That sounds like the ADE effect kicking in. 3-6 months out, exposure to a/the virus.
I’ve been thinking about the 6 month window and I’m thinking that might be a little soon if they really want to take out a lot of people. The majority of the population of the world or even the US or Canada won’t have been vaccinated by July or whenever the 6 months is up. They don’t want to set the kill switch too soon because those who haven’t been vaccinated yet will see what’s happening and just won’t take it. That window would have to be a couple of years out I imagine.