THE FLU and INFLUENZA VIRUSES

The Flu is caused by a virus that belongs to the Family Orthomyxoviridae. There are three different types of influenza virus:

  1. Influenza A viruse: found in many different animals, including ducks, chickens, pigs, whales, horses, and seals. Wild birds are the primary natural reservoir for all subtypes of Influenza A viruses and are thought to be the source of Influenza A viruses in all other animals.

    This virus is divided into subtypes based on the structure of two surface proteins, hemagglutinin (H) and neuraminidase (N). The subtype is designated: H(x)N(x), where x is the specific structure of that surface protein. There are 15 different H-subtypes and 9 different N-subtypes. Thus, there are 135 possible subtypes in Influenza A virus. The current subtypes of Influenza A viruses found in people are A(H1N1) and A(H3N2).

    New recombinant subtypes can be produced when pigs are infected with two different subtypes. The H and N genes of these two subtypes can reassort producing a third, perhaps novel, subtype. Also, the H and N genes can be altered by genetic mutation. Since vaccines are produced against specific subtypes, if new subtypes are generated either by reassortment or mutation the vaccines looses its efficacy against this novel subtype. The result can be a pandemic.

  2. Influenza B virus: found only among humans. Influenza B virus is not divided into subtypes, and

  3. Influenza C virus: also found only in humans. This virus causes a mild respiratory disease in humans, usually children.

The Center for Disease Control reports: "Influenza types A or B viruses cause epidemics of disease almost every winter. In the United States, these winter influenza epidemics can cause illness in 10% to 20% of people and are associated with an average of 36,000 deaths and 114,000 hospitalizations per year. Getting a flu shot can prevent illness from types A and B influenza. Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics. The flu shot does not protect against type C influenza."

During the last 100 years there have been three Flu pandemics:

  1. 1918-19 Spanish flu [influenza A(H1N1)]: caused the highest number of known flu deaths: more than 500,000 people died in the United States, and 20-50 million people may have died worldwide; a death toll greater than that of the First World War. The flu virus that caused it was very deadly. Many died within the first few days after infection and others died of complications soon thereafter. The Spanish flu was unique because almost half of the people who died were young, healthy adults,

  2. 1957-58 Asian flu [influenza A(H2N2)]: caused approximately 70,000 deaths in the United States. The Asian flu was first identified in late February, 1957 in China and spread to the United States by June, 1957, and

  3. 1968-69 Hong Kong flu [influenza A(H3N2)]: caused approximately 34,000 deaths in the United States. This pandemic H3N2 virus was first detected in Hong Kong in early 1968 and spread to the United States later that year. A(H3N2) viruses still circulate today.

Complications from the Flu depends on the age and health of the individual. Young children may develop croup, pneumonia, and middle ear infections. Most deaths in the elderly are attributable to secondary bacterial pneumonia form Staphylococcus aureus, Streptococcus pneumoniae, and Haemophilus influenzae.

In producing a vaccine, the "current" subtypes of Influenza A virus (H1N1 and H3N2) and Influenza B virus are grown separately in the allantois of chick embryos, inactivated, purified, and pooled. The vaccine contains either the inactivated/noninfectious whole virus (subtypes of Influenza A virus (H1N1 and H3N2) and Influenza B virus ) or the surface proteins (H and N) of each virus.