SARS‑CoV‑2 and Mycoplasma pneumoniae co‑infection during the early COVID‑19 pandemic: diagnostic bias leading to delayed recognition

Co‑infections during the early COVID‑19 pandemic were frequently under‑recognized due to diagnostic bias, limited availability of pathogen‑specific testing, and the overwhelming clinical focus on SARS‑CoV‑2. We report a case of co‑infection with SARS‑CoV‑2 and Mycoplasma pneumoniae in a previously healthy 35‑year‑old man who presented with febrile respiratory illness in April 2020. Chest computed tomography demonstrated multiple small ground‑glass opacities, and serology showed a M. pneumoniae PA titer of 1:640, leading clinicians to initially diagnose M. pneumoniae pneumonia. However, when the SARS‑CoV‑2 PCR result was subsequently recognized on day 6 of illness, diagnostic anchoring toward COVID‑19 occurred, and M. pneumoniae was downgraded to a secondary consideration despite supportive radiological and serological evidence. This sequence contributed to premature closure and base‑rate neglect, with co‑infection formally acknowledged only after both results became available. The patient improved with supportive care and appropriate antimicrobial therapy. This case illustrates how cognitive biases and pandemic‑related diagnostic constraints contributed to delayed recognition of co‑infections during the early pandemic. In contrast, current Omicron‑lineage variants exhibit attenuated severity and low ascertainment, making co‑infections in hospitalized patients potentially more clinically significant yet easily overlooked. Awareness of diagnostic pitfalls remains essential for accurate evaluation of respiratory infections in the contemporary era.