The flu, it's genetic too

Certain people exposed to the influenza virus will remain healthy, while others will be less lucky and become very sick and out of commission for several days.

A recent study suggested that these different responses are due to the distinct expression of certain genes involved in defending the body from the influenza virus.

From the end of November to the beginning of March, the cold and low humidity create the perfect breeding ground for the flu to be transmitted. The word "influenza" also comes from the term influenza di freddo (the influence of cold), an Italian expression used in the 18th century to illustrate the predominance of this infection during cold seasons.

It's the droplets caused by coughing and sneezing that are the primary flu-causing agents; a simple cough can create up to 100,000 viral particles and this number can reach two million during a sneeze. The flu is therefore very contagious, infecting between 5-15% of the population each year.

Infection in the cells of the respiratory tract caused by the flu generally causes a number of clinical symptoms, the most common being runny nose, sore throat, fever and general malaise. This response to the virus can vary considerably from one person to another. So, while many people are knocked out for days by the flu, between 30-50% of people are much more tolerant and only present with moderate symptoms. It is therefore likely that these individuals are better able to control infection and eliminate the virus from their system.

To better understand this phenomenon, a group of American researchers examined the expression of many genes in people infected by the flu virus. The H3N2 virus, which is responsible for the seasonal flu, was given nasally to 17 volunteers in good health and blood tests were taken every eight hours for five days following infection. These blood samples were then analyzed.

Of the 17 people infected, nine became sick while the others presented with no symptoms, despite receiving a similar quantity of the virus. The researchers observed that the different responses are directly linked to the spectacular differences in the expression of many genes. In sick people, it is the genes involved in the production of inflammatory compounds and stress response that are primarily active, though they are suppressed in the resistant individual. In these people, the genes activated unleash an anti-inflammatory and antioxidant response instead.

These two "molecular signatures" are so distinct that it is possible to determine if the person will be sick or not only hours after infection.

Even if other studies are needed to better understand these different genetic expressions, these observations could eventually aid in the development of diagnostic tools able to determine the severity of a flu as soon as it appears, a move that could lead to early intervention in the most serious cases.