H5N1 infections rare among bird-exposed workers despite intense contact

Between April 2023 and March 2025, researchers in England tracked 428 workers who had direct contact with birds infected by highly pathogenic avian influenza H5N1. These were people on the front lines of outbreak response—cullers, farm workers, contractors, and animal health officials moving through contaminated premises, handling dead birds, touching infected tissue, breathing the same air as the virus. What they found was reassuring but incomplete: despite the intensity of that exposure, fewer than one in a hundred workers actually became infected.

The study, published in Eurosurveillance, examined a moment when the virus was circulating in a particularly dangerous form. The 2.3.4.4b clade of H5N1 had begun infecting humans in 2024, first in dairy cattle workers in the United States, then spreading elsewhere. In the UK alone, the 2024-25 avian influenza season saw 81 separate outbreaks. The concern was not academic: historically, when H5N1 jumps to humans, it kills roughly six in ten infected people. The fear was that continued circulation and mutation could eventually produce a strain capable of spreading from person to person—the threshold of pandemic.

Of the 862 workers initially exposed at 34 infected premises across England, 428 agreed to participate. Most worked at poultry sites; one worked at an animal rescue facility. The median age was 37. Nearly seven in ten were men. Half were employed by the Animal and Plant Health Agency, the government's outbreak response arm; the rest were contractors. They were asked to provide nasal swabs on day zero and then repeatedly over the following week, and researchers tested those samples for the presence of H5N1 genetic material.

The protective measures in place varied. Eighty-five percent of participants reported wearing complete personal protective equipment—full suits, respirators, gloves—without any breaches while inside the biosecurity zone. But compliance fractured when the work began. During the actual culling operations, 98 percent of government and contractor staff wore full PPE, but only 27 percent of farmers and farm workers did. The exposure itself was nearly universal: 86 percent worked within two meters of infected birds, 81 percent touched the birds or their tissues directly, and 62 percent handled bird waste. Contractors were most likely to have direct hand contact with infected animals; farmers were most likely to touch contaminated litter and eggs.

Six participants tested positive for influenza A(H5) by PCR. But positive detection and actual infection are not the same thing. Three of the six met the study's criteria for confirmed or probable H5N1 infection—two classified as confirmed cases, one as unclear due to timing issues with the sample. The other three appeared to be transient mucosal contamination: the virus had touched the inside of their nose or throat but had not established itself. Two of the six PCR-positive individuals reported mild symptoms. None developed severe illness. None required hospitalization. All six had touched infected birds directly; four had also handled bird waste. Notably, two of the six had not worn full PPE during their exposure.

The protective equipment made a measurable difference. Among workers who reported full PPE use, one percent tested positive. Among those who did not wear full protection, three percent tested positive. Yet the presence of positive tests even among those reporting complete PPE suggested something more complicated was happening—perhaps the equipment was not being put on or removed correctly, perhaps there were breaches workers did not recognize, perhaps the conditions of the work itself made perfect protection impossible. The study's authors noted that culling operations involve long hours in extreme conditions; maintaining flawless technique throughout is not realistic.

No one who tested positive had been vaccinated against seasonal influenza or taken antiviral medication beforehand. Among those who tested negative, 20 percent had received the seasonal flu shot and three percent were taking or had recently taken antivirals as preventive medication. Most striking: no secondary transmission occurred. Contact tracing of the six positive individuals found no evidence that any of them had passed the virus to family members, colleagues, or anyone else. The virus stopped with them.

The findings offer a narrow reassurance. Under the conditions studied—with established outbreak protocols, biosecurity measures, and access to PPE—the risk of severe H5N1 infection in occupationally exposed workers was low. But the study also exposed gaps. Vaccination rates among exposed workers were minimal. The study was smaller than originally planned due to resource constraints and recruitment difficulties. Some workers faced financial hardship if they tested positive and had to isolate, creating a perverse incentive not to participate. As the virus continues to circulate and evolve, maintaining these protections and closing these gaps remains essential.