Study shows why today’s H5N1 flu cases are less serious than past epidemics

In a recent study published in the journal researchers compared viral replication and immune response in human lung organoids (hLO) infected with highly pathogenic avian influenza A (HPAI) viruses of the H5N1 subtype.

In North America, HPAI H5N1 clade 2.3.4.4b viruses have been circulating in birds since 2021 and have also been detected in mammalian species, including atypical hosts such as domestic dairy cattle.

In 2024, HPAI H5N1 clade 2.3.4.4b was detected in cattle and spread to herds in 16 states across the United States (US). The expanded host range and transmissibility of clade 2.3.4.4b viruses have raised critical concerns about their transmission to humans.

As of January 6, 2025, there have been 66 confirmed cases of HPAI H5N1 infection in the United States, many of which have been linked to cattle exposure. Nevertheless, recent outbreaks have led to the detection of human cases associated with poultry.

Moreover, the virus isolated from a Texas dairy farm worker was closely related to cattle viruses, suggesting that it was likely a consequence of direct transmission of the virus from cows to humans. Reported symptoms in these cases ranged from mild respiratory illness to conjunctivitis, and only one fatality was reported, a stark contrast to the 50% mortality rate associated with historical HPAI H5N1 infections.

Investigation and findings

In the present study, researchers assessed viral replication, immune response, and cell survival in human alveolar epithelium infected with historical and modern HPAI H5N1 viruses.

First, alveolar type 2 (AT2) cells from adult stem cell-derived hLOs and induced pluripotent human stem cell-derived lung organoids (ihLO) were infected with three HPAI H5N1 isolates.

These included contemporary isolates from cattle (A/bovine/Ohio/B24OSU-342/2024) and humans (A/Texas/37/2024) and a historical human isolate (A/Vietnam/1203/2004) from a fatal case in 2004.

The team found that the historical isolate replicated higher titers in hLO and ihLO compared to bovine isolates. However, the human isolate from Texas showed increased replication capacity compared to the bovine isolate, likely due to the presence of PB2 E627K mutationwhich has been linked to increased replication in mammalian hosts.

The team then quantified cell death in the lung organoids. Organoids infected with the historical isolate had earlier cell death; infection with other isolates also resulted in cell death, but all three isolates showed similar levels of cell death at 96 h after inoculation, suggesting that external factors may influence pathogenicity in vivo.

Additionally, they quantified the induction of interferon (IFN)-stimulated genes, such as ISG15 and ISG20, and pro-inflammatory cytokines: IFN-β, tumor necrosis factor (TNF)-α, interleukin 6 (IL-6), and IL-1β.

The highest ISG induction was detected in organoids infected with the historical isolate, which was most pronounced in hLO. In contrast, contemporary isolates appeared to suppress ISG responses, particularly in hLO, despite detectable viral replication. This suppression of the ISG response in hLO suggests an adaptation of modern isolates to counteract the human interferon system.

Proinflammatory cytokines showed different patterns: ihLO infected with contemporary isolates and hLO infected with cattle or a historical isolate showed the strongest induction. These differences in immune system activation may partially explain the reduced disease severity observed in modern infections.

Conclusions

In summary, this study assessed viral replication, immune response, and cell survival in human alveolar epithelium infected with HPAI H5N1 virus isolates.

Modern virus isolates showed reduced replication in lung organoids compared to the historical isolate, which convincingly explains why recent cases of human influenza caused by clade 2.3.4.4b viruses resulted in mild disease.

Furthermore, the ability of modern isolates to counteract the human IFN system may contribute to the reduction in disease severity seen with clade 2.3.4.4b viruses. The historical isolate induced significantly higher ISG induction, whereas modern isolates induced reduced induction despite detectable viral replication.

Overall, HPAI H5N1 clade 2.3.4.4b viruses currently circulating in cattle and other mammals appear to cause less severe disease in humans than historical HPAI viruses; however, they should be continuously monitored for changes affecting their transmissibility and pathogenicity.

Chronological review of significant global HPAI outbreaks

1878: First description of bird flu as a “poultry plague” in northern Italy, identified as an infectious disease causing high mortality in poultry.

1959: First known outbreak of H5N1 virus in Scotland affecting chickens.

1997: H5N1 first infects people in Hong Kong, causing 18 infections and 6 deaths; To control the epidemic, approximately 1.3 million chickens are being slaughtered.

2003: Re-emergence of H5N1 virus in humans, cases reported in China and widespread outbreaks in poultry in several Asian countries.

2004: H5N1 is spreading to additional Asian countries, leading to significant outbreaks in poultry and human infections, with significant cases in Vietnam and Thailand.

2005: H5N1 detected in migratory birds at Qinghai Lake in China; the virus spread to Europe, the Middle East and Africa through bird migration.

2006: H5N1 reaches India, North Africa and Europe, causing epidemics in wild bird and domestic poultry populations.

2007: Significant outbreaks of the H5N1 virus are occurring in countries such as Japan, the United Kingdom and Saudi Arabia, affecting both poultry and wild birds.

2008–2019: Multiple HPAI outbreaks worldwide, with different subtypes (e.g. H5N8, H7N9), causing infections in birds and sporadic cases in humans, particularly in Asia.

2020–2024: H5N1 clade 2.3.4.4b becomes dominant, causing widespread outbreaks among wild birds and poultry in Asia, Europe, Africa and the Americas; human cases are still rare but are being closely monitored.