Vaccination and Boosters, Explained
What each one covers, and why the schedule looks the way it does.
What vaccinations actually do
A vaccine introduces the immune system to a pathogen — or a harmless piece of one — before the real thing arrives. The body mounts a response, builds memory cells, and is then primed to react quickly if it ever meets the actual disease. That memory is the whole point: a vaccinated animal's immune system recognises the threat and moves faster than the disease can. The catch is that immune memory fades, which is why the schedule doesn't end at puppyhood or kittenhood — it continues, at longer intervals, for life.
Different vaccines work in slightly different ways. Modified live vaccines contain a weakened form of the organism and typically produce a strong, lasting response. Killed or inactivated vaccines contain dead organisms or isolated proteins; they are often safer for immunocompromised animals but may need more frequent repeating to maintain protection. Your vet will choose between them based on the disease in question, the species, and the individual animal's history.
Dogs, cats, rabbits: what the core vaccines cover
Dogs are routinely protected against distemper, parvovirus, and infectious canine hepatitis — often combined in a single injection — plus leptospirosis, a bacterial infection spread through rat urine and contaminated water. Leptospirosis is the one that tends to surprise people: it's not a virus, so the immunity it produces is shorter-lived than the viral components, and it typically requires an annual booster rather than the three-year cycle the other components can follow. Kennel cough — caused by Bordetella bronchiseptica and parainfluenza virus — is a separate, non-core vaccine, usually given as drops into the nose. Many boarding kennels now require it. Rabies vaccination applies wherever the disease is present or travel is planned; in the UK it is a legal requirement for travel under the pet travel scheme.
Cats are routinely vaccinated against feline herpesvirus and calicivirus, the two main causes of cat flu, and feline panleukopaenia, a serious parvoviral disease. Cats that go outdoors, or that live with other cats, are also commonly offered feline leukaemia virus vaccination. Cat flu vaccines don't guarantee a cat will never develop symptoms — especially from strains they haven't been exposed to — but they substantially reduce severity. That nuance is worth understanding; a vaccinated cat can still seem unwell after a stress event, but the illness is usually milder and shorter.
Rabbits in the UK face two serious threats: myxomatosis and rabbit haemorrhagic disease. There are two known strains of rabbit haemorrhagic disease — the original (RHD1) and a newer variant (RHD2), first identified in France in the early 2010s and now widespread in the UK — and vaccine development has had to keep pace. A combined vaccine that covers myxomatosis, RHD1, and RHD2 is available and is the standard recommendation for pet rabbits. These diseases can reach even indoor rabbits via insects, birds, or contaminated hay, so the "house rabbit" argument for skipping vaccination doesn't hold.
Small pets — guinea pigs, hamsters, rats, mice — currently have no licensed vaccines available in the UK. Their preventive care focuses on environment, diet, and early detection rather than immunisation.
Why the schedule looks the way it does
Puppies and kittens are born with some protection borrowed from their mother, carried in colostrum and early milk. That maternal immunity is a bridge — useful in the first weeks of life, but it also interferes with vaccination, blunting the response if a vaccine is given while maternal antibodies are still circulating. The initial course is therefore given as a series: typically two injections a few weeks apart, designed so that at least one dose lands after maternal immunity has waned. The final injection in the primary course sets the baseline from which boosters are calculated.
After that, the timing is tailored to how long each component's protection lasts — measured through challenge studies and field data accumulated over decades. Some components hold well for three years; others, like leptospirosis, need annual reinforcement. This is why a dog's booster appointment might involve only part of the original combination in any given year, rather than the full course every time.
Titre testing — a blood test that measures circulating antibodies — is occasionally used to assess whether a specific animal still has adequate protection before a booster is given. It is more common in animals with vaccine reactions in their history. It doesn't replace the vet's judgement: high antibody levels aren't the whole picture, and cell-mediated immunity doesn't show up in a titre at all.
If a vaccination course lapses for long enough, some animals will need to restart the primary course rather than simply receiving a single booster. How long is "long enough" varies by vaccine and manufacturer; your vet can tell you where your animal stands.