Influenza, an ever-evolving familiar

Influenza is a contagious disease caused by an RNA virus called influenza virus that causes fever, cough, nasal congestion, muscle aches, and general malaise in people over 80 years old, children under 1 year old, and people who are immunosuppressed. Speaking of particularly serious individuals… There are four types of influenza viruses (A, B, C and D), and from a clinical and epidemiological point of view, the first two are the most important.


Types of influenza viruses

Type A is the most common and causes most flu symptoms. It is characterized by a genome composed of eight RNA segments and a capsid composed of two important proteins: hemagglutinin (HA), which enables it to invade the cells of the infected host, and neuraminidase (NA), which destroys the infected host’s cells. Infected cells continue to spread through the tissue.

Eighteen hemagglutinins and 11 neuraminidase enzymes have been described, which can combine with each other to produce different isoforms. Currently, the most common ones in humans are A(H1N1) and A(H3N2), while there are other subtypes in animals. The one that attracts the most attention is the avian A(H5N1) virus, which causes high mortality. Globally, wild and domestic birds over the past 20 years and marine mammals over the past 2 years. Of note, migratory waterfowl are reservoirs for all influenza A subtypes except A(H17N10) and A(H18N11), which were isolated from bats several years ago.

Type B is the second most important and, like type A, is characterized by eight RNA segments and a capsid formed by hemagglutinin and neuraminidase. However, in this case, they only affect humans and seals, and eventually pigs. There are two lineages called the Victoria lineage and the Yamagata lineage.

Types C and D are less common and do not produce epidemics or major clinical symptoms. They have seven RNA segments. Since 2011, type C has appeared in pigs and humans, and type D has appeared in pigs and cattle.


Its genetic evolution is based on two mechanisms. The first is due to point mutations (errors that occur when transcribing RNA) that accumulate over time. The second mechanism is the recombination of different subtypes of the same type of virus, that is, the exchange of genome segments when two different subtypes infect the same cell.. Recombination leads to important evolutionary leaps that can lead to epidemics.

Influenza generally occurs seasonally, with peak prevalence in winter (between December and February in the Northern Hemisphere, as shown in Figure 1), associated with falling temperatures, maximum time spent in closed spaces with poor ventilation, and time spent with family and friends. Spend the same amount of time together. Social gatherings during the Christmas season. These outbreaks are often caused by various types and subtypes of influenza viruses that change from year to year.

In Figure 2, you can see how over the past decade, three types of viruses have alternately been the primary cause of these seasonal influenza epidemic peaks: A(H1N1), A(H3N2), and B. A dominant virus that usually disappears within a year is of significance in the following years because of its ability to generate population immunity.

despite this, Influenza pandemics occur when new recombinant viruses emerge. Undoubtedly the most famous is the Spanish Flu (misnamed because it originated in a military camp in Kansas) caused by the avian influenza virus A(H1N1).

Other major influenza pandemics include the 1891 Russian flu, possibly caused by an A(H3N8) virus, the 1957 Asian flu, caused by an A(H2N2) avian virus, and the 1968 Hong Kong flu, caused by a recombinant avian virus. ) and the 2009 swine flu, caused by recombinant A(H1N1) viruses from different sources of human, avian and swine viruses.

In 1952, the World Health Organization launched the Global Influenza Surveillance System (GISRS, Global Influenza Surveillance and Response System). The system has been improving over time and, in the case of Spain, has been designated in SiVIRA (Sentinel Surveillance for Acute Respiratory Infections) since 2020. These systems combine syndromic surveillance (i.e., counting patients presenting to primary care and hospital emergency departments with respiratory clinical symptoms) and sentinel networks (networks of diagnostic laboratories selected by primary care physicians to collect representative samples of samples for analysis of pathogens). . Determining the viruses circulating each season in one hemisphere is key to designing influenza vaccines for the next season in the other hemisphere.

The coronavirus pandemic has had a significant impact on seasonal influenza. Due to preventive measures (use of masks, ventilation of closed spaces, increased outdoor activities…), almost no influenza cases were recorded in the 2020-2021 and 2021-2022 seasons, and again in 2022-2023 and 2022. The current 2023-2024 season, the gradual easing of these measures returns us to pre-pandemic epidemiological conditions, with influenza A (H1N1) being the predominant subtype.

Remarkably, the application of these protective measures combined with influenza vaccination was sufficient to nearly eliminate the Yamagata lineage of influenza B viruses.

Another important advance brought about by Covid is the availability in pharmacies of rapid diagnostic tests that allow economic differentiation of different respiratory pathogens (influenza A and B viruses, SARS-CoV-2 and respiratory syncytial virus).

As we can see, there is nothing new under the sun, and when we throw aside the precautions we have taken in the wake of the COVID-19 pandemic, the flu has returned to its usual behavior.

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