My thoughts are that maybe the TB antigens necessary to produce an immune response are proteins; therefore they can be digested in the stomach and small intestine. But I may be wrong though. I am confused why I can't say the same for polio vaccine.
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3 Answers
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There are different polio vaccines - one live (attenuated) vaccine which is given orally and one inactivated, which is injected. The main reason for using the live orally vaccine is that it provides excellent immunity (better than the inactivated) since it uses the natural infection route (oral-faecal) in the body where it enters through cells in the intestine. Besides that, it is also much less expensive than the inactivated form, which is a big thing when doing mass immunisations in developing countries.
The live vaccine, however, may mutate back into a more infectious form as you shed live (attenuated) viruses after the immunisation, so these are not used anymore. Now we are very close to the eradication of the poliovirus. The risk of getting new infections is viewed as being too high these days. See this paper for more information: "Vaccine-derived polioviruses and the endgame strategy for global polio eradication."
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$\begingroup$ Also the injected vaccine often does not stop someone getting infected with polo and passing it on, it "just" stops the person getting ill from polo. $\endgroup$ Jun 6, 2016 at 14:41
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4$\begingroup$ @IanRingrose can you explain your claim, preferably with citations? A vaccine (any vaccine) is meant to stimulate the immune system so it can quickly destroy the infectious agent when it is encountered. "Passing on" a disease means it has taken hold in your body in sufficient amounts that others can be exposed to it, that's what the word "infectious" means. Vaccines stop that transmission. $\endgroup$– MattDMoJun 7, 2016 at 1:25
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$\begingroup$ @MattDMo, Polio is spread by getting into someone’s stomach and coming out of the other end, but needs to get into someone blood to make them ill, The injected vaccine quickly gives a good level of immunity from white blood cells; however it produces very low levels of immunity in the intestine. (As a result, when a person immunized with IPV is infected with wild poliovirus, the virus can still multiply inside the intestines and be shed in the faeces, risking continued circulation) see polioeradication.org/Polioandprevention/Thevaccines/… $\endgroup$ Jun 8, 2016 at 10:43
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$\begingroup$ There are also different types of the oral vaccine, as there are 3 types of polio can including all 3 in the vaccine make it less effective, see polioeradication.org/Polioandprevention/Thevaccines.aspx $\endgroup$ Jun 8, 2016 at 10:48
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$\begingroup$ It comes down to a choose between the vaccine that gives the best protection for the person getting given the vaccine, and the vaccine that is best at stopping ANYONE from getting polo, by stopping polo spreading. $\endgroup$ Jun 8, 2016 at 10:51
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The reason why the oral polio vaccine is not digested in the stomach is that the poliovirus itself has adapted such that it can survive the acidic conditions of the stomach. By using an attenuated (mutated to not cause neurological symptoms) version of the virus, the oral vaccine can also survive the acidic conditions of the stomach. For TB which is transmitted not through ingestion but through inhalation, the bacterium is not resistive to the stomach acid and therefore a different vaccination strategy had to be used which requires injection (which, as described in the answer by Chris, is also less dangerous).
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2$\begingroup$ Note that rotavirus vaccine, given to infants, is also given orally, and it too is a GI tract infection. We think of polio as affecting the nerves, but that's a weird exceptional outcome, it's usually a stomach bug. So when we expect the primary infection to happen in the gut, that's where we prime the immune system. $\endgroup$ Jun 6, 2016 at 17:47
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1$\begingroup$ A point to note that TB is cause by a bacterium and not virus as you've written "the virus is not resistive to the stomach acid..." $\endgroup$ Jun 7, 2016 at 8:42
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$\begingroup$ Also note that breastmilk, while not a vaccine, does contain antibodies, and those do work to reduce the rate of GI illness. So there is an immune system working in the gut, and it can be affected by vaccines and antibodies given orally, they don't get digested before they have a protective effect. $\endgroup$ Jun 7, 2016 at 17:15
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1$\begingroup$ @ABcDexter, Thanks, I corrected the answer accordingly. $\endgroup$– ThawnJun 8, 2016 at 8:13
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$\begingroup$ @swbarnes2 This may be true for newborns but certainly not for adults. All antibody based treatments are given intravenously (see also monoclonal antibody therapy). $\endgroup$– ThawnJun 8, 2016 at 8:19
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As stated by others, polio is primarily a gastro bug, an enterovirus related to Coxsackie and a number of others which produce stomach and respiratory symptoms, as well as various others.
Most polio infections result in nothing worse than a case of "the stomach flu", but in a small percentage of cases the virus escapes from the GI tract and produces neurological symptoms. And in a small percentage of those there is the paralysis that people associate with polio.
Since the route of infection is primarily oral, a live (but "attenuated") polio virus can be administered orally to achieve immunity to the "real" virus. Other diseases are either not spread orally or they do not have live-attenuated vaccines that have been developed. Simply administering the antigens orally would not work, as they would be too dilute and would be quickly neutralized by digestive juices.
