I just typed a great long reply and then suddenly lost it. Arrgh!
No one can say when/if your next bird will get sick with this, but try to put it into perspective in that every living creature will die and it is all a question of timescales and quality of life.
The fact that you have had 7months between your first two outbreaks(deaths) in a flock of youngsters is a positive sign.... usually you get 2 or 3 sick with it within a few weeks of each other. The remaining birds may have some resistance. What is known is that stress can trigger an outbreak, so keeping them as happy and stress free as possible is important. Some things like hormones, seasonal changes and moulting are beyond your control, but good nutrition, plenty to keep them occupied (things to scratch and peck at) and a safe and stable environment (chickens are easily stressed by change) needs to be part of your management system. You might want to look into fermenting their feed if you do not do so already. This helps keep their gut healthy, which means they can absorb the nutrients from their food better. One of the symptoms of Marek's is that it compromises the immune system, so giving foods or supplements to support the immune system can help keep them healthy. Greens hung up for them to peck at, an occasional vitamin supplement in their water etc.
I hope that has given you some hope and food for thought. If you decide that you want to expand your flock at some point in the future perhaps to replace your losses, consider waiting until one of your girls goes broody and giving her some fertile eggs from a breeder with a Marek's resistant flock. Chicks raised by a broody generally have stronger immune systems and they do not suffer integration stress because they are part of the flock from the day they hatch. Not saying that some of them won't get Marek's but they have a better chance in my experience.
I'm not a fan of the vaccination, partly because the vaccine available to backyard flocks is leaky and may well be responsible for the hotter strains of the virus that are rife in the USA. Also, it is important that vaccinated chicks are kept clear of the virus for the first few weeks of life. This means quite strict biosecurity if your main flock has been infected, which is not always possible.
Good luck with your remaining flock and I hope my post has given you some hope.
Regards
Barbara
PS. I broody hatched and raised 56 chicks last year and only 3 showed Marek's symptoms. Two died and one recovered.