A children\u2019s ward for dengue fever in Bangladesh\u2019s capital.<\/span>bdul Goni\/AFP\/Getty via Vox<\/span><\/p>\n<\/div>\n This story was originally published by\u00a0<\/em>Vox<\/a> and is reproduced here as part of the\u00a0<\/em>Climate Desk<\/a>\u00a0collaboration.<\/em><\/p>\n There\u2019s a reason <\/span>dengue infections are also called \u201cbreakbone fever.\u201d<\/p>\n Along with a mild fever, symptoms of the mosquito-borne illness include bone-deep, aching pain in the joints and behind the eyes. In severe cases, blood vessels begin to leak. And in the worst cases, that can lead to organ failure.<\/p>\n More than 14 million people contracted dengue\u00a0last year<\/a>, and the real number is likely several times higher. While it remains most common in South Asia and Latin America, it\u2019s no longer just a tropical disease. Warming temperatures are pushing dengue into southern Europe and the United States. Last year, Texas saw its\u00a0highest case count in two decades<\/a>, including locally acquired infections, meaning the virus is now circulating here, not just arriving with travelers.<\/p>\n The public health tools we have\u2014the\u00a0dengue vaccines<\/a>, bed nets, fogging campaigns, public awareness to drain standing water\u2014are all aimed at keeping mosquitos at bay and preventing infections in the first place. There\u2019s nothing for after: no antivirals\u2014nothing like Paxlovid for Covid, or Tamiflu for the flu, or artemisinin for malaria. Once you\u2019re sick, the strategy is just supportive care and hope.<\/p>\n Earlier this month, though, that changed.<\/p>\n A new antiviral pill for dengue called mosnodenvir showed promising results in\u00a0early phase 2 trials<\/a>. In a study where volunteers were deliberately exposed to dengue, roughly half of those who received the highest dose never got sick at all. For a field that has struggled for decades to find an effective antiviral, it\u2019s the clearest evidence yet that a drug can prevent dengue\u2014and researchers believe the same pill could eventually treat people who are already infected.<\/p>\n But, even before the results were published, Johnson & Johnson, the American pharmaceutical giant that developed mosnodenvir, had already abandoned any efforts to bring the drug to market.<\/p>\n Last year J&J announced it would wind down its dengue antiviral work, with a \u201cstrategic reprioritization\u201d of its research toward non-communicable diseases like cancer and obesity. What this means is that one of the most promising dengue drugs ever tested is now without a pharma sponsor, waiting for someone else to carry it forward.<\/p>\n Andr\u00e9 Siqueira, who heads the dengue program at the Drugs for Neglected Diseases Initiative (DNDi), said mosnodenvir is \u201cvery, very promising\u201d and said he wants to see it pushed into further trials \u201cas quickly as possible.\u201d<\/p>\n But why\u2014if the drug shows much promise\u2014would its maker walk away?<\/p>\n J&J\u2019s exit isn\u2019t an outlier; it\u2019s part of a\u00a0broader<\/a>\u00a0retreat<\/a>\u00a0from infectious disease research across the pharmaceutical industry, as companies shift toward drugs for wealthier markets: cancer, obesity, autoimmune disorders.<\/p>\n Dengue already\u00a0kills thousands every<\/a>\u00a0year, and it\u2019s getting worse. By 2080,\u00a0climate models suggest<\/a>, nearly 60 percent of the world\u2019s population could be living in areas where dengue spreads.<\/p>\n And, in this new world, watching the first antiviral pill that works against dengue get abandoned\u2014while the disease spreads to new continents\u2014reveals the gap between the drugs we need and the drugs the market will deliver.<\/p>\n To test whether<\/span> mosnodenvir actually works, researchers did something uncommon: They deliberately infected people with dengue.<\/p>\n Over the past three years, 31 volunteers in Baltimore and Vermont, in\u00a0what\u2019s called a challenge trial<\/a>, agreed to take a pill for several days and then get injected with a weakened dengue virus. It\u2019s a faster way to get answers than waiting for people to get sick naturally, but it requires volunteers willing to sign up for a controlled case of dengue.<\/p>\n Among people who got the highest dose of mosnodenvir, 6 out of 10 never developed an infection at all. The other four had much lower levels of virus in their blood and milder symptoms than the placebo group, where everyone got sick. At lower doses, the drug delayed infection but didn\u2019t prevent it\u2014a clear signal that the higher dose was doing something real.<\/p>\n \u201cIt\u2019s one of the most beautiful dose-response results I\u2019ve seen,\u201d Anna Durbin, the Johns Hopkins researcher who led the study,\u00a0told Science last month<\/a>.<\/p>\n Then, there\u2019s the field data. In 2023, J&J launched a trial across more than 30 sites in South America and Asia to test whether the drug could protect people in the same household who are at high risk of getting bitten by the same mosquitoes. Among 265 people who received the highest dose, not a single person developed symptomatic dengue. In the placebo group, 60 percent did. (This data hasn\u2019t been formally peer-reviewed yet, but it\u2019s\u00a0posted publicly<\/a>.)<\/p>\n For Neelika Malavige, a prominent dengue researcher at the University of Sri Jayewardenepura in Sri Lanka, the significance goes beyond the numbers. \u201cIt\u2019s a huge scientific breakthrough just doing the study,\u201d she said, referring to the design of the challenge trial itself, which had never been done for a dengue antiviral before. For a disease with no approved treatment, this is as close to proof of concept as it gets.<\/p>\n \u201cThe dengue community may be closer than ever to a long-awaited treatment,\u201d Xuping Xie of the University of Texas Medical Branch wrote in a\u00a0commentary<\/a>\u00a0accompanying the paper.<\/p>\n The trial proved<\/span> that mosnodenvir can prevent infection, a first for any dengue drug. But prevention isn\u2019t what dengue doctors need most. What they need is a treatment, something to give patients who are already sick to keep them from getting worse.<\/p>\n That\u2019s what makes an antiviral so valuable. Prevention strategies have a ceiling; you can reduce mosquito populations, but you can\u2019t eliminate them, and warming temperatures keep pushing them into new territory. A drug that works\u00a0after<\/em>\u00a0exposure would be the first tool that doesn\u2019t depend on stopping the mosquito first.<\/p>\n The hope is that the same drug could do both. Mosnodenvir works by blocking the virus from replicating, and, in theory, that should help whether you take it before you\u2019re infected or shortly after. The question is timing.<\/p>\n That\u2019s where dengue gets tricky. Unlike malaria, where the parasite lingers, and you can kill it with drugs, the dengue virus moves through the body notoriously fast. By the time a patient feels sick enough to see a doctor\u2014usually a few days into the fever\u2014the virus is often already on its way out. The brutal symptoms that follow, the blood vessel leakage and organ damage, are driven largely by the body\u2019s own immune response, not the virus itself.<\/p>\n This is why antivirals have been so hard to develop for dengue. The window to intervene can be narrow, and for many patients, it\u2019s already closing by the time they show up.<\/p>\n The scientists who developed mosnodenvir believe it could work as a treatment. \u201cIf you reduce the amount of replicating virus, you will also reduce the likelihood that the patient evolves towards severe disease,\u201d said Johan Neyts, a virologist at KU Leuven whose lab co-discovered the drug. The logic is in line with how antivirals for, say Covid, work, but this hypothesis hasn\u2019t been tested in humans. Treatment trials were planned in Singapore, but the Covid pandemic made them impossible. By the time restrictions lifted, J&J had already decided to exit.<\/p>\n The dream, Malavige said, is simple, \u201cYou go to the doctor, get yourself tested, the test is positive, you\u2019re given an antiviral, and that\u2019s the end of the story.\u201d The question is whether patients can get there early enough\u2014and whether mosnodenvir can work.<\/p>\n There\u2019s also the question of resistance. In the human challenge trial, genetic mutations emerged in the virus among nearly all the participants who took mosnodenvir\u2014mutations that could, in theory, make the drug less effective over time. And some dengue strains already circulating in nature\u00a0appear to be harder to treat<\/a>\u00a0with this type of drug.<\/p>\n This is a real limitation. Mosnodenvir alone probably isn\u2019t a long-term solution, because, eventually, the virus might adapt. But that problem is a familiar one for drug makers.\u00a0HIV<\/a>\u00a0and\u00a0malaria<\/a>\u00a0both evolved resistance to early drugs, and the answer was combination therapy: multiple drugs that attack the virus in different ways, making it far harder to escape all of them at once.<\/p>\n For that strategy to work with dengue, though, we need more drugs to combine. Mosnodenvir may not be the whole puzzle, but it could be the first piece. \u201cIf people stopped at the first sign of seeing trouble,\u201d Malavige said, \u201cthen the world will not progress.\u201d<\/p>\n Johnson & Johnson\u2019s exit<\/span> follows a well-worn path for big pharma.<\/p>\n Over the past two decades,\u00a0Bristol Myers Squibb<\/a>,\u00a0Novartis<\/a>,\u00a0AstraZeneca<\/a>, and other major drugmakers have all scaled back or abandoned infectious disease research, judging that these drugs simply couldn\u2019t compete with cancer and obesity blockbusters. A\u00a0recent op-ed in the Financial T<\/em>imes<\/a>\u00a0called it a \u201ctextbook market failure.\u201d The public health impact is massive, but the financial returns for addressing them aren\u2019t.<\/p>\n After J&J\u2019s exit, ownership of mosnodenvir is being transferred back to KU Leuven, the Belgian university where the drug was first discovered before J&J licensed it for development. \u201cWe will do all we can to make sure that mosnodenvir is further developed in clinical trials as soon as possible,\u201d said Patrick Chaltin, who directs the university\u2019s drug discovery center. To do that, the university is working with the Wellcome Trust, a major global health funder, to find new partners and funding.<\/p>\n And fortunately, mosnodenvir isn\u2019t the only dengue drug that the pharmaceutical industry is looking into. The Swiss drug maker Novartis is running a phase 2 treatment trial for a different antiviral, and the Serum Institute in India is testing a monoclonal antibody.<\/p>\n Drug development is expensive and uncertain, and the people who need dengue treatments most are not the people who can pay the most. But these steps are encouraging.<\/p>\n In countries where dengue has always circulated\u2014India, Brazil, the Philippines, Sri Lanka\u2014people have learned to live around it, says Malavige. Life bends around when the mosquitoes are biting, and then bends back.<\/p>\n But dengue isn\u2019t locked in those places anymore. Warmer temperatures are carrying the mosquitoes\u2014and the virus\u2014somewhere new every year. And there\u2019s no sign that this expansion is slowing down.<\/p>\n<\/p><\/div>\n![\"A](\"https:\/\/www.motherjones.com\/wp-content\/uploads\/2025\/12\/b005e4dd1660ef63bbc47b458d5844f31624a14b.webp\")