Why does the flu affect some more than others?

It may be due, in part, to childhood exposure to the flu virus. This immunological imprint makes the next contagion more bearable.

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We find ourselves in the middle of the flu season and surely several co-workers have fallen ill, or even ourselves have had to stay at home. It is also quite possible that people around us are not affected at all or have had very mild symptoms. Why does this happen? Why does the flu affect some and not others? Part of the answer, according to a new study published in PLoS Pathogens, is related to the first strain of flu that we get in childhood. Their findings could help improve strategies to minimize the effects of seasonal influenza.

Scientists at the University of California, Los Angeles (UCLA) and the University of Arizona have discovered that exposure to flu viruses during childhood offers people partial protection for the rest of their lives. Biologists call this an immunological imprint.

The discovery undermines the widespread belief that previous exposure to a flu virus protects little or nothing from strains that can pass from animals to humans, such as avian influenza and swine flu. It is precisely these strains that are of greatest global concern because they could mutate and not only jump easily from animals to humans but also spread more rapidly amongst humans.

In the study, scientists studied two subtypes of the influenza virus: H3N2 and H1N1, which are mainly responsible for seasonal influenza outbreaks over the last few decades. The first usually affects the elderly, is the one that causes the most complications and most deaths from the flu. H1N1 usually affects young and middle-aged people and causes fewer deaths.

The researchers analysed the health records that the Arizona Department of Health Services obtains from hospitals and private doctors and detected a pattern. They found that people who were first exposed to the least severe strain, H1N1, during their childhood were less likely to end up in hospital if they came back to H1N1 later than people who were first exposed to H3N2. Those who were first exposed to H3N2 experienced the same thing, that is, they were more protected against this virus throughout their lives. From this they deduced that the protection against future infections is much stronger when one is exposed to strains of the same group that the body has fought before, although it is also true that infection of one strain makes our immune system better prepared to fight infections of another strain.

The study found another pattern that puzzled scientists: people who were exposed when they were younger to H2N2, a strain very close to H1N1, did not have any extra protection against H1N1 when they contracted it during adulthood. It was disconcerting because H2N2 and H1N1 are two subtypes found in the same group and previous studies had shown that exposure to one could, on occasion, significantly protect from the other.

"Our immune system often struggles to recognize and defend itself from closely related seasonal flu strains, although these are essentially the genetic sisters and brothers of the strains that circulated just a few years ago", said lead author Katelyn Gostic, who was a doctoral student at UCLA at the time of the study and is now a postdoctoral fellow at the University of Chicago. "This is disconcerting because our research on avian influenza shows that deep in our immune memory we have a certain capacity to recognize and defend ourselves against third-genetic cousins, related at a distance, of the strains we experienced as children.

Gostic hopes to find clues leading to the development of a universal flu vaccine by studying the differences between immunity against avian influenza and immunity against seasonal influenza. In the first our immune system is able to protect us effectively whilst with the other, not so much.

"The last two flu seasons have been more severe than expected," said Michael Worobey, co-author of the study and head of the department of ecology and evolutionary biology at the University of Arizona. In the 2017-18 season, 80,000 people died in the U.S., more than in the 2009 swine flu pandemic.

Amongst the most effective measures to prevent us from catching the flu and to help spread the virus is getting all the necessary vaccines, covering our mouths when coughing and sneezing with our arms (not our hands), washing our hands frequently with soap and water and maintaining healthy lifestyles.

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