There’s been a lot of talk about the risk factors for developing serious illness from Covid-19. Age, underlying conditions and even pregnancy are just some of the factors that can raise your risk of negative health outcomes.

But a new study suggests the variety and volume of bacteria in the gut – known as the microbiome – may also influence the severity of Covid-19, as well as the magnitude of the immune system response to the infection.

While Covid-19 is primarily a respiratory illness, we know it can also impact other parts of the body.

The gut is the largest immunological organ in the body and past research has shown its importance in regulating the immune system. For this new study, researchers wanted to find out if the gut microbiome might also affect the immune system response to Covid-19 infection. 

Researchers obtained blood and stool samples and medical records from 100 hospital inpatients with laboratory-confirmed Covid-19 between February and May 2020 – and from 78 people without Covid-19 who were taking part in a microbiome study before the pandemic. 

The severity of Covid-19 in these patients was classified as mild (in the absence of x-ray evidence of pneumonia); moderate (where pneumonia with fever and respiratory tract symptoms were detected); severe (if patients found it very difficult to breathe normally); and critical (if they needed mechanical ventilation or experienced organ failure requiring intensive care). 

To characterise the gut microbiome, 41 of the Covid patients provided multiple stool samples while in hospital – of these, 27 provided stool samples up to 30 days after clearance of the virus.

Analysis of the samples showed the make-up of the gut microbiome differed significantly between patients with and without Covid-19.

The research, published online in the journal Gut, found:

• Covid patients had higher numbers of Ruminococcus gnavus, Ruminococcus torques and Bacteroides dorei species than people without the infection. 

• They also had far fewer of the species that can influence immune system response, such as Bifidobacterium adolescentis, Faecalibacterium prausnitzii and Eubacterium rectale.

• Lower numbers of F. prausnitzii and Bifidobacterium bifidum were particularly associated with infection severity.

Dr Jenna Macciochi, an immunologist and the author of Immunity: The Science of Staying Well, tells HuffPost UK the paper “represents another piece in the Covid-gut microbiome puzzle”.

“Earlier studies have shown gut microbiome dysbiosis [a microbial imbalance] in patients with severe Covid but we don’t yet know if this is a predisposing factor or a consequence of severe Covid,” she says. 

“There is also lots of evidence for certain gut microbial species in supporting proper immune function and preventing respiratory tract infections.”

She points out the study is quite small, which is a limitation – and research also focused on species of bacteria which were noted to be different in Covid patients. “It’s very difficult to ascertain the impact of a specific species since we know that the overall output of the microbiome is important and different bugs might do different things in different people,” she said.

This is something the researchers also acknowledged. The gut microbiome varies widely among different populations, they said, so the changes observed in this study may not be applicable to other Covid patients.

“I think the most interesting thing about this study is just the fact that it underscores the importance of the gut microbiome to the immune system,” says Dr Macciochi.

Analysis of blood samples in the study showed the microbial imbalance found in Covid patients was also associated with raised levels of inflammatory cytokines and blood markers of tissue damage.

Researchers said this suggests the gut microbiome might influence the immune system response to Covid-19 infection and potentially its severity and impact, including symptoms widely referred to as long Covid.

“In light of reports that a subset of recovered patients with Covid-19 experience persistent symptoms, such as fatigue, dyspnoea [breathlessness] and joint pains, some over 80 days after initial onset of symptoms, we posit that the dysbiotic [imbalanced] gut microbiome could contribute to immune-related health problems post-Covid-19,” they wrote.

So what can we do?

While the study was observational, researchers pointed to mounting evidence showing that gut microbes are linked to inflammatory diseases within and beyond the gut.

They said: “Bolstering of beneficial gut species depleted in Covid-19 could serve as a novel avenue to mitigate severe disease, underscoring the importance of managing patients’ gut microbiota during and after Covid-19.”

In light of the study, Dr Macciochi suggests we should take care of our gut health “as best we know how”. She recommends diversifying your diet with fibrous plant-based and fermented foods, and avoiding things that could harm the microbiome like antibiotics, stress and over-exercising.

The Covid Symptom Study app also recommends limiting ultra-processed and junk food to keep your gut healthy.