Situation at a glance:
On 4 August 2023, the United States International Health Regulations (IHR) National Focal Point informed PAHO/WHO of a human infection with a novel influenza A(H1N2) variant virus identified in the State of Michigan. This was confirmed by Reverse Transcription Polymerase Chain Reaction at the United States (US) Centers for Disease Control and Prevention (CDC) on August 2.
The case was not hospitalized and is recovering from the illness. Investigations by local public health officials identified swine exposure by the patient at an agricultural fair.
Additional public health response efforts including enhanced surveillance for cases is undergoing. Given the potential impact on public health, human infections with these viruses need to be monitored closely. In this event, there was no reported evidence of sustained human-to-human transmission, the illness was mild and no further influenza variant virus transmission in the community has been identified.
Description of the case:
On 4 August 2023, the United States IHR National Focal Point informed PAHO/WHO of a human infection with a novel influenza A(H1N2) variant virus identified in the State of Michigan and confirmed by the US CDC.
According to the report, the case is under 18 years old, with no comorbidities, resident in the State of Michigan, who developed respiratory illness on 29 July 2023. The case presented with fever, cough, sore throat, muscle aches, headache, shortness of breath, diarrhea, nausea, dizziness, and lethargy. On 29 July, the case sought medical care at an emergency department, and an upper respiratory tract specimen was collected on 30 July. The specimen tested positive for influenza A virus on the same day. On 1 August, the patient received influenza antiviral treatment (Oseltamivir).
On 31 July, the specimen was tested at the Michigan Department of Health and Human Services (MDHHS), and RT-PCR results were positive for influenza A virus but lacked reactivity with diagnostic tests for contemporary human influenza viruses representing either (H1) pdm09 or (H3) subtypes. The specimen was then sent to the US CDC for further testing and received on 2 August. On the same day, RT-PCR analysis of the specimen indicated an influenza A(H1N2) variant (v) virus. The virus was isolated and subsequent analysis including genetic sequencing is underway.
The case was not hospitalized. Investigation by local public health officials identified swine exposure by the patient at an agricultural fair, that took place between 23 and 29 July, within 10 days prior to illness onset. Additional investigation did not identify respiratory illness in any of the patient’s close contacts or household contacts. No additional cases were identified related to this agriculture fair. No person-to-person transmission of influenza A(H1N2)v virus associated with this case has been identified. No additional cases of human infection with A(H1N2)v virus have been identified as of 10 August 2023.
This is the first influenza A(H1N2)v virus infection identified in the United States this year. Since 2005, there have been 512 influenza A variant virus infections (all subtypes), including 37 (human infections with influenza A (H1N2)v viruses reported in the United States.
Epidemiology of the disease:
Swine influenza A(H1N2) viruses circulate in swine populations in many regions of the world. Influenza A viruses that normally circulate in swine are called “variant” influenza viruses when isolated from humans.1 Human infections are usually due to direct or indirect exposure to pigs or contaminated environments. There may be important antigenic and genetic differences between seasonal influenza viruses that circulate worldwide in the human population and influenza viruses that normally circulate in swine. Close monitoring is needed to early identify changes in the virus and/or its transmission to humans.2
Non-seasonal or zoonotic influenza viruses infecting humans may cause disease ranging from mild conjunctivitis to severe pneumonia and even death; usually, these human infections of zoonotic influenza are acquired through direct contact with infected animals or contaminated environments. Since 2005, 37 human infections with influenza A(H1N2)v virus, including this one, have been reported in the United States.
Public health response
The public health response efforts led by local officials included enhanced surveillance for cases. The county health department conducted active case finding by reaching out to fair exhibitors and their families to ascertain additional illnesses. In addition, local providers were notified to watch for respiratory illness in persons who had attended the fair or had recent contact with swine.
WHO risk assessment
Swine influenza A viruses circulate in swine populations in many regions of the world. Depending on geographic location, the genetic characteristics of these viruses differ. When an influenza A virus that normally circulates in swine (but not people) is detected in a person, it is called a “variant influenza virus”. Most human cases are the result of exposure to swine influenza A viruses through contact with infected swine or contaminated environments. Because these viruses continue to be detected in swine populations around the world, further human cases can be expected.
According to the IHR, a human infection caused by a novel influenza A virus subtype is an event that has the potential for high public health impact. A novel influenza A virus is considered to have the potential to cause a pandemic if (1) the virus has demonstrated the capacity to infect and transmit efficiently among humans, and (2) differs from currently circulating seasonal human influenza viruses (i.e., A/H1 or A/H3), circulating widely in the human population such that the hemagglutinin (HA) gene (or protein) is not a mutated form of those; and (3) the population has little to no immunity against it.
To date, only sporadic human infections caused by influenza A(H3N2), A(H1N1) and A(H1N2) variant viruses have been reported in the United States, and there has been no evidence of sustained human-to-human transmission. Human infections with influenza variant viruses tend to result in mild clinical illness, although some cases have been hospitalized with more severe disease. Given the potential impact on public health, human infections with these viruses need to be monitored closely. In this event, there was no reported evidence of sustained human-to-human transmission, the illness was mild and no further influenza variant virus transmission in the community has been identified.
There has been some limited, non-sustained human-to-human transmission of variant influenza A viruses, although ongoing community transmission has not been identified. Current evidence suggests that these viruses have not acquired the ability of sustained transmission among humans. The risk assessment will be reviewed if needed should further epidemiological or virological information become available.
Influenza A viruses that infect pigs are different from human influenza A viruses. Currently, there is no vaccine for Influenza A(H1N2)v infection licensed for use in humans. Thus, influenza vaccines against human influenza viruses are generally not expected to protect people from influenza viruses that normally circulate in pigs.
This case does not change the current WHO recommendations on public health measures and surveillance of seasonal influenza.
WHO does not advise special traveller screening at points of entry or restrictions with regards to the current situation of influenza viruses at the human-animal interface. For recommendations on safe trade in animals and related products from countries affected by these influenza viruses, refer to WOAH guidance.
The public should avoid contact with animals that are sick or dead from unknown causes, including wild animals, and should report dead wild birds and mammals or request their removal by contacting local wildlife or veterinary authorities.
WHO advises that travellers to countries with known outbreaks of animal influenza should avoid farms, contact with animals in live animal markets, entering areas where animals may be slaughtered, or contact with any surfaces that appear to be contaminated with animal excreta. Travelers should also wash their hands often with soap and water. All individuals should follow good food safety and hygiene practices.
Due to the constantly evolving nature of influenza viruses, WHO continues to stress the importance of global surveillance to detect virologic, epidemiologic and clinical changes associated with circulating influenza viruses that may affect human (or animal) health and timely virus sharing for risk assessment. Continued vigilance is needed within affected and neighbouring areas to detect infections in animals and humans. Collaboration between the animal and human health sectors is essential. As the extent of influenza viruses circulation in animals is not clear, epidemiologic and virologic surveillance and the follow-up of suspected human cases should continue systematically. Guidance on investigation of non-seasonal influenza and other emerging acute respiratory diseases has been published on the WHO website here.
Vigilance for the emergence of novel influenza viruses of pandemic potential should be maintained. WHO has developed practical guidance for integrated surveillance in the context of the cocirculation of SARS-CoV-2 and influenza viruses. The guidance is available here.
All human infections caused by a new subtype of influenza virus are notifiable under the International Health Regulations (IHR, 2005). State Parties to the IHR (2005) are required to immediately notify WHO of any laboratory-confirmed case of a recent human infection caused by an influenza A virus with the potential to cause a pandemic. Evidence of illness is not required for this report.
It is critical that these influenza viruses from animals or from people are fully characterized in appropriate animal or human health influenza reference laboratories. Under WHO’s Pandemic Influenza Preparedness (PIP) Framework, Member States are expected to share Influenza Viruses with Pandemic Potential (IVPP) on a regular and timely basis with the Global Influenza Surveillance and Response System (GISRS), a WHO-coordinated network of public health laboratories. The viruses are used by the public health laboratories to assess the risk of pandemic influenza and to develop relevant candidate vaccine viruses.
Source : World Health Organization