COVID-19 complicated with severe M1UK-lineage Streptococcus pyogenes infection in elderly patients: A report of two cases

•Severe pneumonia and pericarditis caused by M1UK-lineage S. pyogenes following COVID-19.•A case of pericarditis due to an M1UK variant is first described.•A prior medical history of head/neck cancer is suggested as a potential risk factor.•This report promotes awareness of potential M1UK infection via the respiratory tract. Case 1: A 71-year-old man was admitted to our hospital with progressive fever and dyspnea, which had developed three days after the onset of COVID-19. Initial chest computed tomography (CT) revealed a pulmonary lesion consistent with a secondary bacterial infection. Streptococcus pyogenes was isolated from cultures of primary sputum collected from the endotracheal tube, and identified as the causative microorganism. Case 2: A 91-year-old man was transferred to our hospital with severe hypotension, which had developed nine days after the onset of COVID-19. A chest CT revealed pericardial effusion and pericardiocentesis yielded purulent fluid. S. pyogenes was isolated from the fluid specimens and was identified as the causative microorganism of the secondary bacterial pericarditis. Isolates from both patients were subsequently identified as M1UK-lineage S. pyogenes via genetic analysis. This report implicates COVID-19 as a potential risk factor for severe M1UK infection via the respiratory tract.