This is p robaby a dumb question but it occurred to me today that I don’t completely understnad why we get immunization injections in the muscle. Wouldn’t it work the same intraveneously? Does it have something to do withthe slow release of bacterium? OR perhaps the need for a smaller needle and in turn less trauma to the patient? Something completely different? Just wondering.
It’s largely a question of balancing speed of onset (as in pain meds) vs duration of effectiveness and minimizing side effects.
There’s a graph showing this better than I can explain it.
Also, some meds are not suitable for intravenous (IV) injection. Anabolic steroids, for example, are about as thick as cold pancake syrup. If you inject that into the bloodstream, it’s likely to clog up the first capillaries it hits, which will be in the lungs, assuming a venous, rather than arterial injection.
Injected intramuscularly, (IM) the stuff is able to diffuse though muscle tissue, where it is more slowly absorbed, tiny bit by tiny bit, by the capillaries in the muscle.
Basically, the route that is chosen for the administration of any vaccine is the one that is believed to combine the maximum safety with the maximum effectiveness.
Here is some pertinent information from the CDC (taken from http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5102a1.htm)
Recommended Routes of Injection and Needle Length
Routes of administration are recommended by the manufacturer for each immunobiologic. Deviation from the recommended route of administration might reduce vaccine efficacy (53,54) or increase local adverse reactions (55–57). Injectable immunobiologics should be administered where the likelihood of local, neural, vascular, or tissue injury is limited. Vaccines containing adjuvants should be injected into the muscle mass; when administered subcutaneously or intradermally, they can cause local irritation, induration, skin discoloration, inflammation, and granuloma formation.
Subcutaneous Injections
Subcutaneous injections usually are administered at a 45-degree angle into the thigh of infants aged <12 months and in the upper-outer triceps area of persons aged >12 months. Subcutaneous injections can be administered into the upper-outer triceps area of an infant, if necessary. A 5/8-inch, 23–25-gauge needle should be inserted into the subcutaneous tissue.
Intramuscular Injections
Intramuscular injections are administered at a 90-degree angle into the anterolateral aspect of the thigh or the deltoid muscle of the upper arm. The buttock should not be used for administration of vaccines or toxoids because of the potential risk of injury to the sciatic nerve (58). In addition, injection into the buttock has been associated with decreased immunogenicity of hepatitis B and rabies vaccines in adults, presumably because of inadvertent subcutaneous injection or injection into deep fat tissue (53,59).
For all intramuscular injections, the needle should be long enough to reach the muscle mass and prevent vaccine from seeping into subcutaneous tissue, but not so long as to involve underlying nerves and blood vessels or bone (54,60–62). Vaccinators should be familiar with the anatomy of the area into which they are injecting vaccine. An individual decision on needle size and site of injection must be made for each person on the basis of age, the volume of the material to be administered, the size of the muscle, and the depth below the muscle surface into which the material is to be injected.
Although certain vaccination specialists advocate aspiration (i.e., the syringe plunger pulled back before injection), no data exist to document the necessity for this procedure. If aspiration results in blood in the needle hub, the needle should be withdrawn and a new site should be selected.
Infants (persons aged <12 months). Among the majority of infants, the anterolateral aspect of the thigh provides the largest muscle mass and is therefore the recommended site for injection. For the majority of infants, a 7/8–1-inch, 22–25-gauge needle is sufficient to penetrate muscle in the infant’s thigh.
Toddlers and Older Children (persons aged >12 months–18 years). The deltoid muscle can be used if the muscle mass is adequate. The needle size can range from 22 to 25 gauge and from 7/8 to 1¼ inches, on the basis of the size of the muscle. For toddlers, the anterolateral thigh can be used, but the needle should be longer, usually 1 inch.
Adults (persons aged >18 years). For adults, the deltoid muscle is recommended for routine intramuscular vaccinations. The anterolateral thigh can be used. The suggested needle size is 1–1½ inches and 22–25 gauge.
Intradermal Injections
Intradermal injections are usually administered on the volar surface of the forearm. With the bevel facing upwards, a 3/8–3/4-inch, 25–27-gauge needle can be inserted into the epidermis at an angle parallel to the long axis of the forearm. The needle should be inserted so that the entire bevel penetrates the skin and the injected solution raises a small bleb. Because of the small amounts of antigen used in intradermal vaccinations, care must be taken not to inject the vaccine subcutaneously because it can result in a suboptimal immunologic response.