I’ve been learning some about rabies and learned about rabies causing hydrophobia. This is just a theory, I’m not saying I know anything about this topic to be knowledgeable, but if we could get someone with rabies to not fear water, could they survive?
Vets and people who work in animal shelters often get the rabies vaccines beforehand. But even if you have been vaccinated previously, you still have to get it again if you are bitten.
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The efficacy of vaccines usually declines over time after administration. The immune system starts to “forget” how to fight a pathogen it doesn’t encounter. It doesn’t completely forget, but it puts the treatment data way back in the archives. So when it encounters the real deal, it can take quite a while to boot up production of antibodies. It also varies by the type of disease.
This is fine for some slow diseases ( which is why sometimes a single vaccination can suffice ), but can be risky if the disease progresses faster than the immune system can ramp up the defenses.
Administering the vaccine as soon as possible after suspected exposure to deadly or highly contagious diseases simply helps the immune system to get the necessary blueprints to get in the fight quicker.
Administering the vaccine before any exposure at regular, long intervals is done to decrease the baseline risk. Sometimes you don’t know you have been infected. Many diseases are not only transmitted by dramatic, obvious vectors. In those cases, it’s definitely better to have some old defense than none at all.
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In addition to what Senshi said, if you have recieved the full course of vaccines (4-5 doses spread over a month), any future bites need only 1-3 doses. Also the time within which you have to take the first dose increases from 24 hours to 2-3 days, which can be quite useful to vets in remote places.