CDC Awaits Clinical Specimen from Fatal Variant Flu Case in Brazil

June 23, 2023 – On June 7, 2023, the Brazilian Ministry of Health (MoH) reported to the World Health Organization (WHO) a fatal human infection with a swine influenza A(H1N1)v virus, a virus that typically spreads in pigs and not people. A clinical specimen collected from the patient is being sent to the United States Centers for Disease Control and Prevention (CDC) for confirmation and further characterization of the virus. While not common, human infections with swine influenza viruses do happen. The specimen is being sent to CDC as the agency’s Influenza Division is one of seven World Health Organization Influenza Collaborating Centers that support global influenza surveillance.

Testing conducted by the Brazil National Influenza Center indicated the virus is genetically related to swine influenza viruses detected previously in swine in Brazil and to previous human infections with A(H1)v viruses detected in Brazil in 2020, 2021, and 2022. CDC will perform additional sequencing of the virus genome in order to look for potential mutations that might make this virus spread more easily from person-to-person or cause more severe disease in people. If virus isolation is successful, additional virologic characterization may be performed.

The patient, who reportedly was severely immunocompromised, lived near a swine farm and had close contact with two family members who worked at the farm. The patient developed symptoms on May 1, was hospitalized with symptoms of an acute respiratory infection on May 3, and died on May 5. People with weakened immune systems are more likely to become seriously ill from any influenza virus infection, including swine influenza viruses, which most often result in mild illness. Investigations conducted by the Brazilian MoH reported that both family members who worked at the swine farm did not develop respiratory symptoms and also tested negative for influenza. In addition, no person-to-person spread has been identified in this case.

While this is the first influenza A(H1N1v) infection reported in Brazil in 2023, sporadic human infections have occurred in the past.

Sporadic infections and even localized outbreaks among people with variant influenza viruses can occur. When this happens, these are called variant virus infections and denoted with the letter “v” in the virus name. In the U.S., 18 influenza A(H1N1)v variant virus infections have been reported since 2010. There have been 475 human infections with other variant flu viruses during that same time.

Most commonly, human infections with variant viruses occur in people with exposure to infected pigs (e.g., children exposed to pigs at an agricultural fair, people who raise pigs, or workers in the swine industry). CDC has issued guidance for people attending settings where swine might be present, including additional precautions for people who are at higher risk of serious flu complications.

This isolated sporadic variant virus infection in Brazil is not thought to pose a risk to the U.S. public. In general, the current risk to the U.S. general public from swine influenza is low, but all influenza viruses have the capacity to change, and that is why it is important to follow up on each of these variant virus infections. CDC continues to monitor closely for variant influenza virus infections and will report cases weekly in FluView and in the Novel Influenza A Virus Infections ( section of FluView Interactive.