Vaccinations are what I would call "a necessary evil". Do they have toxic ingredients? Yes. Can they cause reactions? Yes. Can they fail to protect against a disease? Yes, if not properly administered.
As learned during veterinary school and experienced throughout my work with several species of animals for the last 10 years I can tell you I have a fair understanding of how they work, that I recommend them and that I use them when necessary for myself.
Vaccines are defined as biological preparations that provide active acquired immunity to a particular infectious disease. A vaccine typically contains a part of an agent or a part that resembles a disease-causing microorganism. It is often made from weakened or killed forms of the microbe, its toxins or one of its surface proteins. In other words, because of our incredible immune system and advanced medical technology, we created a solution to prevent highly contagious and highly mortal diseases by preparing our body with antibodies to defend the actual disease if presented. This is actually more complicated than it sounds of course, but you have to know that different diseases are caused by different kinds of agents: viruses, bacteria and fungi. Every kind of agent has a different way to stimulate our immune system to produce a lasting memory of the disease. That process can take different quantities of doses and frequencies to work and the period that they last protecting our body effectively is also highly variable.
Parvovirus and Panleukopenia are among other diseases that can be almost 100% preventable with a proper protocol and handling of vaccinations during the early months of the life of dogs and cats. Parvovirus and Panleukopenia are highly contagious and also have almost a 50% mortality rate in dogs and cats respectively. In places like shelters, rescuers and homes with multiple and constantly changing populations, it is very important to have every member of the population up to date on vaccines in order to avoid an outbreak, unnecessary pain and deaths, plus expensive veterinary bills.
Note how I say "proper protocol and handling of vaccinations" or "properly administered." Vaccines are biological preparations that need to stay within a certain temperature range to conserve the agent responsible for triggering an adequate reaction from the immune system. So if the vaccines are not properly transported, stored and managed, it is like basically injecting an expired medication into an animal.
As a veterinarian, I have personally been exposed to zoonotic diseases such as Rabies and Tetanus after being bitten by a raccoon and being in close contact with an infected monkey respectively. At that time the decision of getting vaccines that were not cruelty-free was difficult to take as a vegan veterinarian, but I had to consider choosing to be alive and healthy in order to help more animals with my skills.
Moreover, as a vaccinated person with the rabies vaccine I was legally allowed to be able to help baby raccoons, bats, foxes and other rabies vectors species. When I volunteered doing spay and neuter surgeries in Puerto Rico, it was highly encouraged to get vaccinated against hepatitis so I took the vaccine. Hence being vaccinated against some diseases can allow us to help more animals or even travel to other countries safely.
If you are worried about over-vaccination and allergic reactions I can tell you this: It is possible to measure the level of antibodies left from a previous vaccine, it is called "titer levels". It is a blood test available for animal companions. It is a bit expensive but it is an option for older animals and can be used to avoid over-vaccinating for certain diseases if the guardian is concerned about that. Vaccine reactions are as possible as any reaction to anything else like medications, insect bites and even food ingredients. In the last five years I have been working with thousands of kittens and puppies, giving them their vaccines after surgery and I have seen a handful of possible reactions to vaccinations. I believe the risk of contracting a pretty devastating disease outweighs the risk of getting a vaccine reaction.
Lastly, not all vaccines are necessary. In the veterinary field we have what we call "core" and "lifestyle" vaccines. Core vaccines should normally be administered to every animal. Lifestyle vaccines, while not required, are strongly recommended depending on geography, lifestyle, and the current prevalence of the disease.
In conclusion, vaccine companies can and absolutely should improve their production process, and hopefully in a few years we can have cruelty-free options to protect our companion animals and ourselves from life-threatening diseases. But as for now, to me they are considered a necessity and there are currently no vegan options, so I use and recommend them for my patients.