The route of administration of vaccines varies according to the type of vaccine. Most vaccines are given by intramuscular injection.
1. By mouth. Oral polio vaccine (OPV) and oral typhoid vaccine are given by mouth.
2. Intramuscular and subcutaneous injection. With the exception of oral vaccines and BCG, all vaccines available currently should be given by intramuscular injection or by deep subcutaneous injection. The site of injection is important: the upper arm (the deltoid region) or the anterolateral aspect of the thigh are strongly recommended, and not the buttock. The injection of vaccine into deep fat in the buttocks is likely, particularly with needles shorter than 5 mm, and there is a lack of phagocytic or antigen-presenting cells in layers of fat. Another factor may involve the rapidity with which antigen becomes available to antigen-processing cells from deposition in fat, leading to delay in presentation to T and B cells. An additional factor may be denaturation of antigen by enzymes as a result of deposition in fat for many hours or days. This is well illustrated in the case of hepatitis B vaccines. There are over 100 reports of low antibody seroconversion rates after hepatitis B immunisation into the buttocks. One comprehensive study in the USA showed that participants who received the vaccine in the deltoid had antibody titres that were up to 17 times higher than those who received the inoculations into the buttock. Furthermore, those who were injected in the buttock were 2-4 times more likely to fail to reach a minimum antibody level of 10mIUl_1 after vaccination. There are also reports which have implicated injection into the buttock as a possible factor in failure of rabies postexposure prophylaxis using a human diploid cell rabies vaccine. Finally, injection into the buttock, particularly at a site other than the upper outer quadrant, carries the risk of damage to the sciatic nerve.
Injection by the deep subcutaneous route and not intramuscularly is recommended for those with blood coagulation defects, e.g. patients with haemophilia.
3. The intradermal route. BCG vaccine is always given intradermally. A number of other vaccines, such as rabies vaccine, may also be given intradermally. However, apart from BCG vaccine, the use of the intradermal route for immunisation is controversial, particularly in the case of hepatitis B vaccine. One of the reasons for using the intradermal route, apart from BCG, is the possibility of reducing the antigen required for immunisation and thereby the cost of the vaccine. Another reason is more rapid presentation of the antigen to the immune system, resulting in a mac-rophage-dependent T-lymophcyte response via specific epidermal cells. In the case of hepatitis B vaccines, the immunogenicity of the vaccine given in doses of 0.1ml (2 |g of antigen protein) has been clearly demonstrated. However, the booster injection at 6 months resulted in antibody titres which were 10 times higher after intramuscular injection than after intradermal inoculation. And there are data indicating that the decline in anti-HBs titre is inversely related to the antibody level attained after completion of primary immunisation. Indeed, trials of intradermal hepatitis B vaccine in children in Gambia have shown a high proportion (59%) of low protective antibody response of less than 10mIUl~i (i.e. failure to respond to the vaccine) in one study, and relatively poor antibody responses, as measured by geometric mean antibody titres after intradermal inoculation, in another study. In addition, intradermal inoculation in another requires skill and inadvertent subcutaneous injection into fat will result in a poor immune response.
Adverse reactions after intradermal injection are not marked, although painful local reactions at the site of injection can be severe. It should also be noted that international and national requirements for vaccine manufacture and licensure require assurance on safety, immunogenicity, and protective efficacy of the recommended dosage by an approved route of administration. It seems imprudent to ignore these requirements and recommendations.
Finally, the use of jet injectors for immunisation is not recommended. Current advice is that until studies clarify the risk of infection (such as with hepatitis B and the human immunodeficiency viruses) by different types of injectors, their use should be restricted to special circumstances where it is essential to immunise a large number of persons within a short time.
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