Avian Influenza A(H5) Outbreak
Current US Outbreak
Since the last update on September 9, 2024:
- CDC has confirmed through genomic sequencing that the Missouri case was infected with H5N1 clade 2.3.4.4b, the clade circulating in poultry and dairy cows.
- Two amino acid sequence changes were identified for the Missouri case; these are not believed to be associated with mammalian adaption, support human-to-human transmission, or impact antiviral effectiveness but may impact the cross-reactivity of the candidate vaccine viruses. CDC is planning to conduct additional antigenic testing to determine how this mutation may impact vaccine efficacy.
- Additional case investigation information has been released by CDC; the adult patient had underlying medical conditions and presented with chest pain, nausea, vomiting, diarrhea and weakness. CDC reports the illness was not severe and he was not treated in the Intensive Care Unit. Two ill close contacts were identified. One was as household contact with symptoms similar to the confirmed case; this person was not hospitalized or tested, and recovered at home. The other was a healthcare worker that tested negative for flu. CDC and the Missouri health department are considering antigen testing.
- CDC and CORI maintain that the risk to the general public remails low at this time, but the risk to farmworkers remains moderate-high and the risk to other people in contact with affected workers and animal populations remains moderate. The confidence in these risk scores is low due to limited information on a potentially new chain of transmission, in combination with limited testing in humans, dairy cows, and other animals. CORI may adjust this assessment based on new information about the chain of transmission for the Missouri case, new results of ongoing serology testing, or identification of additional human cases with no known animal exposures through routine influenza surveillance.
Read more about the current situation and expected impact on special populations in the U.S. below.