Overdue vaccines is a complex one and there is no one correct answer. Is your dogs immunity going to suddenly expire after 1 year count down style!? No, it won't, but it is likely to reduce over time and potentially up to a point where the immune response would convey little protective effect if the animal is challenged by leptospirosis.
Normal core vaccination schedules vary product to product but generally speaking pups get a primary course of 2 injections as youngsters and then a full booster 12 months after that. This usually consists of vaccines covering Distemper, Hepatitis, Parvo virus and a 2 or 4 strain lepto vaccine. Durations of immunity differ from product to product and manufacturer to manufacturer but often will recommend that after the initial booster at 1 year of age DHP revaccination can be done every 3 years. Now there have been instances of dogs that have demonstrated detectable immunity up to 7 years after the initial vaccines. However these are relatively few cases and usually in dogs that either reacted badly to initial vaccines or whose owners do not like the idea of over vaccination. I am not aware of that many good studies demonstrating CONSISTENT extended immunity. Sure it can happen but we don't know which animals will and which wont. Your spaniel might but your lab might not and vice versa. If you are so minded you can perform vaccine titre testing which will give you an idea of their levels of immunity but often costs more than the vaccine!
Now the above is not quite as applicable for lepto. Lepto immunity is much more difficult to assess and vaccinal immunity has been shown to be less long lived. The rough guide that we have been applying has been that up to 6 weeks overdue is not much issue but 3 months is really the max delay if we are to be confident of a good "booster" response. Longer and the recommendation has been to restart.
The other considerations surrounding vaccination are risk vs benefit. Now who is at more risk... Mrs Fenella Von Peaches the chihua-poo-**** for whom the closest she will come to a rodent is a rerun of Tom and Gerry or Badger the patterdale who has killed more rats than Neosorexa?
Peaches likely doesn't need a lepto vaccine, but I'd want to be giving badger one every year! It is fare to say that working dogs are much more likely to be in contact with or at risk from lepto that many "house pets"
The other factor to bare in mind with lepto is that vaccination is not just to prevent/reduce disease it is also to help reduce the likelihood that an animal infected with lepto will become a carrier. Lepto is able to colonise the kidney and essentially live there and shed out in that animals urine, contaminating the environment. Vaccination helps to reduce this.
Now we can get into the big pharma debate. Why are there limited studies into immunity duration? Well studies take a lot of time and money and very often they are only undertaken if there is a point/gain to be made. What is the benefit of a company proving that you don't need their product? They need to prove it works but beyond it working for a reasonable period, they have no interest in proving that. Now I am not aiming to criticise companies, if they dont make money, they dont undertake R&D and you dont get products to use but the reality still stands.
Then finally there is the liability angle. We as vets have to advise and recommend based on what we know and can prove. We can only recommend based on the data we have and are given (often byt he drug companies) and in a world where seemingly people love to play vet-lynch I err on the side of caution. Otherwise if I tell Mr/Mrs Barrister-Lawyer-Professor-of-fine-arts that Miffy will be ok just having a booster even though she is 6 months overdue, and then she gets lepto, guess who gets the unlubricated rod of the law with no leg to stand on!? yeah I do because I didn't give the "correct advice"
Personally, I am more than happy to discuss the pros cons and issues surrounding these situations but then the ultimate decision is up to the owner, I will answer questions and offer guidance but I stress that my recommendation is XYZ and if the owner chooses another path then that is their decision and the consequences are also theirs. But I will say one thing...I much prefer vaccinating for Lepto than treating it (however rarely) as they often just go yellow and then cold!
If you made it this far...hats off to your patience!
. 3 jabs over 120 days 
