Influenza is caused by an RNA virus of the myxovirus group. Influenza viruses can be classified into three types (A, B and C), based on antigenic differences in the nucleoprotein and the matrix protein. Type A and B influenza viruses each contain 8 RNA segments, while type C only has 7 RNA segments. Influenza A is most important and is very pathogenic for man, as well as for animals, for example pigs and horses. Type B influenza causes disease in humans. Influenza C is less severe and has been isolated from humans and pigs. The virus is transmitted through the air, mainly in droplets expelled during coughing and sneezing. The influenza viruses cause an infection of the respiratory tract, that is usually accompanied with coughing, high fever and myalgia. Although an influenza infection does not often lead to the death of the infected individual, the morbidity can be severe. As a consequence thereof influenza epidemics may lead to substantial economic loss. Furthermore, influenza infection can be more dangerous for certain groups of individuals, such as those having suffered from a heart attack, CARA patients or the elderly. A vaccine against influenza is therefore highly desirable.
The influenza A virus contains in its membrane two highly immunogenic, but very variable proteins, the hemagglutinin and the neuraminidase. Due to the variability of these two proteins a broad spectrum, long lasting vaccine against influenza A has so far not been developed. The influenza vaccine commonly used, has to be adapted almost every year to follow the antigenic drift of the virus. In these circumstances the vaccine can protect about 80% of the immunized persons. When more drastic changes occur in the virus, known as antigenic shift, the vaccine is no longer protective.