If you ask an infectious diseases specialist whether we should start worrying about influenza A H5N1, they will probably tell you that we should always be worrying about it. An influenza pandemic has long topped the list of global threats to health, and avian influenza poses a particularly serious concern. H5N1 has infected over 800 people since its identification in 1996, with a mortality rate exceeding 50%. Since 2020, it has become endemic in bird populations, triggering an unparalleled animal pandemic, affecting at least 26 mammal species. There is nothing new in influenza strains evolving, shifting their epidemiological habits, and causing infections in people, although human-to-human transmission of H5N1 is rare. Nonetheless, although developments over the past 3 months may or may not signal the start of a global pandemic, they are at the very least a pressing and unwelcome reminder of the caprices of zoonotic influenza and our continued collective complacency about it.
An unprecedented outbreak of highly pathogenic avian influenza A(H5N1), the first documented infection in dairy cows, was reported on March 25, 2024 in Texas, Kansas, and New Mexico, raising the potential of cow-to-human transmission. Three confirmed human cases in the USA, all farm workers, have been reported so far, with the third person presenting with respiratory symptoms. Genetic analysis shows that the virus lacks changes that would make it better adapted to transmit between people and the risk to human health remains low, but it did detect adaptation to mammalian hosts. Despite calls for increased surveillance, the US response has been slow, with many cases likely going undetected. There has been resistance from the farming industry around testing and prevention, driven by a lack of awareness or understanding around changing practice and fear of trade restrictions and product loss. The US Department of Agriculture (USDA) and the US Centers for Disease Control and Prevention are incentivising dairy producers to supply personal protective equipment and offer laundering services for employees, but the USDA has been accused of being slow in sharing crucial virus sequence data, complicating our understanding of the outbreak. In Canada, early warning systems have been implemented to screen for H5N1 in milk. The UK says it has intensified its response to the US outbreak, but according to a report last month, it is not yet testing cows for the virus.
Action to curb this outbreak is needed urgently, including improving testing, surveillance, and reporting of infected animals and food products; vaccinating animal populations; transparent information sharing; developing and stockpiling human vaccine; and promoting protective measures among farm workers. All countries should build capacities to test, detect, and report infections, cases, and deaths above expected levels and share this information. Countries in need of external assistance to bolster their capacity should receive support.
The need for a robust and coordinated response to highly pathogenic avian influenza A(H5N1), as described in a Comment by Kojima and colleagues, serves to highlight the scale of the failure of countries to secure an international agreement on the pandemic accord. The 77th World Health Assembly, which closed on June 1, was meant to see a finalised draft of an international instrument on pandemic prevention, preparedness, and response. As The Lancet has noted, negotiations have been mired in disagreement and draft provisions have been unjust and inequitable (although changes to the International Health Regulations have been agreed). The process is not over yet—discussions continue with the hope of securing an agreement by May, 2025. Until then, the continued absence of a meaningful and effective accord undermines the ability to respond appropriately to international health threats, such as H5N1. Despite COVID-19, most countries are not prepared for a new pandemic.
Beyond the proximal drivers of outbreaks and potential interventions though, there is a need to confront an uncomfortable truth that the US H5N1 outbreak once again raises. Spillover of zoonoses into human populations stems ultimately from our ways of life and how they shape the human–animal interface. Our diets, our intensive farming practices, our livelihoods, our behaviours, and our cultures. Our exploitation of the natural world and our destruction of the environment. These issues are tractable through interdisciplinary education, intersectoral collaboration, adequate funding, and integrated policies. The concept of One Health, although often acknowledged, is rarely prioritised and operationalised. The result is a missed opportunity to not just respond to pandemic threats, but to prevent them altogether.