flu mutations


First things first, there are different types of flu. There's Influenza A, B, and C.

Influenza C is the least worrisome. It causes illness mostly in small children, which can require hospitalization.

Influenza B is less worrisome. It doesn't cause pandemics, but can cause illnesses requiring hospitalization.

Influenza A is the real problem. Although most cases of the flu aren't that serious, Influenza A can cause pandemics and lead to severe illness.

It includes Swine Flu and Bird Flu.

Influenza Viruses Change, Constantly

Flu viruses constantly change. These changes can be slow and steady or sudden and potentially horrendous.

There are two main types of change 

  • Antigenic Drift

​Drift happens in Influenza A, B, and C.

Antigenic drift occurs slowly. Mutation after mutation occurs, slowly accumulating, slowly drifting the virus away from what it had been. Eventually, the drift will be enough that the immune system that recognized an old version of the virus will not recognize this season's version.

These changes happen frequently enough that your immune system can meet new flu strains each year, even though strains are often related to past strains . Because of this, you need a new flu vaccine each year. The flu vaccine protects you against that season’s three or four most common flu virus strains.

  • Antigenic Shift

Shift only really happens in Influenza A

Antigenic shift occurs suddenly. It is much less common.

Influenza A is found in many animals - birds, pigs, horses, dog, cats, whales, seals, ferrets, and many others - as well as humans. Mixing and matching of strains often occurs between birds -including migratory birds, especially ducks, and poultry - and pigs, often on farms with poultry, and humans.

Shift occurs when two different viruses, likely from different animals, mix in the same cell. These viruses combine to form a new flu subtype, with antigens (that our immune systems use to recognize viruses) from 2 or more strains.

This can create a new flu subtype that our immune system does not recognize. Because most people will not have ever made antibodies to the new subtype, few will be immune and the virus could spread quickly and could cause a very severe flu epidemic or pandemic.

Shift can cause an abrupt change

Antigenic shift can be a huge change in Influenza A. It can result in a new hemagglutinin (HA) and/or neuraminidase (NA) protein on the virus surface. There are at least 18 different HA antigens, which are numbered H1 through H18. There are at least 11 neuraminidase antigens. Influenza A strains are named based on which H or N they contain - like H1N1, H5N1, H3N2, H7N9 etc.

Shift can change the type of Influenza A completely. It can go from H2N2 to H3N2 from acquiring a bird H3 hemagglutinin.

There are different ways Shift can happen

1. A duck infects a pig with influenza. A person transmits influenza to the same pig. The two viruses mix and match genes and end up with a new H and N combination. The strain is then spread from the pig to humans. (This could also happen where the duck and human passed influenza to a chicken instead - or perhaps another animal, since there are many).

2. A strain of bird flu could directly infect a person

3. A strain of bird flu might infect an intermediary animal, like a pig (or from a duck to a chicken), and then this strain hops to people.

This is a big deal

It is thought 1 in 10 or 1 in 20 adults worldwide get the flu each year. 1 in 5 or 3 in 10 children are expected to get the flu each year. If a new strain developed, it might infect more people who might become sicker than they usually would, because they hadn't yet developed an immune response.

Fortunately, Bird Flu H5N1 does not spread easily from person-to-person. There's always a chance one of these shift events could lead to a strain that is new, deadly, and transmits easily from person to person. Hopefully, this doesn't happen. This is why there is so much thought and work put into vaccine design and distribution.

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