People who have severe long COVID have a very different oral and pulmonary microbiome than those with mild symptoms, and the differences are linked to specific symptoms, which scientists say may lead to a clearer picture of the condition and better ways to treat it.

The findings come from an observational study by the longitudinal, multicenter UK Post-hospitalisation COVID-19 (PHOSP-COVID) Collaborative Group and published yesterday in the European Respiratory Journal.

The researchers recruited 771 adults, grouping them by severity of ongoing health problems and symptom subgroups. Six months after hospital release, the team took saliva samples from the patients’ mouths and sputum samples from their lungs for sequencing and bacterial-diversity analysis, and participants completed symptom questionnaires.

The study participants had been released from one of 83 UK hospitals after COVID-related admission from February 2020 to March 2021. The average participant age was 57.5 years, and 38.5% were women.

“Acute COVID-19 is a pulmonary disease with the infection phase known to alter the oral and airway microbiota, with shifts in microbial composition linked to disease severity and immune responses,” the authors wrote. “These microbial communities are not isolated; they interact with other organ systems through communication pathways such as the gut-lung and the oral-brain axes, which have been associated with other chronic diseases.”

In the group with very severe long-COVID symptoms, the oral microbiota was significantly less diverse than that in the group with mild symptoms at six months post-hospitalization. This group had the highest frequency of breathlessness, fatigue, impaired lung function, and reduced exercise capacity.