The flu vaccination changes every year to attack different strains of flu.
As a result, the BG impact may vary year on year.
In the UK, it is strongly recommended that people with diabetes take the flu vaccination as it may make diabetes more difficult to manage (but, my understanding is that we are at no more risk) and, therefore, are provided it free of charge through the NHS.
I have taken advantage of this for the last 14 years on the basis that one or two days of potentially higher BG is better than a few weeks of full blown flu. Most years, I have seen no impact but there have been a couple when my BG has been raised a little for a couple of days.
Given the vaccination targets the most likely flu viruses each year, it is not fool proof - it is possible a flu virus may get through. But the risk is significantly reduced.