The mysteries of seasonal flu

Some years yes, others do not; Why is the flu virus affecting us different every year and the vaccine does not always work well?

The two questions have the same answer: The flu virus (called Influenza) is constantly changing.

These constant changes occur through two phenomena of which we will talk to you today: Antigenic drift and antigenic shift. If you do not remember what an antigen is or how the immune system and vaccines work in general, we recommendyou: Vaccines: How do they really work?

There are four different genera of influenza: A, B, C and D. What changes most drastically and is more virulent is the A, which we present below.

antigen hemmaglutinin neuraminidase lipid layer proteic RNA in capsid 8 fragments the flu variants are named after the antigens H and N

When we talk about changes, we refer to the variations suffered by the two antigens: Hemaglutinin (H) and Neuraminidase (N) proteins. Our immune system needs to detect these antigens to end with the virus, but if they have changed a lot since the last time they were recognized, the body needs to become immunized again and the previous defenses no longer serve.

Which two reasons cause these changes?

  • First of all, the antigenic drift consists in small changes to the antigens H and N caused by mutations in the viral genome. The genetic material of the Influenza virus is formed by RNA rather than DNA, making it more susceptible to changes. These small changes can be accumulated until the immune system no longer recognizes the resulting virus.
  • The antigenic shift is the name given to a radical change in the composition of the virus. It is not caused by specific mutations but by regroupings between the human influenza virus and that of another species. Precisely, the natural hosts of the influenza A virus are aquatic birds. These birds act as reservoirs and mutation/change spots. The antigenic shift typically occurs in intermediaries such as pigs, which are naturally infected by the two variants (avian and human) and allow regrouping.

cycle of avian and human flu, antigenic drift and shift caused by accumulation of mutations and reordering between the two viruses. Swine, Humans, domestic and migratory birds

How is the flu vaccine prepared?

At this point, what’s left to discuss is why some years the vaccines are more effective than others. The flu vaccine is formulated each year by WHO. WHO has a network of national influenza centers and collaborating centers around the world, where the circulating flu viruses are tracked. With the data they obtain, WHO selects the most common viruses and forecasts which ones will most likely affect the following year. This is done twice a year, in February for the countries of the northern hemisphere and in September for those in the southern hemisphere.

Based on the recommendations, a mixture is made between these variants and a safer laboratory virus. As we said, a predictive vaccine is produced. Therefore, the virus affecting the population each season is sometimes different to the predictions so the vaccine does not work efficiently.

At the historical level, what antigenic changes have caused great pandemics *?

  • 1580. First flu pandemic in Europe. The Pope of Rome granted it to the influence of the stars Mars and Venus. That’s why the influenza virus is called Influenza.
  • 1889-1890. Russian flu pandemic. It is not clear what virus caused it.
  • 1918-1919. Spanish flu pandemic. H1N1 virus. It is estimated that it killed between 20 and 50 million people. It it’s called “Spanish” because Spain was the only country that was not at war and published it in the newspapers.
  • 1957-1958. Asian swelling. H2N2 virus. It left about 4 million victims.
  • 1968-1969. Swelling of Hong Kong. H3N2 virus. Third 20th century pandemic in Europe with 2 million victims.

Other new influenza pandemics have been avian influenza (2005, H5N1 virus) and influenza A (2009-2010, H1N1 virus). The “A flu” left 18,000 deaths around the world. Seasonal flu drops between 250,000 and 500,000 every year.

* A pandemic is a disease with cases around the world. An epidemic is when it occurs in regions or more localized areas.

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