Contrary to popular belief using e-cigs with pg does not stop you from getting the flu. I know this because I am just now recovering from the flu and I vape my head off daily. So yeah. Sorry but it's a myth.
If I remember correctly, the flu shot is only about 70% effective and that's only if the flu doesn't mutate. PG may not stop all the viral infections but it certainly can't hurt. I haven't even got a sniffle this year and that's unusual for me (knock on wood). Here's what Google has Propylene Glycol influenza - Google Search , I sincerely hope you feel better soon. My brother caught a bad one earlier this season and he said his throat was so raw he couldn't vape (I'm not sure how much he vapes as opposed to smoking).