Flu vaccine.
After coming home from WWI, in 1918, my great grandfather died of flu, his wife nearly died. Also I’d take a copy of the NYT from Dec 7, 1941.
Something like that might change the timeline. Really not sure about this part but I'm wondering if a person living in 1918 would be able to handle a modern day flu shot because immune systems might be different these days. Cheers!