Bacteremic Group A Streptococcal Co-Infection in Two Adults with H3N2 Influenza in Japan, 2025-2026: A Case Series.

Abstract

Two adult cases of influenza with bacteremia due to Group A Streptococcus pyogenes (GAS) in the 2025-2026 season, which was a major season of H3N2 subclade (subclade K) type influenza in Japan, are presented. Case 1: A 52-year-old man with no underlying diseases and no vaccination history was admitted as influenza A. The onset was five days ago, and GAS was isolated from his blood. He was in shock with lobular pneumonia, high inflammation, and renal dysfunction, and finally he died although intensive care and appropriate antiviral/antibiotic treatment. Case 2: A 68-year-old woman with no underlying disease was admitted as influenza A. She received laninamivir inhalation on Day -1 and had history of the influenza vaccine for the season. She was also in shock and renal dysfunction status but did not show obvious pneumonia although Streptococcus pneumoniae (S. pneumoniae) was isolated from her sputum. In addition, GAS was also isolated from her blood on admission. She finally recovered by intensive care and appropriate antibiotic treatment. Both patients showed septic status and acute kidney injury (AKI) on admission; however, one patient survived and the other did not. These cases highlight the severity of influenza-GAS co-infection during a season in which H3N2 subclade K predominated. Early antiviral treatment within 48 hours, vaccination, and high index of suspicion for invasive GAS in influenza patients with shock will be needed.