I agree, viruses mutate, but what I was trying to say was that if this was the form of the virus in the vaccine, they would be immune, so it isn't the form they were vaccinated for. Calici virus is one of the worst offenders, it has more forms than can ever be in a vaccine, they just keep trying to immunize against the most deadly ones. I don't consider that vaccine failure, just virus superiority. And then there's the corona virus that mutates to FIP............ugh.With respect, this isn't true. Feline viruses mutate just like human viruses. Feline vaccines fail just like human vaccines.