What might be a reason for a late reaction to a vaccine in pets?

A late reaction to a vaccine in pets may be attributed to suboptimal storage conditions of the vaccine. Vaccines contain live or inactivated pathogens, adjuvants, and stabilizers that are carefully formulated to elicit an appropriate immune response. If a vaccine is not stored at the correct temperature or is left exposed to light or moisture for too long, it can degrade, leading to decreased effectiveness or unexpected immune reactions when administered. This degradation may not manifest immediately but can cause a delayed immune response, leading to a late reaction after the vaccine has been given.

While stress can influence the immune system, it would not typically cause a late reaction specifically related to vaccination. Immediate interaction with infectious agents or normal immune system function would not explain a late reaction, as they pertain to immediate responses rather than delayed responses resulting from improper vaccine handling.