The abundance of a specific intestinal bacteria known to suppresses the binding of the coronavirus to human cell receptors is likely to play a role in the low COVID-19 mortality rates seen in Asia and Northern Europe, according to a study led by a team of researchers at Nagoya University.

Many scientists have speculated there may be an X-factor when it comes to the low death rates from COVID-19 in Asia, including Japan, and some countries in Northern Europe such as Finland. Genetic and immunological differences, as well as the taking of the BCG vaccine in early childhood to protect against tuberculosis, have often been cited as possible reasons. An in-vitro study by the Kyoto Prefectural University of Medicine has also shown that compounds in green tea known as catechins significantly reduced the infectiousness of the coronavirus.

The highly contagious omicron variant is known to lead to less serious cases than the delta variant, and the mortality rate has been lowered to be more or less comparable to that of influenza.

But the death rate from COVID-19 tends to go up with age. Other risk factors include obesity, diabetes, smoking and a history of respiratory infections, though COVID-19 vaccination curbs the possibility of severe illness and death.

To shed light on what the mysterious factor may be behind the low death rates in some countries, Nagoya University scientists analyzed raw sequencing data of gut micro-organisms in 953 healthy subjects in 10 countries from a public database.

The team analyzed the relationship between the composition of intestinal bacteria and the mortality rates of COVID-19, applying an advanced machine learning model in February 2021, when vaccines were not yet commonly available. It analyzed 30 important intestinal bacteria and found that having the lowest amount of one called collinsella was the highest predictive factor behind high COVID-19 mortality rates, with a markedly high statistical significance.

The scientists then categorized the data into five types of gut bacteriological ecosystems, called enterotypes, based on the compositional similarity of their micro-organisms. They compared them with the mortality rates of the 10 countries and found that the level of collinsella was negatively correlated with the mortality.