Influenza, the ABCs of viruses

Three types of influenza viruses are known, called A, B and C. They have different characteristics and different dangerousness. Let’s get to know them better.

The history of influenza began in 1933, in Great Britain, when the first influenza virus that affects humans was identified.

In fact, influenza viruses had already been isolated in chickens or pigs before, and of course influenza, as a disease, had existed for centuries.

From the 30s to today, scientists have identified multiple types of viruses, which have been classified into three families: type A, B and C.

The three families in detail

• Type A viruses are the most common. They circulate in the human body, but also in that of other animal species, such as birds, pigs and horses. Essentially they are avian viruses, some of which have adapted to humans, but can also pass through pigs, as seems to have happened in 2010 with the new influenza originating in Mexico (H1N1). They are responsible for the most serious forms of influenza and can cause epidemics and pandemics.
• Type B viruses circulate only in the human body. They too can cause epidemics, but they do not seem to be able to induce pandemics, i.e. large-scale infections. They generally result in a less severe form of influenza than that due to type A viruses, although they have the potential to become dangerous if they mutate.
• Type C viruses are not linked to pandemics or large epidemics, and usually cause asymptomatic or very mild infections, therefore with little clinical relevance.

While type B and C viruses have no subtypes, type A viruses have different H or N proteins on their surface, and are therefore divided into subtypes.

To date, there are three subtypes of A viruses, related to serious pandemics of human influenza, namely the H1N1 viruses, linked to the flu called Spanish, which gave rise to the pandemic that occurred in 1918, the H2N2 (cause of the Asiatica pandemic of 1957) and the H3N2 viruses also called Hong Kong, responsible for the 1968 pandemic.

Dangerous mutations

One of the peculiar characteristics of influenza viruses is their ability to mutate. The mutations mainly affect surface and H and N antigens, making it more difficult for our immune system to recognize the virus.

Changes can occur through two different mechanisms:

• The first, called antigenic drift (or antigenic drift), occurs continuously in both A and B viruses, is gradual and leads to small mutations in surface protein structures. It is the phenomenon responsible for seasonal virus variations.
• The second, called antigenic shift, occurs only occasionally (every 10-30 years), affects only the A virus and produces new viral varieties, with one or both surface structures H and N completely replaced due to an “exchange” between a human and an animal virus.

These new viruses, therefore, completely unknown to the human immune system, are able to transmit quickly and over great distances in all age groups, this could cause a pandemic (an epidemic that spreads on a global scale).

When the simultaneous change of the two surface antigens occurs, the influence is particularly dangerous, as happened in 1918 with the H1N1 virus and in 1957, when the H2N2 subtype appeared, but also in 1968, when Hong Kong occurred, due to the H3N2 subtype, less severe than the previous ones because the shift involved only hemagglutinin (H) and antibodies against neuroamidase N2 were present in part of the population.

Fortunately, the most frequent mutations are the minor ones, which produce small variants of viruses A and B, to which the population has a certain degree of immunity.