Slow for what? For the development of protective antibodies.

Some of these outbreaks come with the wind and die with the wind. The man-hours of vaccination available may be far less than needed to stop an epidemic. Incidentally, climate deserves better appreciation. Times over, Mother Nature not man has snuffed out an epidemic.

Is your purpose to protect an individual or a population? What is the host, man or horses? What is the virus ? What is the locality and season of the year? What is the disease history of recent years? What facilities are at your disposal ?

The Mexican reality for the human population of 100 million demands a DEN1-4 plus WNV chimera vaccine, whereas for the equine population of 10 million demands a VEEV plus WNV plus EEEV (plus others) chimera vaccine. The development from formalin-killed to attenuated vaccine might be a costly and prolonged development. However, killed vaccines may be highly satisfactory.