On January 23, 2024, the Anhui provincial CDC received a cooperation report from the Zhejiang provincial CDC and subsequently initiated a retrospective epidemiological study to investigate the epidemiology, clinical characteristics, and etiology involved. On December 1, the patient reported experiencing fever symptoms, although the temperature was not measured; these symptoms persisted without improvement on December 2. Due to a worsening condition, the patient was admitted to the Guangde County Traditional Chinese Medicine Hospital on the morning of December 3 with suspected pneumonia. On January 25, sequencing conducted by the China CDC revealed that the patient’s sample contained the complete genome sequences of both the seasonal influenza A (H3N2) virus and the influenza A (H10N5) virus.

According to the guidelines from the China CDC, close contacts are defined as: 1) medical personnel or other caregivers; 2) household members and individuals with close contact, and 3) others determined by on-site investigators. During the epidemiological investigation, health officials conducted retrospective health follow-ups on 30 close contacts, including 15 medical staff from Guangde Hospital of Traditional Chinese Medicine, 10 staff members from the same ward, 2 neighbors, and 3 family members. None of these individuals showed signs of infection, and all tested negative for the virus using nucleic acid assays. All close contacts last interacted with the cases between December 1–17, 2023. Investigators collected 189 samples from various sources, including live poultry, environmental sites, and individuals in locations linked to the patient, such as their home, adjacent households, supermarkets, mobile vendors, live duck wholesale markets, and stand-alone live chicken vendors. These samples were sent to the CDC, with 167 completing laboratory testing. Analysis revealed 28 positive results: 14 for H10N5, 3 for N5 nucleic acid fragments, and 11 for other subtypes including H5, H6, H9, N1, N2, and N6. Additionally, 30 serum samples from close contacts were subjected to pseudovirus-based neutralization tests for H10N5 antibodies, all of which yielded negative results.