Vaccination

VACCINATION

Vaccination is undoubtedly the most successful measure against microbial infections, and particularly viral infections. Remarkably, human protection against smallpox was achieved by Jenner in 1796 with the inoculation of cowpox, well before the ‘germ theory’ of disease was postulated. A worldwide vaccination program initiated in 1966 led to the eradication of smallpox in 1980. The poliovirus is almost completely eradicated following an intensive worldwide vaccination programme by the World Health Organization.

Vaccines are preparations containing antigens that elicit a specific and active immunity against an infecting agent; they can induce the innate and the adaptive (cellular, humoral) parts of the immune system. There are currently a number of viral vaccines available against a diverse range of human viruses (Table 5.6). The success of vaccination relies on the prevention of a disease from occurring. Vaccination is particularly indicated to protect persons at risk (e.g. hepatitis B, varicella zoster vaccines for healthcare workers), prior to travelling (hepatitis A virus, Japanese encephalitis, yellow fever, tickborne encephalitis virus) or to prevent cancer (human papillomavirus vaccine preventing cervival cancer). A vaccine can also be given to protect from a viral outbreak (e.g. measles, mumps and rubella vaccine—MMR) or following exposure to rabies.

Viral vaccines are prepared using different methods, the most common being the use of live attenuated viruses, inactivated viruses or the use of viral components. Viruses or their components can be prepared from animals, fertilized hen’s eggs, in suitable cell cultures (see section 5) or in suitable tissues, or by culture of genetically engineered cells. MMR vaccine and live (oral) poliomyelitis vaccines are based on the use of live attenuated viruses, which are not as virulent as the original virus. The attenuated viruses will cause a strong immune response without causing the disease. Hepatitis A virus and influenza vaccines rely on chemically inactivated (formaldehyde, propiolactone) virus particles or components (surface antigens). Hepatitis B virus vaccine is a recombinant vaccine where the viral DNA encoding for a virus surface antigen (HBsAg) is expressed in yeast (Saccharomyces cerevisiae) or mammalian cells (Chinese hamster ovary cells or other suitable cell lines). The surface antigen is then purified. Human papillomavirus (HPV) vaccine contains virus-like particles and recombinant capsid protein expressed in yeast or using a baculo-virus as an expression system.

It should be noted that most viral vaccines contain one or more adjuvants, e.g. aluminium salts (antigens are absorbed to the aluminium salts), monophosphoryl lipid A, to increase or modulate the host immune response to the antigens.

Immunoglobulin may play a role in the protection of patients with a compromised immunity against viral infections. Human normal immunoglobulin (HNIG) is prepared from a pool of donated human plasma that has been checked to be non-reactive for hepatitis B surface antigen, hepatitis C virus and HIV (types 1 and 2) but contains immunoglobulin G (IgG) and antibodies against viruses that are prevalent in the general population including hepatitis A, measles, mumps, rubella and varicella. Intramuscular normal immunoglobulin is thus used to protect against hepatitis A virus in immuno-compromised patients visiting areas where the disease is highly endemic and to protect against or attenuate measles infection in immuno-compromised patients. It can also be used for pregnant women against rubella virus, where the risk of termination of pregnancy is unacceptable.

Disease-specific immunoglobulins are prepared from a pool of plasma obtained from specific human donors who have high levels of the specific antibody required. Disease-specific hepatitis B immunoglobulin is used following accidental inoculation by a risk material (e.g. needle stick injury) or for infants born from mothers infected with the virus. Disease-specific rabies immunoglobulin is available following the bite of an animal suspected of carrying the disease or originating from an area where the disease is endemic. Disease-specific varicella zoster immunoglobulin is indicated for individuals who are at a high risk such as neonates whose mothers develop chickenpox, or for those exposed to the virus while requiring intensive care or prolonged special care, and for immuno-compromised individuals.