Why the Lepto vaccine may be risky for chihuahuas: 

Increased side effects:

Smaller dogs, including toy breeds like Chihuahuas, have a disproportionately higher risk of experiencing side effects from the Lepto vaccine, with reactions ranging from mild lethargy and fever to severe events like anaphylaxis (shock).  

Potential for autoimmune disease

Some dogs may develop autoimmune disease after receiving the Lepto vaccine.

Limited Protection:

The current Lepto vaccine only protects against a limited number of Lepto serovars (types), meaning a dog remains vulnerable to other strains of bacteria.

Short lived immunity:

The protection offered by the vaccine is not long lasting, requiring re-vaccination yearly to maintain immunity.

Why some Vets and Breeders are hesitant:

High reactivity. The Lepto vaccine has a high rate of adverse reactions compared to other vaccines.