Isn't it - if you're testing negative via lft then you're not contagious? You could have it but if you're vaccinated then your immune system could be working like mad to stop the virus replicating to the point where you don't become contagious to others.
I think it's something to do with 'viral load'.
There's an incubation period for all viruses but if your vaccinated your body could just be staying on top of it, especially if you had a small viral load when catching it.
Recently my baby and I were out with friends, the next day my friend text to say she had covid. We weren't sitting near her but we were all in the same room, so I braced myself. 3 days later my unvaccinated baby started showing symptoms and a strong positive LFT. I was also experiencing symptoms but testing negative. Both had a PCR - baby was positive. I was negative.
Baby is unwell and I'm obviously caring for her, not giving a crap about myself as she's laid in my arms coughing and sneezing all over my face.
3 days later I am strong positive.
Omicron has a fast incubation time. My arm chair theory (I.e no credibility to it at all!) Is that we both were exposed to the virus from my friend. My vaccine worked though and at the time my unvaccinated baby came down with it my vaccine was standing strong but I still felt ropey because my immune system was still launching an attack. However, being exposed to such a huge and prolonged viral load in my face via my baby the virus took over, my immune system couldn't cope, the virus started to replicate and I came down with covid (i.e a breakthrough infection) where I tested positive.
That's my theory! I definitely had symptoms when my baby started with it but I felt better the following day, but then I properly came down with it.
Just an anecdotal theory based on some reading around the topic. I'm no doctor or scientist!