Way of the mouth
Oral vaccines are convenient to administer, and are preferred to injections. But a more important consideration for scientists while developing any vaccine is to ensure long-term protection.
The key to prolonged immune protection by vaccination lies in the body's ability to develop an 'immuno-logical memory' - which means that the immune system must remember the earlier encounter with antigens, the molecules that make up the pathogens. Normally, this is achieved by the proliferation of a class of white blood cells called lymphocytes (also called memory cells) following the first encounter with the antigen. Memory cells have a higher affinity for the antigen and can be activated even at lower antigen concentrations when encountered again. Scientists have been worried as different vaccine formulations differ in their ability to induce such a memory, especially in the long term.
However, oral vaccines seem to make life simpler for scientists as well. Evidence from several clinical and experimental studies reviewed in Vaccine (1992) suggests that immunological memory may be stimulated more effectively by oral vaccines. In a more recent study published in Immunology (1995), it was shown that memory cells from the intestinal mucosa of mice retained the ability to respond to pathogens even two years after oral immunisation.
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