There are some confounding factors to that though, I think? Infectivity post mortem (as seen with plague) and time lag between infectivity and symptoms appearing allowing for longer periods of asymptomatic transmission. Both variables that mean the spread of a mutation and it's lethality need not be linked.
Mass distribution of non-sterilizing vaccines which reduce symptoms (via blood/serum antibodies) but don't prevent infection and transmission (lack of nasal/mucous antibodies) can increase spread of variants that would otherwise cause symptoms and self-isolation of the infected person.
Vaccination reduces the infection rate. Among vaccinated individuals who suffer breakthrough infection the time at peak viral load is significantly reduced.
> Even if they’re not showing symptoms, fully vaccinated people should “get tested 3-5 days after exposure to someone with suspected or confirmed COVID-19 and wear a mask in public indoor settings for 14 days after exposure or until they receive a negative test result,” ... “Our updated guidance recommends vaccinated people get tested upon exposure regardless of symptoms,” CDC Director Rochelle Walensky, MD, told The New York Times
