Prophylactic vaccinations

The basis of influenza prevention is protective vaccinations. The key to their action is stimulation of the immune system of the body to the production of antibodies against viral antigens. The body’s own antibodies (not the vaccine itself!) are able to neutralise the wild virus particles in the layer of mucus covering the respiratory epithelium, after the virus enters the human respiratory tract. The disease is blocked even before it starts and the antigen-antibody complexes are rapidly removed with the mucus. The vaccine changes the immunity status of the body from the status before contact with the virus (“naive” status) to the immune status, and leaves the body resistance as if it underwent the disease.

To attain this objective, the vaccine must contain virus particles (fragments) in the inactivated form (i.e. dead). In vaccine production, large quantities of the virus are obtained on chicken embryos as the source of cells. The virus is then thickened and purified, and the viral RNA is inactivated. This “whole virus vaccine” is then further processed.

The basic rule of vaccinations: the patient is administered viral hemagglutynin and neuraminidase; then the immune system produces antibodies: anti-hemagglutynin and anti-neuraminidase which ensure protection if a live virus will try to attack. The body is immune if it possesses the sufficient amount of antibodies protecting against the merger of the virus with the susceptible cell.

History of the influenza vaccine

1933	Discovery of human type A influenza virus
1935-1937	Firs experiments with the raw material containing the virus
1940	Discovery of the human type B influenza virus
1943-1945	First extensive clinical studies on the whole virus vaccine in the American army
1945	First registration of the influenza vaccine (USA)
1964	First experimental split virion vaccine
1966	Considerable progress in vaccine purification (zonal centrifugation)
1976	First experimental subunit vaccine (registered in 1980)
1995	First experimental liposome vaccine (registered in 1998)

In 1966, a new purification method (zonal centrifugation) became available. This method makes it possible to remove most of the pyrogenic impurities (e.g. egg protein, cell remnants). Figure 11 presents an electron microscopic photograph of the influenza vaccine (A) before and (B) after the use of zonal centrifugation. In photograph (A), the vaccine seems to contain mainly impurities (common adverse effects are caused by such vaccine) and sometimes a virus particle, in photograph (B), the vaccine consists of a large number of virus particles practically without impurities (much less adverse effects caused by the vaccine).

A. B.

Types of influenza vaccines

Split virion vaccines

To lower the internal reactogenicity of the influenza vaccine further, split virion vaccines were developed in 1960s. The particles were “split” and the viral envelope could be partially separated from the membrane fragments containing significant membrane proteins.

Subunit vaccines

In 1970s, it became clear that antibodies directed against the membrane hemagglutinin protein (and to a lesser extent against neuraminidase) were significant in the development of immunity against infections. Therefore, the vaccine containing only hemagglutinin and neuraminidase, without other ingredients of the virus, was developed. After the membrane and detergent are removed, isolated subunits of hemagglutinin and neuraminidase spontaneously form rosettes.[1] This is the subunit vaccine.

[1] These rosettes are very important for the formation of antibodies. The immune system cells prefer antigens presented in the spherical form. Small isolated protein particles have weak immunising properties, i.e. they are not easily recognised by the immune system. In this form, the subunit vaccine would elicit a weak immune response. But the rosette form is for the immune system as a small sphere, which leads to the production of antibodies similar to their product after the vaccine containing the whole virus is administered.

Adjuvant subunit vaccine

Adjuvants are substances which enhanced the immune response to hemagglutynin and neuraminidase in persons who otherwise show only weak response. The MF59 vaccine (registered for the first time in 1997) is a subunit vaccine in hydroethanolic emulsion of squalene. Droplets of squalene oil are used as vehicles of viral antigens hemagglutinin and neuraminidase.

Virosomal vaccine (registered for the first time 1998)

The combination of the liposome and viral antigens is called a virosome. Liposomes are small empty spheres consisting of a non-toxic bi-layer lipid membrane. Viral antigens hemagglutynin and neuraminidase are anchored in this lipid membrane, which imitates the natural virus particle.