There is a very strong version of feline calici virus called VS - FCV infection. (cat-health-guide.org)
Remember, it is possible for a cat to be infected with feline calici virus and have no symptoms at all. (cat-health-guide.org)
Veterinarians are worried about this virus becoming an epidemic because most cats are not vaccinated against the mutant strain of calici virus. (itchmo.com)