March 6, 2021

Influenza A (H3N2) variant virus – United States of America

On 13 January, 2021, a child under 18 years of age in Wisconsin developed respiratory disease. A respiratory specimen was collected on 14 January. Real-time reverse transcriptase polymerase chain reaction (RT-PCR) testing conducted at the Wisconsin State Laboratory of Hygiene indicated a presumptive positive influenza A(H3N2) variant virus infection. The specimen was forwarded to the Influenza Division of the Centers for Disease Control and Prevention (CDC) on 21 January for further testing. On 22 January, CDC confirmed an influenza A (H3N2)v virus infection using RT-PCR and genome sequence analysis. Investigation into the source of the infection has been completed and revealed that the child lives on a farm with swine present. Sampling of the swine on the property for influenza virus has not yet been conducted but is planned. Five family members of the patient reported respiratory illness during the investigation and were tested for influenza; all tested negative. The patient was prescribed antiviral treatment and was not hospitalized and has made a full recovery. No human to human transmission has been identified associated with this investigation.

Sequencing of the virus by CDC revealed it is similar to A (H3N2) viruses circulating in swine in the mid-western United States during 2019-2020. Viruses related to this A (H3N2)v virus were previously circulating as human seasonal A (H3N2) viruses until around 2010-2011 when they entered the USA swine population. Thus, past vaccination or infection with human seasonal A (H3N2) virus is likely to offer some protection in humans.

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