CPR Renewal
Online Training
New NRP Class
Pandemic Influenza – 1 Contact Hour
Influenza Types and Subtypes
There are three types of influenza viruses: A, B and C, all of which can infect humans. Influenza type A viruses are further divided into subtypes, based on two proteins on the surface of the virus. These proteins are hemagglutinin (HA) and neuraminidase (NA), displayed on the virus’ shell. (Hemagglutinin is sometimes called the “spike” the virus uses to enter a cell, while neuraminidase is the “helicopter blade” that chops off receptors, allowing newly made virus to escape.)
Subtypes of influenza A virus are named according to these proteins. There are sixteen known HA subtypes and nine known NA subtypes of influenza A viruses. Many different combinations of HA and NA proteins are possible, each combination representing a different subtype. For example, an “H7N2 virus” designates an influenza A subtype that has an HA 7 protein and an NA 2 protein. Similarly, an “H5N1″ virus has an HA 5 protein and an NA 1 protein.
Influenza A viruses have infected many different animals, including ducks, chickens, pigs, whales, horses, and seals. However, certain subtypes of influenza A virus are specific to certain species, except for birds, which are hosts to all known subtypes of influenza A. Subtypes that have caused widespread illness in people either in the past or currently are H3N2, H2N2, H1N1 and H1N2.
Antigenic Drift and Shift
Influenza type A viruses undergo two kinds of changes. One is a series of mutations that occurs over time and causes a gradual evolution of the virus. This is called antigenic “drift.” The other kind of change is an abrupt, major alteration in the hemagglutinin and/or the neuraminidase proteins. This is called antigenic “shift,” which produces a novel influenza A subtype.
A global influenza pandemic may then occur if three conditions are met:
- This new subtype is introduced into the human population.
- The virus causes serious illness in humans.
- The virus can spread easily from person to person in a sustained manner.
Influenza type B viruses are usually found only in humans. Unlike influenza A viruses, these viruses are not classified according to subtype. Influenza B viruses can cause morbidity and mortality among humans, but in general are associated with less severe epidemics than influenza A viruses. Influenza type B viruses can cause human epidemics, but, unlike type A influenza, influenza type B viruses change only by the more gradual process of antigenic drift, and therefore do not cause pandemics.
Influenza type C viruses cause mild illness in humans and do not cause epidemics or pandemics. These viruses are not classified according to subtype.
Influenza Strains
Influenza B viruses and subtypes of influenza A virus are further characterized into strains. There are many different strains of influenza B viruses and of influenza A subtypes. New strains of influenza viruses appear and replace older strains. This process occurs through antigenic drift. When a new strain of human influenza virus emerges, antibody protection that may have developed after infection or vaccination with an older strain may not provide protection against the new strain. Therefore, influenza vaccines must be updated on a yearly basis to keep up with the rapid evolution of influenza viruses.
Swine Influenza A (H1N1) and Avian Flu
Influenza A (H1N1 – previously known as swine flu) is caused by the influenza strain called H1N1, which appears to be easily passed from person to person and can quickly infect large numbers. The most common method of transmission is airborne, and it is also possible to become infected by touching a surface with the virus on it and then touching one’s mouth or nose.
Most flu viruses have two genetic elements, but H1N1 has four: two types of swine flu, a bird flu and some human flu genes. However, recent genetic sequencing analyses of this flu have suggested that it is not as virulent as previously suspected. It lacks certain proteins and amino acids that would make it as deadly as some other flus. And it appears similar enough to other common strains that most people may have some immunity.
In contrast to Influenza A (H1N1), the lethal avian flu that has kept world health authorities anxious for years is caused by the H5N1 influenza virus. It has killed 257 of the 421 people who have contracted it, or 61 percent. But it has shown very little ability to pass from person to person, mainly infecting poultry, and some experts have suggested that there may be something about the H5N1 virus that makes it inherently less transmissible among people. Most human cases of H5N1 virus infection are thought to have occurred as a result of direct contact with sick or dead infected poultry.
