Key points
• The overall objective of continual global surveillance for human infection with avian influenza A(H5) viruses is to detect and characterize any influenza A(H5) viruses infecting humans in order to:
- (1) promptly trigger public health control and response actions,
- (2) assess the trends of such infections and the public health risks posed (including the risk of a pandemic); and
- (3) inform global pandemic preparedness activities.
• Specific surveillance objectives include rapidly detecting human cases of influenza A(H5) virus infection, monitoring the incidence of new cases over time and geographical distribution, assessing and monitoring changes in transmission patterns to promptly detect any unusual events that may signal human-to-human transmission of the virus, characterizing and monitoring changes in any influenza A(H5) viruses infecting humans relative to those circulating in animals to inform control strategies, describing the clinical presentation of illness and identifying risk factors for infection and severe outcomes.
• Close collaboration with the animal health and environment sectors is essential to understand the extent of the risk of human exposures, to target enhanced surveillance and case finding activities, and to prevent and control the spread of influenza A viruses in animals.
• Under the International Health Regulations (IHR) (2005) (1), States Parties are required to notify WHO within 24 hours of any laboratory-confirmed case of human influenza caused by a new subtype according to the WHO case definition (2). Human infection caused by a new subtype has been established as being unusual or unexpected and may have serious public health impact. For this reason, even a single case of human infection with a new influenza subtype that fulfils the WHO case definition must always be notified immediately to WHO, regardless of the context in which it occurs. For events involving suspected cases of human influenza caused by a new subtype (e.g., in the absence of laboratory confirmation), States Parties are required to carry out an assessment of such events according to the decision instrument contained in Annex 2 of the IHR (2005), and then to notify WHO of all qualifying events within 24 hours of such an assessment. Notifications and other event-related communications under the IHR are carried out, by the most efficient means of communication available, between the National IHR Focal Point on behalf of the State Party concerned and the WHO IHR Contact Point at the respective WHO Regional Office.
Background and rationale
The avian influenza A(H5N1) epizootic has led to unprecedented numbers of deaths in wild birds; outbreaks and culling in domestic poultry; and A(H5N1) infections in mammals, including humans. Such human infections remain rare and thus far have been associated with exposure to infected animals or to contaminated environments, without subsequent sustained human-to-human transmission. However, A(H5N1) viruses pose a significant public health risk, with human infections often causing severe disease and high mortality. In addition, such viruses have the potential to adapt to humans and with pandemic potential.
Other influenza A(H5) virus subtypes, such as A(H5N2), A(H5N6) and (H5N8), have also been detected in birds and mammals, including in humans. The current influenza A(H5) situation warrants intense global monitoring and a coordinated global response (3).
Due to the potential significant risk to human health, and the far-reaching implications of the disease for the health of wild birds and other animal populations, a “One Health” approach is essential in effectively tackling avian influenza. Close collaboration with the animal health and environment sectors is vital for understanding the extent of the risk of human exposures, and for preventing and controlling the spread of A(H5) and other influenza A viruses in animals. In addition to surveillance approaches at the human-animal-environment interface, it is recommended that countries, through their National Influenza Centres (NICs) and other influenza laboratories within the WHO Global Influenza Surveillance and Response System (GISRS), remain alert to the possibility of human influenza A virus infections of zoonotic origin.
Following prompt testing, early and appropriate clinical management should be initiated, and precautionary measures put in place to assess and prevent potential further spread among humans and animals. Epidemiological and virological surveillance, and the follow-up of suspected and confirmed human cases, should be conducted systematically. (4, 5) The WHO implementation guidance on surveillance for human infection with influenza A(H5) viruses is summarized below and will be updated as the situation evolves or as more information becomes available.
Surveillance objectives
Overall objective of continual global surveillance To promptly trigger public health control and response actions; to detect and characterize any influenza A(H5) viruses infecting humans to assess the public health risks posed (including the risk of a pandemic), and to inform global pandemic influenza preparedness.
Within these overall objectives, the specific objectives of such surveillance are to:
1. rapidly detect any human cases of A(H5) virus infection; o assess and monitor changes in virus transmission patterns and promptly detect any unusual events that may signal human-to-human transmission of the virus; o monitor the incidence of new cases over time and geographical distribution;
2. characterize and monitor changes in any A(H5) viruses infecting humans relative to those in animals to inform control strategies; and
3. describe the clinical presentation of illness and identify risk factors for infection and severe outcomes.
Surveillance and investigation of human infections with A(H5) viruses
For all countries
• A collaborative, One Health approach to surveillance is needed to identify when humans could be at risk of zoonotic influenza A virus infections, detect human cases when they occur and monitor for human-to-human virus transmission.
• Use information gathered from animal health surveillance on the circulation of influenza A viruses in animals to inform the risk assessment and targeted surveillance of human populations and guide appropriate measures. If timely and of good quality, such information can inform the investigation of respiratory events reported from health care or community settings and unexpected or unexplained changes in trends observed in public health surveillance systems for acute respiratory illnesses.
• Vigilance for the emergence of novel influenza viruses of pandemic potential should be maintained at all times. In the context of the co-circulation of SARS- CoV-2 and influenza viruses, WHO has published practical guidance for integrated surveillance (6).
• To detect human cases, event-based and indicator-based surveillance are important. Approaches to surveillance should include respiratory event-based surveillance at health facilities, community event-based surveillance where appropriate, establishment of nationally notifiable diseases and conditions reporting, the use of laboratory networks. These approaches can be complemented with the monitoring of acute respiratory disease trends and influenza detections in indicator-based surveillance, public media campaigns, social media monitoring and targeted surveillance among at-risk populations.
o It is essential to have a monitoring system for possible human-to-human virus transmission in place to enable a rapid transition to more intensive case detection and to provide a platform for further investigations (7).
• Raise awareness and be prepared for the possibility of human infections with any novel influenza virus of zoonotic origin, including influenza A(H5) viruses.
• In hospital settings, clinicians should be alerted and consider testing patients with severe unexplained acute respiratory illness for influenza, especially if:
(1) the patient had, in the 14 days prior to illness onset, lived in or travelled to an area in which influenza A(H5) infections had recently been detected in humans and/or animals;
(2) the patient had been exposed to live or dead wild or domestic animals, or to environments such as exhibitions, markets or farms where live animals are kept or sold; or
(3) the patient had been exposed to other individuals with recent acute respiratory illness who had such histories or exposures as outlined in (1) or (2).
o Test any health care worker who develops an acute respiratory illness or conjunctivitis and has been caring for patients with severe unexplained acute respiratory illness.
o In settings where there may be limited access to health care, or areas known to be at risk of influenza infections and outbreaks in animals, community representatives should be trained to report clusters1 of respiratory illness, illness in people exposed to infected or potentially infected animals or outbreaks in animals, through a standard reporting channel.
o Increase awareness among laboratories of the importance of molecular detection of influenza A(H5) viruses in human clinical specimens (respiratory and conjunctival) and of collaboration with an affiliated NIC and provide guidance on immediate shipping of positive but un-subtypeable influenza A or A(H5)-positive specimens to a WHO Collaborating Centre on influenza for detailed virus characterization.
o Maintain trained rapid response teams and establish protocols in advance for outbreak investigation, and active and passive case finding, including through contact tracing, for all clusters of unexplained acute respiratory illness (8, 9).
For countries with A(H5) viruses suspected or detected in in birds or mammals, in addition to all of the above
• Countries should have in place an approach for assessing and monitoring the health of individuals at risk of potential exposure to influenza A(H5) viruses. This may include individuals who work in the poultry or other livestock industry or fur farms or zoos (including farmers and veterinarians), visit animal farms or premises in the course of their work (such as animal and public health responders), transport or sell live poultry or other animals or carcasses, slaughter or are involved in culling/depopulating/disposing of poultry or other animals or in the decontamination of contaminated premises. Additionally, individuals may have nonoccupational potential exposure to A(H5) viruses in the course of interacting with infected or potentially infected animals.
• Raise awareness among clinicians and other health care workers of the possibility of human infection with avian influenza A(H5) viruses to facilitate early clinical suspicion and diagnosis, isolation of patients with suspected A(H5) virus infection, correct use of recommended personal protective equipment and prompt initiation of antiviral treatment.
• In outpatient settings, clinicians should consider testing for influenza in patients with acute respiratory infection or influenza-like illness or conjunctivitis if the patient has been exposed to influenza-infected (or presumed to be infected) birds or other animals in the 14 days prior to illness onset, as described below.
• Countries should define, based on their available capacity and infrastructure, which syndromes that clinicians should test, or refer for testing, to ensure that the health system can effectively manage the testing process without overburdening facilities or resources.
Case definitions
The case definitions provided below are used for surveillance purposes and to standardize case classification and are not intended to provide complete descriptions of disease in patients or to guide clinical management. They are also not intended to be used for additional, more sensitive case finding during outbreak investigations, which may include investigating any at risk individual with signs or symptoms of acute respiratory illness. National authorities may develop other case definitions for other objectives and testing strategies. The case definitions may change as new information about the disease, epidemiology, or the viruses become available. WHO clinical practice guidelines for influenza have been published separately (10). Clinical decisions concerning the treatment, care or triaging of people potentially infected with an influenza A(H5) virus should be based on clinical judgement and epidemiological reasoning.
While most patients infected with influenza A(H5) viruses present with fever, cough and lower respiratory tract symptoms, the clinical spectrum is broad and can include mild symptoms (such as upper respiratory tract symptoms or conjunctivitis only) without fever.
Suspected influenza A(H5) case definition
A person presenting with unexplained acute respiratory illness with fever (> 38 °C) or cough, shortness of breath or difficulty breathing or conjunctivitis.
AND
One or more of the following exposures in the 14 days prior to symptom onset:
• Close contact (within 1 metre) with a person (for example, caring for, speaking with or touching) who is a suspected or confirmed avian influenza A(H5) case.
• Exposures in an area where avian influenza A(H5) virus infections in animals or humans have been suspected or confirmed, such as:
o close contact (within 1 metre) with live, sick or dead infected animals or animal products, or consumption or handling of raw uncooked meat, unpasteurized milk or other raw animal meat or products;
o direct exposure to surfaces that could be contaminated with infected animal products or with water contaminated with such products (such as wastewater from a live bird market or slaughtering facility); or
o visiting or working at a live animal market, farm, zoo or other setting with infected animals.
• Handling samples (animal or human) suspected of containing avian influenza A(H5) virus in a laboratory or other setting.
Confirmed case definition
1. A person with a laboratory-confirmed infection with an avian influenza A(H5) virus. A laboratory-confirmed infection is considered if it has been confirmed by positive results from polymerase chain reaction (PCR), virus isolation, or serological testing of paired acute and convalescent serum.
Serologic testing of paired acute and convalescent serum specimens:
• Serological confirmation of an A(H5) case requires paired sera collection (one acute, one convalescent specimen), with a ≥ 4-fold rise in neutralizing antibody titres (or equivalent) to an influenza A(H5) virus2 that is antigenically similar to the virus the person was exposed to, with a convalescent neutralizing titre ≥ 1:40. Acute serum should be collected within 7 days of symptom onset; convalescent serum should be collected ≥ 21 days (ideally 21–28 days) after symptom onset.
Serologic testing of a single convalescent serum specimen, when the following are met:
• The criteria for seropositivity of an A(H5) infection using a single convalescent serum specimen, collected at ≥ 21 days after symptom onset or exposure includes a neutralizing antibody titre ≥ 1:40 to an influenza A(H5) virus; and
• A positive result using a different serological assay such as a hemagglutination inhibition (HI) antibody titre ≥ 1:40, or an influenza A(H5)-specific positive result from another immunological assay such as an enzyme-linked immunosorbent assay (ELISA), a multiplex binding antibody assay, or similar binding antibody assay; and
• In all assays mentioned above, sera are tested against an influenza A(H5) virus(es)4 or antigen(s) antigenically similar to the virus the person was exposed to; and
• The person has an epidemiological link3 to a laboratory-confirmed human case.
Testing
All individuals meeting the suspected surveillance case definition or other locally adapted case definitions for other objectives should be tested according to local protocols.
• The types of samples to be collected for the diagnosis of viral infections of the upper and lower respiratory tract are described in the WHO Manual for the laboratory diagnosis and virological surveillance of influenza (11). In cases presenting with conjunctivitis, conjunctival specimens should be collected.
o WHO information for the molecular detection of influenza viruses can be found on the WHO website (12).
o All influenza A positive specimens that are not able to be subtyped should be sent immediately to a National Influenza Centre (NIC) if originally tested elsewhere, and from the NIC to a WHO Collaborating Centre of GISRS (13) for further analysis in line with the relevant WHO operational guidance (14) under their Terms of Reference (15).
o Virus isolation from specimens suspected or confirmed to contain avian influenza A(H5) virus is not recommended, unless it is performed at a WHO influenza CC or a WHO H5 Reference Laboratory of GISRS, due to the biosafety requirements.
o Serologic testing is strongly recommended to be performed or directly supported by, or performed in collaboration with, a WHO CC or H5 Reference Laboratory of GISRS.
o Contact WHO Global Influenza Programme (GISRS-WHOhq@WHO.int) for support of serology testing for A(H5) and other help to confirm a human infection with an avian influenza A(H5) virus.
• Testing of asymptomatic exposed individuals could also be considered on a case- by-case basis, depending on available resources and based on an exposure risk assessment and testing objectives (for example, as part of an outbreak investigation or special study to assess asymptomatic transmission). In this context, the testing of respiratory samples for viable and replicating viruses needs to be paired with serological testing of acute and convalescent serum samples.
Investigation of confirmed cases and monitoring of exposed individuals
• All confirmed human cases of influenza A(H5) infection should be further investigated and closely monitored, and contacts also monitored to detect and rapidly interrupt potential humanto-human virus transmission and to better understand exposure risks. More detailed guidance can be found in the WHO Protocol to investigate non-seasonal influenza and other emerging acute respiratory diseases (9). In addition, various protocols under WHO influenza investigations and studies (Unity Studies) are currently being updated. When sharing influenza A(H5)-positive specimens, the relevant WHO operational guidance should be followed (14).
• Case definitions for additional case finding should be developed locally and may be shaped by information obtained from the interview with the confirmed case(s).
• The specific public health actions that should be implemented immediately include:
o testing for cases of human infection with animal influenza A viruses using appropriate investigation and laboratory protocols;
o assessing exposure to animals and travel history of confirmed cases;
o identification and monitoring of household and other close contacts of a confirmed case (including health care personnel) and active searching for other cases; and
o early detection of any unusual respiratory disease events that could signal person-to-person transmission of the virus.
• Public health and animal health authorities should conduct joint investigations of human cases of novel influenza A virus infection (zoonotic influenza). This will involve assessing the role of local animals as sources of exposure, understanding patterns of illnesses or death in local animals and determining whether animal influenza viruses are circulating in local animals so that appropriate control measures can be implemented to reduce the risk of continued human exposure.
Reporting under IHR and information sharing
Under the International Health Regulations (IHR) (2005), States Parties are required to notify WHO within 24 hours of any laboratory-confirmed case of human influenza caused by a new subtype according to the WHO criteria for IHR notification (2). Human influenza caused by a new subtype has been established as being unusual or unexpected and may have serious public health impact. For this reason, even a single case of human infection with a new influenza subtype that fulfils the WHO case definition must always be notified immediately to WHO, regardless of the context in which it occurs. For events involving suspected cases of human influenza caused by a new subtype (e.g., in the absence of laboratory confirmation), States Parties are required to carry out an assessment of such events according to the decision instrument contained in Annex 2 of the IHR (2005), and then to notify WHO of all qualifying events within 24 hours of such an assessment. Notifications and other event-related communications under the IHR are carried out, by the most efficient means of communication available, between the National IHR Focal Point on behalf of the State Party concerned and the WHO IHR Contact Point at the respective WHO Regional Office.
A minimum data set reporting form for human infection with an influenza virus with pandemic potential is available in the Annex of this document. As specified in Article 6.2 of the IHR (2005), the notification must always include or be followed by timely and ongoing communication of accurate and sufficiently detailed public health information about the event as well as the health measures implemented in response to the event. As the event unfolds, more information may become available, and the State Party must continue to share the relevant public health information to allow WHO to conduct its risk assessment with respect to the ongoing event in collaboration with the notifying State Party.
WHO has published the WHO case definition for human infections with avian influenza A(H5) virus requiring notification under IHR (2005) (16). The results of ongoing surveillance activities, and of studies or other research activities, should also be communicated to WHO in a timely manner to inform global risk assessment and guidance.
Information on human infections and information not under the IHR reporting requirements (for example, findings from seroprevalence studies) that might be of public health importance, should be rapidly shared with GISRS for risk assessment purposes, via WHO CCs, WHO regional officers or the Global influenza Programme. For example, if a single serum specimen tests positive in a serology assay but does not meet the notification requirements under IHR as mentioned above, it is strongly recommended to communicate this information to a WHO CC of GISRS for surveillance and risk assessment purposes. This includes situations where a single convalescent serum specimen tests positive by microneutralization assay and another assay, such as ELISA, but the individual from whom the specimen was taken did not have an epidemiological link to a confirmed A(H5) human case, even though they may have had exposure to A(H5)-infected animals or contaminated environments.
Wastewater surveillance
Although Influenza A viruses can be detected in wastewater (and can be distinguished from influenza B viruses), most of the laboratory assays used cannot distinguish between different influenza A virus subtypes. It is also currently not possible to determine the source of an influenza A virus in wastewater (human waste, animal waste or other) or to know how many cases must occur in an area before influenza viruses can be detected through wastewater surveillance. If used, wastewater and environmental surveillance should be integrated as part of multimodal influenza surveillance (17).
Methods
This guidance is based on guidance previously developed by WHO for other zoonotic influenza subtypes and considers the information reported on human infections with influenza A(H5) viruses to WHO and GISRS. The guidance also incorporates information from other WHO products that have become available since previous versions of surveillance guidance for other zoonotic influenza subtypes were published.
Contributors
This surveillance guidance was developed by the World Health Organization (WHO) Global Influenza Programme through a process of review and consultation with internal and external experts. WHO expresses its gratitude to those who reviewed the document for their efforts, experience and insights. This surveillance guidance was adapted from previous guidance by Aspen Hammond of the WHO Global Influenza Programme.
WHO staff and consultants who contributed to the development of this guidance in 2024 include: Vanessa Cozza (WHO headquarters, Global Influenza Programme), Helge Hollmeyer (WHO headquarters, IHR Secretariat), Joshua Mott (WHO headquarters, Epidemic & Pandemic Preparedness and Prevention), Sergejs Nikisins (WHO headquarters, Global Influenza Programme), Sarika Patel (WHO Country office Cambodia), Dmitriy Pereyaslov (WHO headquarters, Global Influenza Programme), Angel Rodriguez (WHO Regional Office for the Americas), Melissa Rolfes (WHO headquarters, Global Influenza Programme), Magdi Samaan (WHO headquarters, Global Influenza Programme), Maria Van Kerkhove, Marc-Alain Widdowson (WHO Regional Office for Europe), Reina Yamaji (WHO headquarters, Global Influenza Programme) and Wenqing Zhang (WHO headquarters, Global Influenza Programme). Technical experts from the following WHO Collaborating Centres of the Global Influenza Surveillance and Response System (GISRS) (13, 18) contributed to this document through their review of the draft document in 2024: WHO Collaborating Centre for Reference and Research on Influenza Victorian Infectious Diseases Reference Laboratory, The Peter Doherty Institute for Infection & Immunity, Australia; WHO Collaborating Centre for Reference and Research on Influenza, National Institute for Viral Disease Control and Prevention, Chinese Center for Disease Control and Prevention (CDCD), China; WHO Collaborating Centre for Reference and Research on Influenza, National Institute of Infectious Diseases (NIID), Japan; WHO Collaborating Centre for Reference and Research on Influenza, The Francis Crick Institute, United Kingdom of Great Britain and Northern Ireland; WHO Collaborating Centre for the Surveillance, Epidemiology and Control of Influenza, Centers for Disease Control and Prevention, USA; WHO Collaborating Center for Studies on the Ecology of Influenza in Animals, St. Jude Children's Research Hospital, USA; and the WHO Collaborating Centre for Studies on Influenza at the Animalhuman Interface, State Research Center of Virology and Biotechnology "VECTOR", Rospotrebnadzor, Russian Federation. Declaration of interests Technical experts represented institutions designated as WHO Collaborating Centres of the Global Influenza Surveillance and Response System (GISRS) and no declarations of interest were required from them for their review of the draft document.
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{1} A “cluster” is defined as two or more persons with onset of symptoms within the same 14-day period and who are associated with a specific setting, such as a classroom, workplace, household, extended family, hospital, other residential institution, military barracks or recreational camp.
{2} Wild type virus is preferred.
{3} This may include close contact such as providing care for the patient, including as a health care worker or family member, or other similarly close physical contact, or staying at the same place (e.g. lived with, visited) as a confirmed case while the case was symptomatic.
References
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18. WHO Collaborating Centres Global database [website]. Geneva: World Health Organization (https://apps.who.int/whocc/, accessed 7 November 2024.
Further reading
• Current information on animal influenza events reported to the World Organisation for Animal Health (WOAH) can be found at: WAHIS: World Animal Health Information System [website]. World Organisation for Animal Health (https://wahis.woah.org/#/home).
• The results of human A(H5) surveillance and public health risk assessments and related resources can be found at: Human-animal interface [website]. Geneva: World Health Organization (https://www.who.int/teams/global-influenza-programme/avian-influenza).
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Source: World Health Organization, https://www.who.int/publications/i/item/surveillance-for-human-infections-with-avian-influenza-a(-h5)--viruses
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