FVRCP vaccine question

Each of the two fosters I have got one FVRCP vaccine at around 4 months ago when they were 4'ish months old. As far as I can tell, they haven't had any boosters.

Isn't this a series of shots? They're about 9 months old now. Would continuing the series even work at this point or has too much time passed?

One of them keeps getting a reoccurring cold (sneezing, runny eye). The FRCV shot he got was modified live vaccine. Would it be advisable to give him boosters considering his health issues? He also keeps getting diarrhea which we get to stop but then starts up again.

 

Here is what UCDavis says. Specifically look at note 1.

II. FELINE
A. Vaccination Schedule


Age Vaccine
6-9 weeks FVRCP
9-12 weeks FVRCP, FeLV
12-14 weeks FVRCP, FeLV
12 weeks Rabies


Note:

1) Kittens should receive FVRCP at 3-4 week intervals until the age of 12-14 weeks. Vaccination of kittens less than 6 weeks of age is not recommended. Adult cats ( > 6 months old) with no known vaccination history, regardless of age, should receive 1 dose of FVRCP. A booster vaccination should follow 1 year later, then at 3-year intervals.
2) Kittens should receive 2 doses of the FeLV vaccine. The first dose should be at 9 weeks of age or older. The second dose should follow 3-4 weeks after the first dose (i.e., at 9 weeks, then at 12 weeks of age). Booster vaccinations should be given 1 year later, then at 3-year intervals. FeLV vaccine is not recommended for cats kept strictly indoors. FeLV vaccine is recommended for all cats allowed to roam outdoors.

Cats previously vaccinated with other FeLV vaccines and requiring a booster immunization will only need to receive a single immunization with the product used by the VMTH. Adult cats not previously vaccinated for FeLV should receive the two-dose primary immunization series. Booster vaccinations should follow after 1 year, then at 3-year intervals.

3) Rabies vaccination is recommended for all at-risk cats and where it is required by law. The vaccination regimen is the same as for dogs, i.e., initial immunization, a booster 1 year later, and subsequent re-vaccinations at 3-year intervals.


From:
https://www.vmth.ucdavis.edu/vmthold/in ... nproto.htm

 

I have strong opinions on vaccinations, Rule #1 never vaccinate in the scruff. I don't really do vaccinations for my indoor only cats except Abigail. She has herpes so I do opt to get her the FVRCP vaccine annually to help alleviate some of her symptoms. I may stop that though since it doesn't seem to be helping much anyway. Just wanted to give you some food for thought.
And about the diarhea..could you add probiotics to his food? Or fortiflora? Goodluck on everything!