Not everyone gets sick from a flu shot. Allergies do not impact all people - nor do they impact, equally those who suffer from allergic reactions. . .Immunities are not equal. While response same in extremes (freezing temperatures freeze), people do not experience/respond 'equally' at same stress temperatures of cold/heat. (Have only too look at how people dress on a frigid day to see that - or a hot day, for that matter.)
As well, the nervous system/immune system of newborns, babies (nor children/adults/elderly for that matter) are not equal. Nor, in many cases, are the vaccines/antidotes given to enhance/protect such. Beyond mind/body variables; the same chemotherapy that might kill one patient; cures another.
Meantime, to appreciate a nexus of health and disease; of biology, human error, conspiracy and truth along with the power of silence, suggest a reading - no matter which side of debate you hold to - Dr. Mary's Monkey/Edward T. Haslam. It IS excruciatingly detailed in it's documentation and is available - with reviews - at 'Amazon.com'.
Or, for starters, just google SV 40 (Simian Virus 40).
I saw a couple of articles in the WSJ maybe 5-10 years ago, in the science section, about work being started on variations in individual response -- I remember one was specifically on response to toxins (apparently it's true that "one man's meat is another man's poison"); I think the other had something to do with individual variations in the genome with a view toward tailoring some treatments to individuals. I was cheered -- personally, I don't seem to respond the expected way to lots of things, like some OTC medicines. Haven't heard anything about such work since though! :(