https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7199677/
https://www.ahajournals.org/doi/10.1161/circ.144.suppl_1.107...
https://www.cdc.gov/mmwr/volumes/70/wr/mm7035e5.htm
(influenza vaccine effect - https://pubmed.ncbi.nlm.nih.gov/15976761/) et al
I'm not sure, specifically, where the 30x claim comes from. There are a myriad of studies showing a measurable increase.