OMG I am so sorry to hear this. Looking on NOAH, they list the risk of adverse events to be very low indeed.
For TriCat Trio, the possible side-effects are as follows:
"A slight painful swelling may be observed at the injection site for 1-2 days. A slight transient rise in body temperature (up to 40°C) may occur for 1-2 days. In some cases sneezing, coughing, nasal discharge, and a slight dullness or reduced appetite may be observed for up to 2 days post vaccination. In very rare cases, the vaccine may cause hypersensitivity reactions (pruritus, dyspnoea, vomiting, diarrhoea and collapse)."
https://www.noahcompendium.co.uk/?id=-455549
For FeLV
"Transient and small thickening or nodule, approximately 5-10 mm in size, may be observed at the injection site and disappear within 2 to 6 weeks without treatment. Occasionally, systemic reactions (hyperthermia, anorexia, lethargy) may occur within one or two days after vaccine administration.
Where Nobivac FeLV has been used to reconstitute cat vaccines in the Nobivac range containing feline calicivirus (F9 strain), feline rhinotracheitis virus (G2620A strain) and/or feline panleucopenia virus (MW-1 or Bristol strain) prior to inoculation, a small nodule at the site of vaccination is frequently observed. It can persist for up to 18 days post-inoculation.
Occasionally, the nodule may be painful for up to 6 days after injection. A transient rise in body temperature or lameness may occur and last up to 3 days post vaccination. In some cases, a slight dullness or reduced appetite may be observed for up to 1 day post vaccination.
In the rare event of hypersensitivity reaction following vaccination, administer an antihistamine, corticosteroid or adrenaline without delay and by the most-immediate route.
The frequency of adverse reactions is defined using the following convention:
- very common (more than 1 in 10 animals treated displaying adverse reactions)
- common (more than 1 but less than 10 animals in 100 animals treated)
- uncommon (more than 1 but less than 10 animals in 1,000 animals treated)
- rare (more than 1 but less than 10 animals in 10,000 animals treated)
- very rare (less than 1 animal in 10,000 animals treated, including isolated reports)."
https://www.noahcompendium.co.uk/?id=-455441
I would get the medical notes from the vet straight away to see how they managed the incident. I would also contact the manufacturer to explain what happened. They need to know.
And I would contact the regulators, the MHRA and the EMA (even though we've left the EU, we are still following their approvals for 2 years). They need to know in case this is a systemic issue.
I am so very, very sorry.