The make-up of the gut microbiome may be linked to a person's risk of developing long COVID many months after initial infection with SARS-CoV-2, the virus responsible for COVID-19 infection, suggests a study.

Fatigue, muscle weakness, and insomnia are the most commonly reported long COVID symptoms. According to researchers from the University of Hong Kong, microbiome 'profiling' might help identify those who are most susceptible to developing the condition.

The findings, published online in the journal Gut, showed that 81 bacterial species were associated with different categories of long COVID and many species were associated with more than two categories of persistent symptoms.

For example, at 6 months, persistent respiratory symptoms were strongly associated with several opportunistic 'unfriendly' microbes, including Streptococcus anginosus, Streptococcus vestibularis, Streptococcus gordonii, and Clostridium disporicum. And several species known to boost immunity, including Bifidobacterium pseudocatenulatum, F. prausnitzii, R. inulinivorans and Roseburia hominis, were depleted in those with long COVID at 6 months.

Similarly, several 'unfriendly' bacteria species were associated with poorer performance on the 6-minute walk test among those with long COVID. "Altered gut microbiome composition is strongly associated with persistent symptoms in patients with COVID-19 up to 6 months after clearance of SARS-CoV-2 virus," said Professor Siew C. Ng, Department of Medicine and Therapeutics.

"Considering the millions of people infected during the ongoing pandemic, our findings are a strong impetus for consideration of microbiota modulation to facilitate timely recovery and reduce the burden of post-acute COVID-19 syndrome," Siew added.

The team tracked changes in the gut microbiome of 106 patients with varying degrees of COVID-19 severity, treated at 3 different hospitals between February and August 2020, and in a comparison group of 68 people who didn't have Covid-19, over the same period.

They did this by analysing participants' stool samples. Long COVID was reported in 86 of these patients at three months and in 81 at six months. The most common symptoms at 6 months were fatigue (31 per cent), poor memory (28 per cent), hair loss (22 per cent), anxiety (21 per cent), and sleep disturbances (21 per cent).

Among the 68 patients with Covid-19 whose stool samples were analysed at six months, 50 had long COVID. At 6 months, patients with long COVID had significantly fewer 'friendly' F. prausnitzii, and Blautia obeum and a greater abundance of 'unfriendly' Ruminococcus gnavus and Bacteroides vulgatus than people who hadn't had Covid-19.

On the other hand, the gut microbiome of those who didn't develop long COVID showed only 25 changes in bacteria species at hospital admission, and this recovered completely after 6 months. "This is an observational study, and as such can't establish the cause," the researchers said.