On Apr. 12, 2024, the U.S. Centers for Disease Control and Prevention (CDC) announced that Vietnam had reported the country’s first human infection with a low pathogenic avian influenza (LPAI) A(H9N2) virus. This is a different subtype of avian influenza than the highly pathogenic avian influenza (HPAI) A(H5N1) virus that is most commonly reported globally and which is causing outbreaks in poultry and cattle in the U.S. currently. Wild water birds are hosts to avian influenza viruses, and infected birds shed virus in their saliva, mucous and feces. Rare human infections with avian influenza viruses can happen when virus gets into a personﾒs eyes, nose or mouth, or is inhaled.

Human infections with A(H9N2) viruses have been reported sporadically in more than 100 people since 1998 in China, and in Hong Kong, Bangladesh, Cambodia, Egypt, India, Oman, Pakistan, and Senegal. Although a small number of deaths have been reported, infections with (H9N2) avian influenza viruses have mostly resulted in mild upper respiratory tract illness symptoms. All A(H9) viruses identified worldwide in wild birds and poultry are LPAI viruses and cause few signs of disease in infected wild birds. A(H9N2) viruses have been detected in bird populations in Asia, Europe, the Middle East and Africa, and notably, A(H9N2) viruses are the most commonly identified avian influenza viruses in Vietnam, representing more than 50% of all subtyped detections from live bird markets in that country. Antibodies to H9 viruses have also been previously reported in Vietnamese poultry workers.

The infection in Vietnam occurred in an adult with underlying medical conditions who was hospitalized on March 16, 2024, and the patient remains hospitalized for monitoring and treatment. On March 23, 2024, the Hospital for Tropical Diseases supported by the Oxford University Clinical Research Unit (OUCRU) sequenced a portion of the genome with results similar to earlier influenza A(H9) viruses. An original sample from March 23 was sent to Pasteur Institute Ho Chi Minh (PI-HCM) on April 1, and PI-HCM also detected influenza A(H9) viruses.