Influenza A(H1N1) variant virus – Viet Nam

On 19 August 2024, the Viet Nam National Focal Point (NFP) for International Health Regulations (IHR) notified the World Health Organization (WHO) of a laboratory-confirmed human case of infection with swine-origin influenza A(H1N1) variant (v) virus, in the province of Son La, a northern mountainous province in Viet Nam bordering the Lao People’s Democratic Republic. This is the first-ever reported human infection caused by influenza A(H1N1)v virus in Viet Nam. The source of exposure to the virus is currently unknown. According to the International Health Regulations (IHR) 2005, a human infection caused by a novel influenza A virus subtype is an event that has the potential for high public health impact and must be notified to WHO. Based on the currently available information, WHO assesses the current risk to the general population posed by this virus as low. WHO continues to stress the importance of global surveillance to detect virological, epidemiological and clinical changes associated with circulating influenza viruses that may affect human (or animal) health, and timely virus sharing for risk assessment.

On 19 August 2024, the Viet Nam IHR NFP notified WHO of a laboratory-confirmed human infection with a swine-origin influenza A(H1N1)v virus. The patient was a 70-year-old female with underlying medical conditions, from Son La province, a northern mountainous province in Viet Nam bordering the Lao People’s Democratic Republic.

The patient developed fever, fatigue, and loss of appetite one week after returning from spending a month in her home village in Hung Yen province. On 1 June 2024, she was admitted to a district hospital due to persistent symptoms and was diagnosed with pneumonia in addition to her underlying medical conditions. On 5 June, a rapid test confirmed influenza A. On the same day she was transferred to the National Hospital for Tropical Diseases (NHTD) where she underwent treatment for six days but died on 11 June.

A sample collected on 5 June was tested through reverse transcription-polymerase chain reaction (RT-PCR) which confirmed influenza A virus on 18 June, but subtyping was not determined. On 1 August, genomic sequencing identified influenza A(H1N1)v virus, which was confirmed by the WHO Collaborating Centre (WHO CC) for influenza at the United States Centers for Disease Control and Prevention (US CDC) on 5 August. Further characterization of the virus is ongoing.

Epidemiological investigations revealed that the patient lived alone and had limited contact with a few villagers and caregivers during her illness in Son La province. From June to 11 August, there have been no reports of respiratory symptoms among contacts, including healthcare workers of the case, or outbreaks in the community in Son La where the patient lived. There have been no disease outbreaks among livestock, including pigs, around the case’s residence in Son La. There have been no official or unofficial reports from the case’s home village in Hung Yen province. At the time of reporting, the source of exposure to the virus remains unknown.

Influenza A (H1) viruses circulate and cause disease in swine populations in most regions of the world. Of the many subtypes, A(H1N1), A(H1N2), and A(H3N2) are major subtypes of swine influenza A viruses in pigs which occasionally infect humans, usually after direct exposure to pigs or indirectly through contaminated environments. When an influenza virus that normally circulates in swine is detected in a person, it is called a “variant influenza virus”. Human infections with variant viruses tend to result in mild clinical illness, although some cases have been hospitalized with more severe disease and some have been fatal.

Swine influenza viruses have been detected in pigs in Viet Nam since 2010, according to results of active surveillance of influenza A viruses of swine. However, this is the first human influenza A(H1N1)v infection identified in Viet Nam.

The public health sector of Viet Nam has taken the following response actions:

• The national authorities have called for enhanced surveillance, human-animal health coordination, and outbreak investigation, particularly at the subnational level.
• The Son La Provincial Center for Disease Control (PCDC) are collaborating with local public health and animal health officials for further investigation and contact tracing.
• On 11 August, Son La PCDC submitted an official report on their retrospective case investigation, prevention, and control activities, along with recommendations to local health and animal health authorities.

Human infections with swine-origin influenza viruses have been reported in recent years from many countries. Most human cases with influenza A(H1N1)v virus infection result from exposure to swine influenza viruses through direct contact with infected swine or indirectly through contaminated environments. However, a few cases have been reported without an apparent source of exposure to swine in the weeks prior to illness onset. Because these viruses continue to be detected in swine populations worldwide, further human cases following direct or indirect contact with infected swine can be expected.

Limited, non-sustained human-to-human transmission of variant influenza viruses has been described, although ongoing community transmission has never been identified. Current evidence suggests that these viruses have not acquired the capacity for sustained transmission among humans. According to the information available thus far, no further human cases of infection with A(H1N1)v viruses associated with this case have been detected.

Based on the available information, WHO assesses the current risk to the general population posed by this virus to be low. Further virus characterization is ongoing. The risk assessment will be reviewed should further epidemiological or virological information become available.

Surveillance:

• This case does not change the current WHO recommendations on public health measures and seasonal influenza surveillance.
• WHO does not advise special traveler screening at points of entry or restrictions concerning the current situation of influenza viruses at the human-animal interface.
• Due to the constantly evolving nature of influenza viruses, WHO continues to stress the importance of global surveillance to detect virological, epidemiological and clinical changes associated with circulating influenza viruses that may affect human (or animal) health, and timely virus sharing for risk assessment.

Notification and investigation:

• All human infections caused by a novel influenza subtype are notifiable under the International Health Regulations (IHR), and States 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.
• In the case of a confirmed or suspected human infection caused by a novel influenza virus with pandemic potential, including a variant virus, a thorough epidemiologic investigation including history of animal exposure and travel should be conducted. Contact tracing should be initiated. The epidemiologic investigation should include early identification of unusual respiratory events that could signal person-to-person transmission of the novel virus. Clinical samples collected from the case should be tested and sent to a WHO Collaborating Centre for further characterization.

Travel and trade:

• WHO does not recommend any travel and/or trade restrictions for Viet Nam based on the currently available information.
• Proper hygiene measures should be adhered to, such as regular hand washing with soap before and after touching animals, and avoiding contact with sick animals.

Prevention measures for travelers:

• Travelers 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 feces or other excreta. Travelers should also wash their hands often with soap and water. Travelers should follow good food safety and good food hygiene practices.
• Influenza viruses that infect pigs are different from human influenza viruses. There is no vaccine for zoonotic influenza infection licensed for use in humans. However, the development of influenza candidate vaccine viruses for novel zoonotic influenza, coordinated by WHO, remains an essential component of the overall global strategy for influenza pandemic preparedness. Vaccines against human influenza viruses are generally not expected to protect people from infection with influenza viruses that normally circulate in pigs. Nevertheless, WHO recommends seasonal influenza vaccination to avoid severe illness from infection with circulating influenza viruses.

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Citable reference: World Health Organization (4 September 2024). Disease Outbreak News; Influenza A(H1N1) variant virus – Viet Nam. Available at: https://www.who.int/emergencies/disease-outbreak-news/item/2024-DON532