United states: The most recent flu pandemic—2009’s H1N1 “swine flu”—was, in absolute terms, a public-health crisis. By scientists’ best estimates, roughly 200,000 to 300,000 people around the world died; countless more fell sick. Kids, younger adults, and pregnant people were hit especially hard.
However, it could have been far more catastrophic. Of the documented flu pandemics, 2009’s had the lowest mortality; the preceding H1N1 pandemic, which began in 1918, infected approximately 500 million people worldwide, resulting in at least 50 million deaths. Even some recent seasonal flus have killed more people than swine flu did.
With swine flu, “we got lucky,” Seema Lakdawala, a virologist at Emory University, told. H5N1 avian flu, which has been transmitting wildly among animals, has not yet spread in earnest among humans. Should that change, though, the world’s next flu pandemic might not afford the same break.
Swine flu caught scientists by surprise. At the time, many researchers were dead certain that an H5N1, erupting out of somewhere in Asia, would be the next Big Bad Flu. Their focus was on birds; hardly anyone was watching the pigs. But the virus, a descendant of the devastating flu strain that caused the 1918 pandemic, found its way into swine and rapidly gained the ability to hack into human airway cells. It was also great at traveling airborne—features that made it well positioned to wreak global havoc, Lakdawala said.
By the time experts caught on to swine flu’s true threat, “we were already seeing a ton of human cases,” Nahid Bhadelia, the founding director of the Boston University Center on Emerging Infectious Diseases, told. Researchers had to scramble to catch up. But testing was intermittent, and reporting of cases was inconsistent, making it difficult for scientists to get a handle on the virus’s spread. Months passed before the rollout of a new vaccine began, and uptake was meager. Even in well-resourced countries such as the US, few protections hindered the virus’s initial onslaught.
However, the direst outcomes never materialized—reasons for which remain elusive to experts. Undoubtedly, compared to the 1918 pandemic or those of the 1950s and ’60s, modern medicine was better equipped for flu testing and treatment; while vaccine uptake has never been flawless, the availability of vaccines provided increased overall protection, noted Sam Scarpino, an infectious-disease modeler and director of AI and life sciences at Northeastern University.
Subtler influences might have also contributed. Other H1N1 viruses have circulated globally since the late 1970s, potentially bestowing a degree of immunity on much of the population, explained Troy Sutton, a virologist at Pennsylvania State University. Particularly older individuals might have had additional defenses due to exposure to H1N1 strains from the early 20th century. (Post-1918 pandemic, variants of that virus lingered and continued to permeate the population for decades.) These extra safeguards might elucidate why younger people were severely impacted in 2009, according to Lakdawala.
Similar factors might influence an H5N1 epidemic. However, 2009 serves as an imperfect model—particularly given the many uncertainties surrounding this new avian flu. Genuine human-to-human transmission of H5N1 remains a remote possibility: the virus would likely need significant evolutionary modifications to its genome, possibly transforming into an almost unrecognizable form. This complicates any forecasts regarding how a future outbreak might develop.
Nevertheless, experts are monitoring several factors that could heighten H5N1’s risks. For example, no variants of H5N1 flu have established a persistent presence in humans, implying “there’s very little immunity in the community,” according to Michael Osterholm, the director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
Exposure to other flu strains might provide limited defense. Lakdawala and Sutton have conducted experiments with ferrets, which contract and spread flu similarly to humans. Preliminary findings indicate that ferrets with prior exposure to seasonal-flu strains suffer milder illness when infected with this specific H5N1. However, ferrets without prior flu exposure—which would be akin to very young children—fare worse, more severely than with the 2009 H1N1, which Lakdawala described as “scary.”
It’s too early to say how those results would translate into people, for whom data are sparse. Since this H5N1 virus was first detected in the 1990s, scientists have recorded hundreds of human cases, nearly half of whom have died. But experts caution strongly against reading too much into those stats: No one can be certain how many people the virus has actually infected, making it impossible to estimate a true fatality rate. The virus has also shape-shifted over decades—and the versions of it that killed those people did not seem capable of spreading among them. As Sutton pointed out, past experiments suggest that the mutations that