Influenza

There are three main types of influenza virus (Types A, B, and C) that cause infection in humans and these are further characterized into subtypes and strains. The continued emergence of new flu strains each year is due to the ability of the influenza virus to mutate slowly (through small genetic changes called antigenic drift) and quickly through a process called reassortment. Antigenic drift is responsible for the seasonal variations every year and reassortment is responsible for the development of new strains that can cause pandemics.

Influenza type A (Flu A) viruses are especially prone to reassortment due to their wide host range (humans, dogs, birds, pigs, horses, whales, seals, and other animals). Specifically, the Flu A genome is made up of eight loosely linked segments, each of which harbors at least one important gene. Those genes direct the expression of the major viral proteins such as hemagglutinin (HA) and neuraminidase (NA). In the process of viral reproduction, the linkages between the eight segments of the Flu A genome break apart. Since it is possible for two different Flu A strains to infect a cell simultaneously, some of the genetic segments from one strain can be swapped with another during reproduction. For instance, if a human flu virus and a bird flu virus infect a person, reassortment can intermingle genes from both viruses during replication and create a virus with a protein against which humans have little or no immunity, plus human influenza genes that are more likely to cause sustained human-to-human transmission. In contrast, Influenza B (Flu B) and C viruses do not cause pandemics, most likely due to their limited host range of only humans.

Flu A virus is the most common flu virus infecting humans, animals, and birds. It is divided into subtypes, based on the nature of their surface glycoproteins, HA and NA. There are 18 different HAs and 11 NAs that are distinguishable serologically (antibodies to one virus subtype do not react with another). In comparison, Flu B infection mostly occurs in humans and is divided into lineages and strains. Currently circulating influenza B viruses belong to one of the two lineages: B/Victoria and B/Yamagata. This virus is responsible for significant morbidity which is why the seasonal trivalent influenza vaccine contains Flu B as an integral component. Unlike Flu A or B, Influenza C viruses only cause a mild respiratory illness in humans and secondary complications are rare. Flu C is structurally different from Flu A and B viruses and contains a glycoprotein called HEF (hemagglutinin-esterase-fusion).

Influenza viruses are mostly spread by aerosolization made when an infected person coughs or sneezes. Complications usually arise from bacterial infections of the lower respiratory tract and signs of a secondary respiratory infection often appear just as the infected person seems to be recovering. The elderly and the chronically ill are at greater risk for secondary infection and other complications. Children can also experience a rare, but serious complication called Reye’s syndrome.