The highly pathogenic avian influenza virus A (HPAI) H5N1 was first identified in a farmed goose in 1996 from China (1). High pathogenic avian influenza viruses cause severe disease in poultry and represent a smaller proportion of avian influenzas (2). In 2008, the H5Nx acquired the function to reassort its neuraminidase (N) and created the 2.3.4.4 H5Nx clade. Initial outbreaks of 2.3.4.4b began in H5N8 and H5N6 in 2016 (3). However, in 2020, whole genome sequencing (WGS) conducted in the Netherlands detected a new H5N1 clade, 2.3.4.4b in wild birds along the Adriatic flyway (4), which reassorted from H5N8 2.3.4.4b clade (3).

Currently, the 2.3.4.4b HPAI H5N1 outbreak is the dominant circulating strain in the panzootic outbreak. This clade has been responsible for large outbreaks within avian species and spill over into human cases has occurred (5-7). The earliest reported human case was in 2021 (5-7).

Highly pathogenic avian influenza H5N1 clade 2.3.4.4b

The first detected human case of H5N1 clade 2.3.4.4b was within the United Kingdom in 2021 (5-7) following a large epizootic outbreak within the country in both wild and domestic birds.

Zoonotic: Host species range between domestic and wild avian species, with an increase in wild outbreaks in waterfowl.

The first human case was detected in 2021 (5-7) following the detection in birds in 2020 (3).

Ongoing

H5N1 has been reported throughout the globe in both domestic and wild birds in every continent except Oceania and Antarctica (8). Since the avian (HPAI) H5N1 outbreak in 2020, the reported human cases of this clade have been within the United Kingdom (UK), the United States (US), China, Spain, Vietnam, Ecuador, and Chile (9).

There have been 11 human 2.3.4.4b H5N1 cases since the initial reports in 2021 (10).

No difference in clinical features have been identified between 2.3.4.4b and prior clades in regard to human infection. However, early clinical manifestations of human H5N1 infection can include (11, 12):

The estimated incubation period of all clades of H5N1 in humans is typically from two to five days, though longer durations have been recorded (13). The dominant mode of transmission to humans is through the unprotected mucous membrane exposed to saliva, mucus, or faeces from birds infected with the H5N1 virus, or via inhalation or contact with infected surfaces (14). H5N1 can persist on surfaces 2.5 times longer compared to other avian influenzas and is able to survive on plastic surfaces and skin for approximately 26 and 4.5 hours respectively, while also being resistant to ethanol (14, 15).

Demographic and disease details for the twelve reported human cases are provided in Table 1.

The case fatality rate (CFR) for the 2.3.4.4b cases is 9% (n=1) (27). However, the overall CFR since 2003 for all H5N1 human cases, regardless of clade, is 52% (n=458) (27).

Severe cases of H5N1 human infection may present with respiratory complications such as dyspnoea and tachypnoea (11). Infection can progress rapidly to pneumonia, pneumothorax, and acute respiratory distress syndrome (ARDS), which can lead to respiratory failure and death (11, 28, 29). Extrapulmonary complications may also occur, including encephalitis, renal disease, heart failure, multiorgan failure, and disseminated intravascular coagulation (11).

A 2006 study of 65 H5N1 human cases found that 41 (63%) cases required advanced organ support, with over half (54%) of such cases developing ARDS (28). However, it should be noted that complication rates in such cases may represent only severe disease, as asymptomatic or mild infections may not be reported (28).

The 2.3.4.4b case in Chile exhibited two mutations adaptations of concern, both within the PB2 gene, PB2-D701N and PB2-Q591K. The PB2-D701N substitution has been previously seen in both human H5N1 and H5N6 mutations and has been linked to increased severity (30, 31). The PB2-Q591K mutation has increased viral replication in mammals (32). These adaptations were not seen within local poultry or in the mammalian outbreaks surrounding the region, which left researchers to assume these mutations most likely came from host adaptations after the infection.

Most recently, a study(33) was output studying the pathogenicity and transmission on the new lineage in North America using two strains of 2.3.4.4b in a wigeon from 2021 and a bald eagle from 2022. In addition, two animal models were used for virus transmission, ferrets and leghorn chickens. Of the sequenced, they identified four distinct viral genotypes, which displayed variations in their nucleoprotein (NP) and polymerase gene combinations, which originated from either North America or Eurasia (33). Results revealed that reassortants containing increasing numbers of North American gene segments exhibited enhanced virulence with neurological involvement in mammalian models (33) and the lineage has a tendency to reassort and target the brain and spinal cord (33). The findings emphasize the rapid dynamics of a natural system and indicate the growing potential for additional reassortment and phenotypic variations. This risk is amplified by the unprecedented global spread of these viruses.

For this clade and all other H5N1 clades, the most effective prevention method for humans is to avoid direct contact with birds (34). The United States Center for Disease Control (CDC) (31) and World Organization for Animal Health (WOAH) (35) recommends that birds should be observed from a distance where possible, as infected birds may be asymptomatic. Individuals who have direct contact with birds (e.g., those working or living on a farm, hunters, bird hobbyists, etc.) should use personal protective equipment (PPE), including gloves and a mask (a N95 or a fitted surgical mask is recommended) (34, 35). Bodily fluids such as saliva, mucous, and faeces are often highly contaminated with virus, and thus should be avoided or handled using PPE (34, 35). Individuals who are exposed to infected birds for extended periods of time, in close contact and without protection, are most at risk of infection. If individuals are in direct contact with birds or their bodily fluids, it is very important that they wash their hands with soap and water prior to touching their mouth, eyes, or nose (34). Any potentially contaminated clothing should be changed and there should be efforts to prevent infection of healthy domestic birds by wild birds (34, 35).

Several H5N1 vaccine candidates for both humans and avians have been approved by the US Food and Drug Administration (FDA) (36). For human vaccination, two candidate vaccine viruses (CVVs) have been created in the United States, which use a strain closely related to clade 2.3.4.4b (A/Astrakhan/3212/2020-like), CDC (i.e., IDCDC-RG71A) and U.S. FDA (CBER-RG8A) (34). Unfortunately, no identical clade-specific vaccines have been created and the manufacturing process is often time-consuming as given vaccines are produced by incubating each dose in an egg, limiting their effectiveness in an emergency situation (36).

Lastly, control and prevention for poultry is needed to reduce transmission in avians and to humans. Poultry vaccination is still slowly being accepted globally as a preventative measure but there are concerns that poultry vaccination does not stop transmission but rather masks symptoms (37). Many countries share that concern and have long-standing trade restrictions/bans on poultry who have been vaccinated. The European Union ‘harmonised’ these laws by allowing vaccinated poultry to be traded (37). The United States, Australia, and a few European countries still do not authorise the use of vaccination and opt for culling techniques (37). Despite these concerns, countries have previously had large success from mass vaccination of poultry (38).

The treatment for all clades of H5N1 human infection is the same, and the key recommendation is to commence antiviral treatment as soon as possible (34). The current recommendation is to use neuraminidase inhibitors, such as oseltamivir, peramivir, and zanamivir, which are effective against most avian influenzas and have been shown to reduce viral replication and improve patient outcomes (39-41). However, some evidence of antiviral resistance has been found in lineages of H5N1 (29, 42) so continual monitoring for antiviral resistance is imperative. Oseltamivir resistance has also been reported (41).

Treatment with antivirals should last at least 5 days and can continue if there has been little clinical response. Corticosteroids should not be administered routinely unless clinically indicated. Recent H5N1 viruses have been shown to be resistant to adamantane antiviral drugs, including amantadine and rimantadine, therefore these should not be administered as the only course of treatment (41).

Since 2003, a total of 874 human cases of H5N1 has been reported across 23 countries (27). Large human outbreaks have simultaneously occurred with a rise in avian H5N1 cases, such as in Egypt during the winter season of 2014-2015. Similar to the current outbreak, cases in both wild and domestic birds were incredibly high and H5N1 was declared enzootic in 2008 (43). Inadequate control measures led to genetic drift in the hemagglutinin (HA) gene (43). Additional mutations found within the human cases were novel or rare within Egypt (43).

No human-to-human transmission has been identified but there have been examples of probable non-sustained outbreaks in Thailand in 2004 (44), Indonesia in 2006 (45), China in 2007 (46) and Pakistan in 2007 (47).The most concerning was the 2006 North Sumatra, Indonesia cluster, which consisted of eight cases across an extended family (45, 48). The index case had exposure to poultry and came into contact with 20 extended family members, some from different households (45, 48). Of the eight cases, seven lived in three adjacent houses but one lived approximately 10 kilometres away (45, 48). The secondary attack rate for this outbreak was estimated to be 29% (45), and could have been more detrimental if occurring in a less remote village.

Currently, there has not been an outbreak between humans with this clade with all of the 2.3.4.4b cases having linkages to poultry. However, as cases increase in more avian species and more spill over occurs, the likelihood of genetic mutations to adapt to mammals will increase.

Since the virus attaches to upper respiratory tract receptors that are more prevalent in birds than mammals, H5N1 tends to not affect mammals. However, on this occasion, several mammalian species, such as foxes, cats, ferrets, seals, and dolphins, have contracted 2.3.4.4b, presumably through contact with infected birds. Recently, two outbreaks of concern in mammals have arisen, with potential mammal-to-mammal transmission in minks and sea lions both from the clade of 2.3.4.4b.

In October 2022, workers at a mink farm in Spain observed a rise in the mortality rate of the minks from a baseline of 0.25% per week to 0.77%, with minks testing positive for H5N1 (49). In subsequent weeks, the disease seemed to spread from two to four pens, where all the animals became infected and died. Unfortunately, the situation worsened, even with restrictions, and more animals fell sick, forcing the workers to cull all 51,986 minks on the farm. Genetic sequencing revealed a PB2 gene mutation (T217A) (50), which has public health implications as it is present in the avian-like PB2 gene of the 2009 H1N1 pandemic flu virus and has characteristics that enable recognition by human airway receptor cells. However, no humans working in the farm or conducting culling tested positive (49). Human transmission, however, remains a concern.

An additional example of potential mammal-to-mammal transmission was recently observed in Peru with infected sea lions. Initial reports arose in November 2022 of deaths in three sea lions and one dolphin which tested positive for H5N1 after reports of a mass outbreak in birds in the region (51). By February 2023, the outbreak had spread to 585 sea lion reported deaths and more than 55,000 wild birds (52). In early March 2023, that number again grew to 3,487 reported sea lion deaths in seven protected regions along the Peruvian coastline (53). By the end of March, the sea lion deaths had spread to Chile with approximately 500 seals reported as dead from 2.3.4.4b (54). In less than two weeks, the death count had increased to 1,535 sea lions and by 14 April 2023 (54), more than 3300 seals had reportedly tested positive for 2.3.4.4b (55). In the month of May the death toll had increased to 5860 sea lion deaths encompassing 11 regions (56) in mid-May and, most recently, more than 8000 sea lions have died in 13 regions with 44 positive samples (57) taken. One region, previously unaffected, Aysén (57), which is incredibly south was registered this past week. The true number of affected mammals is said to be higher while Servicio Nacional de Pesca y Acuicultura (SERNAPESCA) are investigating. With a large concentration of mammal cases and a strain lineage that has a tendency to reassort, there is a greater likelihood for mutation of the PB2 gene, which is one of the requirements for human-to-human transmission to occur (58).

In order to prevent human cases, countries with large avian outbreaks and robust surveillance systems have begun implementing routine asymptomatic testing of those participating in culling and those exposed to wild bird cases, such as the United States (59) and European countries (60, 61).

The number of global avian H5N1 cases, including those affecting both wild and domestic birds, has been rapidly increasing. With large avian H5N1 case numbers, spill over has begun into mammals, including humans. While human-to-human transmission has yet to be identified, there have been probable non-sustained outbreaks that have occurred in areas of high avian transmission.

According to genetic analysis conducted on the viruses found in European birds affected during the winter season, H5N1 appears to have a higher affinity for avian receptors (58). However, certain markers have been identified in the PB2 protein of the viruses found in mammals that are associated with increased virulence and replication in these animals (58). These changes were rarely observed before 2020 and likely emerged after transmission to mammals, posing potential public health concerns (58).

While most H5N1 cases in mammals are found in animals that hunt or scavenge infected birds or dead animals, there are two above mentioned unusual events in a Spanish mink farm and in Peruvian sea lions. The occurrence of these events, along with mutations that make the virus more adaptable to mammals, and serologic evidence of infection in wild boar and pigs (58), are worrisome and warrant close monitoring. The occurrence of asymptomatic infection is still debated as positive cases may be due to contamination of the nasal mucosa rather than active infection (62). However, with the rise of panzootic outbreaks and their subsequent spill over to mammals, it is important to maintain adequate close monitoring on all potential human exposures to environmental contamination. Early monitoring systems, like EPIWATCH, can identify potential hot spots for earlier control measures to be implemented. Official government reporting is often delayed due to many factors including, validation of notifications, case confirmation and sequencing. With countries experiencing large outbreaks, many report avian cases in aggregate form, which makes identifying individual source outbreaks to be difficult and time consuming. Open-source intelligence, such as EPIWATCH, can aid in timeliness of reporting, which is often times needed for rapid surveillance and prevention methods to be implemented. Birds have been found to be natural hosts for a vast range of avian influenza subtypes (63). Enhanced monitoring and surveillance systems are therefore extremely important given the ability of the virus to reassort, heightening its pandemic potential (63).

As the global spread increases, countries are looking into new prevention and control methods. Vaccination has been considered as an additional tool to control the spread of the disease. While some countries have been vaccinating flocks for some time, others do not vaccinate due to the potential trade implications (8). Recently, the United States changed their stance and has begun vaccinating endangered species, such as the condor (64). It is essential to recognize the amplified risk posed by the unprecedented global spread of this lineage and the emphasises for continual vigilance and research to mitigate the potential impact of these evolving 2.3.4.4b strains.

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Funding was provided by The Balvi Filantropic Fund.

1. Organization WH. H5N1 highly pathogenic avian influenza: Timeline of major events. 2013.

2. Center for Disease Control. Influenza Type A Viruses 2022 [Available from: https://www.cdc.gov/flu/avianflu/influenza-a-virus-subtypes.htm.

3. Xie R, Edwards KM, Wille M, Wei X, Wong S-S, Zanin M, et al. The episodic resurgence of highly pathogenic avian influenza H5 virus. bioRxiv. 2022:2022.12.18.520670.

4. El-Shesheny R, Moatasim Y, Mahmoud SH, Song Y, El Taweel A, Gomaa M, et al. Highly Pathogenic Avian Influenza A(H5N1) Virus Clade 2.3.4.4b in Wild Birds and Live Bird Markets, Egypt. Pathogens. 2022;12(1).

5. Oliver I, Roberts J, Brown CS, Byrne AM, Mellon D, Hansen R, et al. A case of avian influenza A(H5N1) in England, January 2022. Euro Surveill. 2022;27(5).

6. Grierson J. First UK person to catch H5N1 bird flu strain is named. The Guardian. 2022.

7. World Health Organization. Assessment of risk associated with recent influenza A(H5N1) clade 2.3.4.4b viruses World Health Organization; 2022.

8. Wille M, Barr I, Klaassen M. Bird flu, human cases and the risk to Australia. Doherty Institute; 2023.

9. Prevention CfDCa. Past Reported Global Human Cases with Highly Pathogenic Avian Influenza A(H5N1) (HPAI H5N1) by Country, 1997-2023 2023 [updated 2023. Available from: https://www.cdc.gov/flu/avianflu/chart-epi-curve-ah5n1.html.

10. Center for Disease Control. Past Reported Global Human Cases with Highly Pathogenic Avian Influenza A(H5N1) (HPAI H5N1) by Country, 1997-2023 2023 [Available from: https://www.cdc.gov/flu/avianflu/chart-epi-curve-ah5n1.html.

11. Uyeki TM. Human Infection with Highly Pathogenic Avian Influenza A (H5N1) Virus: Review of Clinical Issues. Clinical Infectious Diseases. 2009;49(2):279-90.

12. World Health Organization. Influenza: H5N1. 2012.

13. Uyeki TM. Human infection with highly pathogenic avian influenza A (H5N1) virus: review of clinical issues. Clin Infect Dis. 2009;49(2):279-90.

14. Center for Disease Control. Transmission of Avian Influenza A Viruses Between Animals and People. 2022.

15. Bandou R, Hirose R, Nakaya T, Miyazaki H, Watanabe N, Yoshida T, et al. Higher Viral Stability and Ethanol Resistance of Avian Influenza A(H5N1) Virus on Human Skin. Emerg Infect Dis. 2022;28(3):639-49.

16. World Health Organization. Avian Influenza A (H5N1) - United States of America. 2022.

17. Pitt D. First new U.S. case of human bird flu confirmed in Colorado prison. PBS News Hour. 2022.

18. World Health Organization. Avian Influenza Weekly Update Number 872 2022.

19. World Health Organization. Disease Outbreak News; Avian Influenza A (H5N1) – Spain. 2022.

20. Wappes J. Vietnam reports first avian flu infection in 8 years. CIDRAP. 2022.

21. World Health Organization. Human infection caused by avian influenza A(H5) - Ecuador. 2023.

22. Ecuador: first case of avian influenza in a nine-year-old girl: Poultry Med; 2023 [updated 2023. Available from: https://www.poultrymed.com/Poultrymed/Templates/showpage.asp?DBID=1&LNGID=1&TMID=178&FID=5008&PID=0&IID=83089.

23. China reports new H5N1 avian flu case [press release]. 2023.

24. World Health Organization. Human infection caused by Avian Influenza A (H5) - Chile. 2023.

25. United Kingdom Health Security Agency. Bird flu (avian influenza): latest situation in England 2023 [Available from: https://www.gov.uk/government/news/bird-flu-avian-influenza-latest-situation-in-england#:~:text=Highly%20pathogenic%20avian%20influenza%20(%20HPAI%20)%20H5N1%20was%20confirmed%20in%20commercial,surveillance%20zone%20has%20been%20revoked.

26. World Health Organization. Avian Influenza A(H5N1) - United Kingdom of Great Britain and Northern Ireland. 2023.

27. World Health Organization. Highly pathogenic avian influenza A (H5N1) virus infection in farmed minks, Spain, October 2022. 2022.

28. Gruber PC, Gomersall CD, Joynt GM. Avian influenza (H5N1): implications for intensive care. Intensive Care Med. 2006;32(6):823-9.

29. Earhart KC, Elsayed NM, Saad MD, Gubareva LV, Nayel A, Deyde VM, et al. Oseltamivir resistance mutation N294S in human influenza A(H5N1) virus in Egypt. Journal of Infection and Public Health. 2009;2(2):74-80.

30. Liu S, Zhu W, Feng Z, Gao R, Guo J, Li X, et al. Substitution of D701N in the PB2 protein could enhance the viral replication and pathogenicity of Eurasian avian-like H1N1 swine influenza viruses. Emerging Microbes & Infections. 2018;7(1):1-10.

31. Nieto A, Pozo F, Vidal-García M, Omeñaca M, Casas I, Falcón A. Identification of Rare PB2-D701N Mutation from a Patient with Severe Influenza: Contribution of the PB2-D701N Mutation to the Pathogenicity of Human Influenza. Front Microbiol. 2017;8:575.

32. Wang C, Lee HH, Yang ZF, Mok CK, Zhang Z. PB2-Q591K Mutation Determines the Pathogenicity of Avian H9N2 Influenza Viruses for Mammalian Species. PLoS One. 2016;11(9):e0162163.

33. Kandeil A, Patton C, Jones JC, Jeevan T, Harrington WN, Trifkovic S, et al. Rapid evolution of A(H5N1) influenza viruses after intercontinental spread to North America. Nature Communications. 2023;14(1):3082.

34. Center for Disease Control. Prevention and Antiviral Treamtnet of Bird Flu Viruses in People 2022. [updated 2022 Oct 31]. [Cited 2022 Mar 24]. Available from: https://www.cdc.gov/flu/avianflu/prevention.htm#contact-infected-birds-become-sick.

35. Health WOoA. Prevention and Control 2023 [Available from: https://www.woah.org/en/disease/avian-influenza/#ui-id-3.

36. Joi P. Why bird flu vaccines need urgent R&D [Internet]. Gavi; [updated 2023 February 15]. [Cited 2023 Mar 24]. Available from: https://www.gavi.org/vaccineswork/why-bird-flu-vaccines-need-urgent-rd.

37. Stokstad E. Wrestling with bird flu, Europe considers once-taboo vaccines. Science. 2022.

38. Swayne DE, Spackman E, Pantin-Jackwood M. Success factors for avian influenza vaccine use in poultry and potential impact at the wild bird-agricultural interface. Ecohealth. 2014;11(1):94-108.

39. Center for Disease Control. Treatment of Avian Influenza A Viruses in People. 2018.

40. Smith JR. Oseltamivir in human avian influenza infection. J Antimicrob Chemother. 2010;65 Suppl 2(Suppl 2):ii25-ii33.

41. WHO. Influenza (Avian and other zoonotic) [Fact sheet]. [updated 2018 Nov 13]. [Cited 2023 Mar 24]. Available from: https://www.who.int/news-room/fact-sheets/detail/influenza-(avian-and-other-zoonotic).

42. Govorkova EA, Baranovich T, Seiler P, Armstrong J, Burnham A, Guan Y, et al. Antiviral resistance among highly pathogenic influenza A (H5N1) viruses isolated worldwide in 2002-2012 shows need for continued monitoring. Antiviral Res. 2013;98(2):297-304.

43. Kayali G, Kandeil A, El-Shesheny R, Kayed AS, Maatouq AM, Cai Z, et al. Avian influenza A (H5N1) virus in Egypt. Emerging infectious diseases. 2016;22(3):379.

44. Ungchusak K, Auewarakul P, Dowell SF, Kitphati R, Auwanit W, Puthavathana P, et al. Probable Person-to-Person Transmission of Avian Influenza A (H5N1). New England Journal of Medicine. 2005;352(4):333-40.

45. Yang Y, Halloran ME, Sugimoto JD, Longini IM, Jr. Detecting human-to-human transmission of avian influenza A (H5N1). Emerg Infect Dis. 2007;13(9):1348-53.

46. Wang H, Feng Z, Shu Y, Yu H, Zhou L, Zu R, et al. Probable limited person-to-person transmission of highly pathogenic avian influenza A (H5N1) virus in China. The Lancet. 2008;371(9622):1427-34.

47. Human cases of avian influenza A (H5N1) in North-West Frontier Province, Pakistan, October-November 2007. Wkly Epidemiol Rec. 2008;83(40):359-64.

48. Butler D. Family tragedy spotlights flu mutations. Nature. 2006;442(7099):114-6.

49. Agüero M, Monne I, Sánchez A, Zecchin B, Fusaro A, Ruano MJ, et al. Highly pathogenic avian influenza A(H5N1) virus infection in farmed minks, Spain, October 2022. Eurosurveillance. 2023;28(3):2300001.

50. KUPFERSCHMIDT K. ‘Incredibly concerning’: Bird flu outbreak at Spanish mink farm triggers pandemic fears. Science. 2023.

51. Leguia M, Garcia-Glaessner A, Muñoz-Saavedra B, Juarez D, Barrera P, Calvo-Mac C, et al. Highly pathogenic avian influenza A (H5N1) in marine mammals and seabirds in Peru. bioRxiv. 2023:2023.03.03.531008.

52. News Desk. Peru: At least 585 sea lions and about 55,000 birds have died from avian flu in protected natural areas. Outbreak News Today. 2023.

53. News Desk. Peru reports death of nearly 3,500 sea lions due to H5N1 bird flu. Outbreak News Today. 2023.

54. BNO News. More than 1,500 sea lions in Chile die of H5N1 bird flu. BNO News. 2023.

55. Robles C. More than 3,300 sea lions in Chile die of H5N1 bird flu. BNO. 2023.

56. Sernapesca. Sernapesca reports the death of more than seven thousand marine animals killed by bird flu throughout the country. 2023.

57. Sernapesca. Sernapesca confirms the first case of Avian Influenza in marine mammals in the Aysén region. 2023.

58. Authority EFS, Prevention ECfD, Control, Influenza EURLfA, Adlhoch C, Fusaro A, et al. Avian influenza overview December 2022–March 2023. EFSA Journal. 2023;21(3):e07917.

59. Centers for Disease Control and Prevention. Interim Guidance on Testing and Specimen Collection for Patients with Suspected Infection with Novel Influenza A Viruses with the Potential to Cause Severe Disease in Humans 2022.

60. United Kingdom Health Security Agency. UKHSA’s asymptomatic avian influenza surveillance programme 2023 [Available from: https://ukhsa.blog.gov.uk/2023/06/06/ukhsas-asymptomatic-avian-influenza-surveillance-programme/.

61. Aznar E, Casas I, González Praetorius A, Ruano Ramos MJ, Pozo F, Sierra Moros MJ, et al. Influenza A(H5N1) detection in two asymptomatic poultry farm workers in Spain, September to October 2022: suspected environmental contamination. Eurosurveillance. 2023;28(8):2300107.

62. Chen X, Wang W, Wang Y, Lai S, Yang J, Cowling BJ, et al. Serological evidence of human infections with highly pathogenic avian influenza A(H5N1) virus: a systematic review and meta-analysis. BMC Medicine. 2020;18(1):377.

63. Huang J, Li K, Xiao S, Hu J, Yin Y, Zhang J, et al. Global epidemiology of animal influenza infections with explicit virus subtypes until 2016: A spatio-temporal descriptive analysis. One Health. 2023;16:100514.

64. Kozlov M. US will vaccinate birds against avian flu for first time — what researchers think. Nature. 2023.