Well, I don't know about vaccines for humans, but I CAN comment on vaccines in cats and dogs.

Basically, every pet cat and dog should be vaccinated for rabies. This is not because we think the animal is going to contract rabies - though there have been plenty of documented cases of both species being exposed and contracting it, either via and outdoor wild animal or a wild animal such as an infected bat or mouse getting into the house. I don't know about other countries, but in most states in the U.S., if someone is bitten by an unvaccinated cat or dog, it is required by law that a police report is filed, the animal is quarantined for at least 10 days, and a subsequent rabies vaccine is given after the 10 day period if the animal doesn't show any signs of the disease. Because of the zoonotic potential of rabies (humans can get it), no risks are taken. Every state is different in their rabies laws, but most states (and cities for that matter) do require an animal to be up to date on the vaccine unless otherwise noted and discontinued by a veterinarian due to a medical problem.

Most rabies vaccines are given annually, though there are a few that are licensed for 3 years. The hospital where I work used to use the 3 yr vaccine since we originally thought less vaccinating was better, but the 3 yr rabies vaccine is a killed virus, and therefore contains adjuvants to help increase the immune response. These adjuvants in killed vaccines where found to cause a specific type of cancer called a fibrosarcoma - which is extremely fast-growing. The tumors were found to contain an increased amount of aluminum which is from the adjuvant in the vaccine - therefore, the use of adjuvants in vaccines were linked to fibrosarcomas.

Since then, we've switched rabies vaccine products to a recombinant form of live canarypox (not a killed virus) which doesn't contain adjuvants. But this product is only licensed for annual use, not every 3 years. But it's actually safer to use the non-adjuvanted vaccine annually than it is to use the adjuvanted vaccine every 3 years.

Most killed virus vaccines contain adjuvants because a killed virus does not stimulate the immune system very well - which, as we all know, is the whole point of a vaccine. It stimulates an immune response by the body, creating an immunity against that specific virus. So adjuvants are added to create an appropriate immune response by the body to create immunity. Modified live vaccines disable the virus but it's not completely killed, therefore some of these vaccines do not contain adjuvants, but the risk of actually causing the disease you are vaccinating for is increasing, but still rare.

It depends on where you live and potential exposure of your animal for what vaccines will be recommended. So your veterinarian should sit down with you and discuss your pet's lifestyle to determine what vaccines are recommended and what vaccines are not. It is the goal not to over-vaccinate but still provide the highest protection possible.

For example, if you live near a woods or swampy area, your vet may recommend the Lyme disease vaccine for your dog, since the risk of coming in contact with ticks which transmit this deadly disease are far greater than a dog that lives in the city. Or if your cat goes outside unsupervised, it's recommended to have the feline leukemia vaccine up to date since your cat may by exposed to infected cats - but this vaccine is not recommended for indoor-only cats.

So a specific vaccination program should be tailor-fitted to your animal, depending on risk factors, and medical history. Sometimes veterinarians will discontinue vaccines in geriatric animals due to their decreased immune system and lack of risk factors. It all depends.