorous testing practices, which have been catching silent cases that other places might miss. All of Omicron’s iterations also remain capable of triggering severe disease and long COVID. And there are still plenty of worrying omens that climbing cases could reach a horrific peak, sit on a prolonged plateau, or both.

    One of China’s biggest weak spots is its immunity, or lack thereof. Although more than 90 percent of all people in the country have received at least two COVID shots, those over the age of 80 were not prioritized in the country’s initial rollout, and their rate of dual-dose coverage hovers around just 66 percent. An even paltrier fraction of older people have received a third dose, which the World Health Organization recommends for better protection. Chinese officials have vowed to buoy those numbers in the weeks ahead. But vaccination sites have been tougher to access than testing sites, and with few freedoms offered to the immunized, “the incentive structure is not built,” says Xi Chen, a global-health expert at Yale. Some residents are also distrustful of COVID vaccines. Even some health-care workers are wary of delivering the shots, Chen told me, because they’re fearful of liability for side effects.

    Regardless of the progress China makes in plugging the holes in its immunity shield, COVID vaccines won’t prevent all infections. China’s shots, most of which are based on chemically inactivated particles of the 2020 version of SARS-CoV-2, seem to be less effective and less durable than mRNA recipes, especially against Omicron variants. And many of China’s residents received their third doses many months ago. That means even people who are currently counted as “boosted” aren’t as protected as they could be.

    All of this and more could position China to be worse off than other places—among them, Australia, New Zealand, and Singapore—that have navigated out of a zero-COVID state, says Caitlin Rivers, a senior scholar at the Johns Hopkins Center for Health Security. Australia, for instance, didn’t soften its mitigations until it had achieved high levels of vaccine coverage among older adults, Rivers told me. China has also clung to its zero-COVID philosophy far longer than any other nation, leaving itself to contend with variants that are better at spreading than those that came before. Other countries charted their own path out of their restrictions; China is being forced into an unplanned exit.

    What Hong Kong endured earlier this year may hint at what’s ahead. “They had a really, really bad wave,” Kayoko Shioda, an epidemiologist at Emory University, told me—far dwarfing the four that the city had battled previously. Researchers have estimated that nearly half the city’s population—more than 3 million people—ended up catching the virus. More than 9,000 residents died. And Hong Kong was, in some respects, in a better place to ease its restrictions than the mainland is. This past winter and spring, the city’s main adversary was BA.2, a less vaccine-evasive Omicron subvariant than the ones circulating now; officials had Pfizer’s mRNA-based shot on hand, and quickly began offering fourth doses. Hong Kong also has more ICU beds per capita. Map a new Omicron outbreak onto mainland China, and the prognosis is poor: A recent modeling paper estimated that the country could experience up to 1.55 million deaths in the span of just a few months. (Other analyses offer less pessimistic estimates.)

    Lackluster vaccination isn’t China’s only issue. The country has accumulated almost no infection-induced immunity that might otherwise have updated people’s bodies on recent coronavirus strains. The country’s health-care system is also ill-equipped to handle a surge in demand: For every 100,000 Chinese residents, just 3.6 ICU beds exist, concentrated in wealthier cities; in an out-of-control-infection scenario, even a variant with a relatively low severe-disease risk would prove disastrous, Chen told me. Nor does the system have the slack to accommodate a rush of patients. China’s culture of care seeking is such that “even when you have minor illness, you seek help in urban health centers,” Huang told me, and not enough efforts have been made to bolster triage protocols. More health-care workers may become infected; patients may be more likely to slip through the cracks. Next month’s Lunar New Year celebration, too, could spark further spread. And as the weather cools and restrictions relax, other respiratory viruses, such as RSV and flu, could drive epidemics of their own.

    That said, spikes of illness are unlikely to peak across China at the same time, which could offer some relief. The country’s coming surge “could be explosive,” Cobey told me, “or it could be more of a slow burn.” Already, the country is displaying a patchwork of waxing and waning regulations across jurisdictions, as some cities tighten their restrictions to combat the virus while others loosen up. Experts told me that more measures may return as cases ratchet up—and unlike people in many other countries, the Chinese may be more eager to readopt them to quash a ballooning outbreak.

    Read: China’s zero tolerance for Xi’s COVID restrictions

    A major COVID outbreak in China would also have unpredictable effects on the virus. The world’s most populous country includes a large number of immunocompromised people, who can harbor the virus for months—chronic infections that are thought to have produced variants of concern before. The world may be about to witness “a billion or more opportunities for the virus to evolve,” Cowling told me. In the coming months, the coronavirus could also exploit the Chinese’s close interactions with farmed animals, such as raccoon dogs and mink (both of which can be infected by SARS-CoV-2), and become enmeshed in local fauna. “We’ve certainly seen animal reservoirs becoming established in other parts of the world,” Worobey told me. “We should expect the same thing there.”

    Then again, the risk of new variants spinning out of a Chinese outbreak may be a bit less than it seems, Abdool Karim and other experts told me. China has stuck with zero COVID so long that its population has, by and large, never encountered Omicron subvariants; people’s immune systems remain trained almost exclusively on the original version of the coronavirus, raising only defenses that currently circulating strains can easily get around. It’s possible that “there will be less pressure for the virus to evolve to evade immunity further,” says Emma Hodcroft, a molecular epidemiologist at the University of Bern; and any new versions of the virus that do emerge might not fare particularly well outside of China. In other words, the virus could end up trapped in the very country that tried to keep it out the longest. Still, with so many people susceptible, Cobey told me, there are zero guarantees.

    Either way, viral evolution will plod on—and as it does, the rest of the world may struggle to track it in real time, especially as the cadence of Chinese testing ebbs. Cowling worries that China will have trouble monitoring the number of cases in the country, much less which subvariants are causing them. “There’s going to be a challenge in having situational awareness,” he told me. Shioda, too, worries that China will remain tight-lipped about the scale of the outbreak, a pattern that could have serious implications for residents as well.

    Even without a spike in severe disease, a wide-ranging outbreak is likely to put immense strain on China—which may weigh heavily on its economy and residents for years to come. After the SARS outbreak that began in 2002, rates of burnout and post-traumatic stress among health-care workers in affected countries swelled. Chinese citizens have not experienced an epidemic of this scale in recent memory, Chen told me. “A lot of people think it is over, that they can go back to their normal lives.” But once SARS-CoV-2 embeds itself in the country, it won’t be apt to leave. There will not be any going back to normal, not after this.

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    Katherine J. Wu

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  • Polio Is Exploiting a Very Human Weakness

    Polio Is Exploiting a Very Human Weakness

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    In 1988, the World Health Assembly announced a very ambitious goal: Polio was to be vanquished by the year 2000. It was a reach, sure, but feasible. Although highly infectious, polioviruses affect only people, and don’t hide out in wild animals; with two extraordinarily effective vaccines in regular use, they should be possible to snuff out. Thanks to a global inoculation campaign, infections had, for years, been going down, down, down.

    But 2000 came and went, as did a second deadline, in 2005, and a third, in 2012, and so on. The world will almost certainly miss an upcoming target at the end of 2023 too. In theory, eradication is still in sight: The virus remains endemic in just two countries—Pakistan and Afghanistan—and two of the three types of wild poliovirus that once troubled humanity are gone. And yet, polio cases are creeping up in several countries that had eliminated them, including the United Kingdom, Israel, and the United States. Earlier this year, New York detected America’s first paralytic polio case in nearly a decade; last week, the governor declared a state of emergency over a fast-ballooning outbreak.

    This is the cruel logic of viruses: Give them enough time—leave enough hosts for them to infect—and they will eventually find a way to spread again. “You have to stop transmission everywhere, all at the same time,” says Kimberly Thompson, a health economist and the president of the nonprofit Kid Risk. Which means eradication will demand a near-perfect syncing of vaccine supply, access, equity, political will, public enthusiasm, and more. To beat the virus, population immunity must outlast it.

    Right now, though, the world’s immunological shield is too porous to stop polio’s spread. At the center of the new epidemics are vaccine-derived polioviruses that have begun to paralyze unimmunized people in places where immunity is low—a snag in the eradication campaign that also happens to be tightly linked to one of its most essential tools. Vaccine performance has always depended on both technology and human behavior. But in this case especially, because of the nature of the foe at hand, those twin pillars must line up as precisely as possible or risk a further backslide into a dangerous past.

    In the grand plan for eradication, our two primary polio vaccines were always meant to complement each other. One, an ultra-effective oral formulation, is powerful and long-lasting enough to quash wild-poliovirus transmission—the perfect “workhorse” for a global vaccination campaign, says Adam Lauring, an evolutionary virologist at the University of Michigan. The other, a supersafe injectable, sweeps in after its colleague has halted outbreaks one country at a time, maintaining a high level of immunity in post-elimination nations while the rest of the world catches up.

    For decades, the shot, chaser approach found remarkable success. In the 1980s, wild poliovirus struck an estimated 300,000 to 400,000 people each year; by 2021, the numbers had plummeted to single digits. But recently, as vaccine coverage in various countries has stalled or slipped, the loopholes in this vaccination tactic have begun to show themselves and grow.

    The oral polio vaccine (OPV), delivered as drops in the mouth, is one of the most effective inoculations in the world’s roster. It contains weakened forms of polioviruses that have been altered away from their paralysis-causing forms but still mimic a wild infection so well that they can stop people from spreading wild pathogens for years, even decades. In the weeks after people receive the vaccine, they can also pass the weakened virus to others in the community, helping protect them too. And OPV’s transportability, low price point, and ease of administration make it a “gold standard for outbreak interruption,” says Ananda Bandyopadhyay, the deputy director for the polio team at the Bill & Melinda Gates Foundation. Since its mid-20th-century debut, OPV has helped dozens of countries—including the U.S.—eliminate the virus. Those nations were then able to phase out OPV and switch to inoculating people with the injected vaccine.

    But OPV’s most potent superpower is also its greatest weakness. Given enough time and opportunity to spread and reproduce, the neutered virus within the vaccine can regain the ability to invade the nervous system and cause paralysis in unvaccinated or immunocompromised people (or in very, very rare cases, the vaccine recipient themselves). Just a small handful of genetic modifications—three or fewer—can spark a reversion, and the mutants, which are “better at replicating” than their kin, can take over fast, says Raul Andino, a virologist at UC San Francisco. In recent years, a few thousand cases of vaccine-derived polio have been detected around the world, far outstripping the toll of wild viruses; dozens of countries, the U.S. now among them, are battling such outbreaks, and the numbers seem to be only going up. Vaccine-derived polio is still a true rarity: Billions of oral vaccines have been delivered since the global campaign began. But it underscores “the real problem” with OPV, Lauring told me. “You’re fighting fire with fire.”

    Read: The epidemic that preyed on children

    The injected polio vaccine, or IPV, which contains only chemically inactivated versions of the virus, carries none of that risk. To purge all polio cases, “you have to stop using oral polio vaccine,” Thompson told me, and transition the entire globe to IPV. (Post-eradication, countries would need to keep IPV in their routine immunization schedule for at least 10 years, experts have said.) But the injected vaccine has a different drawback. Although the shot can very effectively stave off paralysis, IPV doesn’t elicit the kind of immunity that stops people from getting infected with polioviruses and then passing them on. In places that rely on injected vaccines, “even immune individuals can participate in transmission,” Thompson told me. Which opens up a vulnerability when too many people have skipped both types of vaccines: Paralyzing polioviruses erupt out of communities where the oral vaccine is still in use—then can spread in undervaccinated areas. It might be tempting to blame OPV for our troubles. But that’s not the main threat, Bandyopadhyay told me. “It’s the lack of adequate vaccination.”

    As things stand, the goal in the endemic countries of Pakistan and Afghanistan remains achieving sufficiently high vaccine coverage, Bandyopadhyay said. But many of the communities in these nations are rural or nomadic, and tough to reach even with convenient drop-in-the-mouth vaccines. Civil and political unrest, misinformation, natural disasters, and most recently, the COVID pandemic have raised additional hurdles. So have intermittent bans on house-to-house vaccination in Afghanistan, says John Vertefeuille, the chief of the polio-eradication branch at the CDC. Cases of wild polio have experienced a recent jump in Pakistan, and have also been imported into the non-endemic countries of Malawi and Mozambique.

    But the toll of those outbreaks—all featuring type 1 polio—currently pales in comparison with those featuring vaccine-derived type 2. The last case of wild type 2 polio was detected in 1999, but that version of the virus has persisted in its modified form in oral polio vaccines. And when it reverts to its dangerous form, it gains particularly infectious oomph, allowing it to spread unchecked wherever immunity is low. Some 30 countries around the world are battling outbreaks of poliovirus whose origin can be traced back to the oral inoculations; vaccine-derived type 2 is what’s been circulating in Jerusalem, London, and New York, where it ultimately paralyzed an unvaccinated young man. The extent to which the virus is churning in other parts of the country isn’t fully known; routine immunization has dropped since the COVID pandemic’s start, and the U.S. hasn’t regularly surveyed its wastewater for the pathogen.

    The success of these vaccine-derived viruses is largely the result of our own hubris—of a failure, experts told me, to sync the world’s efforts. In 2016, 17 years after the last wild type-2 case had been seen, officials decided to pivot to a new version of OPV that would protect against just types 1 and 3, a sort of trial run for the eventual obsolescence of OPV. But the move may have been premature. The switch wasn’t coordinated enough; in too many pockets of the world, type-2 polio, from the three-part oral vaccine, was still moseying about. The result was disastrous. “We opened up an immunity gap,” Thompson told me. Into it, fast-mutating vaccine-derived type-2 viruses spilled, surging onto a global landscape populated with growing numbers of children who lacked protection against it.

    A new oral vaccine, listed for emergency use by the WHO in 2020, could help get the global campaign back on track. The fresh formulation, developed in part by Andino and his colleagues, still relies on the immunity-boosting powers of weakened, replicating polioviruses. But the pathogens within have had their genetic blueprints further tweaked. “We mucked around” with the structure of poliovirus, Andino told me, and figured out a way to make a modified version of type 2 that’s far stabler. It’s much less likely to mutate away from its domesticated, non-paralyzing state, or swap genes with related viruses that could grant the same gifts.

    Technologically, the new oral vaccine, nicknamed nOPV2, seems to be as close to a slam dunk as immunizations can get. “To me, it’s just super cool,” Lauring told me. “You keep all the good things about OPV but mitigate this evolutionary risk.” In the year and a half since the vaccine’s world premiere, some 450 million doses of nOPV2 have found their way into children in 22 countries—and a whopping zero cases of vaccine-derived paralysis have followed.

    But nOPV2 is “not a silver bullet,” Andino said. The vaccine covers just one of the three poliovirus types, which means it can’t yet fully replace the original oral recipe. (Trials for type-1 and -3 versions are ongoing, and even after those recipes are ready for prime time, researchers will have to confirm that the vaccine still works as expected when the three recipes are mixed.) The vaccine’s precise clinical costs are also still a shade unclear. nOPV2 is a safer oral polio vaccine, but it’s still an oral polio vaccine, chock-full of active viral particles. “You can think of it as more attenuated,” Thompson said. “But I don’t think anybody expects that it won’t have any potential to evolve.” And nOPV2’s existence doesn’t change the fact that the world will still have to undergo a total, coordinated switch to IPV before eradication is won.

    As has been the case with COVID vaccines, and so many others, the primary problem isn’t the technology at all—but how humans have deployed it, or failed to. “Vaccine sitting in a vial, no matter how genetically stable and how effective it is, that’s not going to solve the problem of the outbreaks,” Bandyopadhyay said. “It’s really vaccination and getting to that last child in that last community.”

    Read: How the poor get blamed for disease

    If dwindling vaccination trends don’t reverse, even our current vaccination strategies could require a rough reboot. In 2013, health officials in Israel—which had, for years prior, run a successful IPV-only campaign for its children—detected wild type-1 virus, imported from abroad, in the country’s sewage, and decided to roll out another round of oral vaccines to kids under 10. Within a few weeks, nearly 80 percent of the targeted population had gotten a dose. Even “polio-free countries are not polio-risk-free,” Bandyopadhyay told me. The situation in New York is different, in part because type-1 polio causes paralysis more often than type-2 does. But should circumstances grow more dire—should substantial outbreaks start elsewhere in the country, should the nation fail to bring IPV coverage back to properly protective levels—America, too, “may have to consider adding OPV as a supplement,” says Purvi Parikh, an immunologist and a physician at NYU, “especially in rural areas” where emergency injected-vaccine campaigns may be tough. Such an approach would be a pretty extreme move, and a “very big political undertaking,” Thompson said, requiring a pivot back to a vaccine that was phased out of use decades ago. And even then, there’s no guarantee that Americans would take the offered oral drops.

    The CDC, for now, is not eager for such a change. Noting that most people in the U.S. are vaccinated against polio, Katherina Grusich, an agency spokesperson, told me that the CDC has no plans to add OPV or nOPV to the American regimen. “We are a long way from reaching for that,” she said.

    But this week, the U.S. joined the WHO’s list of about 30 nations with circulating vaccine-derived-poliovirus outbreaks. The country could have avoided this unfortunate honor had it kept shot uptake more uniformly high. It’s true, as Grusich pointed out, that more than 90 percent of young American children have received IPV. But they are not distributed evenly, which opens up vulnerabilities for the virus to exploit. Here, the U.S., in a sense, had one job: maintain its polio-free status while the rest of the world joined in. That it did not is an admonition, and a reminder of how unmerciful the virus can be. Polio, a fast mutator, preys on human negligence; the vaccines that guard against it contain both a form of protection and a catch that reinforces how risky treating these tools as a discretionary measure can be.

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    Katherine J. Wu

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