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handle is hein.crs/govesjc0001 and id is 1 raw text is: H5N1 Avian Influenza: The Human Health Response Since early 2022, a U.S. H5N1 avian influenza (bird flu) outbreak in poultry and wild birds has led to outbreaks in dairy herds. Increasing cases of transmission from animals to humans occurred in 2024. H5N1 influenza is a subtype of influenza A virus that causes highly pathogenic avian influenza, a form of influenza that is highly contagious and deadly to birds. The virus has adapted to spread among certain mammal species such as cattle and cats. Currently, H5N1 influenza is not known to spread easily from human to human. To date, most human H5N1 influenza cases have been associated with animal exposures, with very few cases due to human-to-human transmission. Yet many uncertainties persist around if and when H5N1 influenza virus might adapt to spread more easily among humans, potentially resulting in a pandemic. In this case, most people would not have prior immunity to H5N1 influenza virus, which means the virus could spread quickly. The virus's growing spread among animals creates more opportunities for a strain with human pandemic potential to emerge. Concerns about H5N1 influenza have long driven U.S. federal pandemic preparedness policy. Some of the first H5N1 influenza outbreaks in Asia in 2003-2004 led the Bush Administration and Congress to pursue policy initiatives that inform preparedness for today's situation. Therefore, the United States is prepared for H5N1 influenza in some ways that it was not for the Coronavirus Disease 2019 (COVID-19) pandemic. Most notably, there are U.S. Food and Drug Administration (FDA)-licensed vaccines and vaccine components stockpiled for H5N1 influenza. Still, there are many unknowns about H5N1 influenza and the threat it may potentially pose to humans. U.S. federal health agencies have been helping state, local, tribal, and territorial (SLTT) public health agencies respond to the current situation with a goal of preventing and controlling spread among humans. Federal agencies are also preparing for the possibility of the virus adapting to spread more easily among humans. For information on the dairy herd response, see CRS In Focus IF12837, H5N1 HPAI Continues to Spread in Dairy Herds. HItorca Cases and Outbreaks Since the first H5N1 influenza human cases in 1997, the virus has caused periodic illness in humans as it has circulated globally. These human cases were mostly associated with animal exposures, with some limited close human-to-human spread in household and hospital settings. Historically, H5N1 influenza has generated concern because of its high severe illness and death rate among reported human cases. Between November 2003 and May 2019 there was a 53% mortality rate among reported human cases. It is widely understood that this reported mortality rate may not reflect the true historical mortality rate in humans. These data tend to capture more severe cases, as February 4, 2025 many patients with mild illness are unlikely to seek testing; therefore, their cases go unreported. Still, some experts think that even with a true mortality rate of 5%, the virus would pose a major human health threat if it were to evolve the ability for human-to-human transmission. Current Human Heath Situaton The Centers for Disease Control and Prevention (CDC) states that the current public health risk to the general public is low, but is closely monitoring the situation for any changes. The risk is higher for those with regular exposure to infected animals. CDC has received reports of 67 confirmed cases in 10 states since 2024 (data as of January 17, 2025), most associated with agricultural animal exposure. Two of these cases have unknown exposure sources. There are also seven additional probable cases. The majority of reported U.S. cases have involved mild illness with symptoms such as conjunctivitis (eye redness and discharge) and some mild respiratory symptoms. One U.S. patient has died from H5N1 influenza. It is unclear if the relatively mild illness cases are due to (1) a circulating strain that causes milder disease than prior strains, (2) the amount of virus the cases were exposed to, or (3) early detection and treatment of illness, or other possible reasons. US. Pubic Health Response In the U.S. federalist system of government, SLTT governments have primary legal authority and responsibility for responding to human infectious disease threats in their respective jurisdictions. Federal agencies may assist SLTT government agencies and may help coordinate across states. Federal agencies also have legal authority to prevent introduction of an infectious disease threat into the United States or transmission across state lines. Federal pandemic planning has long recognized that the general public may look to the federal government for leadership during a major infectious disease event. SLTT governments have some infectious disease control laws, capabilities, and expertise, though the specifics vary among jurisdictions. Federal agencies assist SLTT public health agencies with infectious disease control through technical assistance, deployed staff and resources, and several ongoing grant programs focused on preparedness. For example, CDC's Public Health Emergency Preparedness cooperative agreement (a grant program) requires funded jurisdictions (all states, territories, and certain localities) to plan for a large-scale pandemic influenza response. At the federal level, four U.S. Department of Health and Human Services (HHS) agencies were involved in a response team for H5N1, as of May 2024: Administration for Strategic Preparedness and Response (ASPR): ASPR leads federal public health