[wittyreference]

There's cross-reactivity, and they target both Influenza A and Influenza B strains (usually), so you'll get broader protection than strictly the strains most targeted by the vaccine. That said, you can ask the pharmacist for a copy of the FDA package insert, or google it yourself for the specific vaccine they're giving you. The package insert, under "DESCRIPTION" (Section 11), should tell you the precise strains that the antigens were developed from.

For example, google "FLUVIRIN fda package insert",and the one for the 2017/2018 flu season on page 8 you'll find "A/Singapore/GP1908/2015,IVR-180 (an A/Michigan/45/2015 (H1N1)pdm09- like virus; A/Hong Kong/4801/2014, NYMC X-263B (H3N2) (an A/Hong Kong/4801/2014- like virus); and B/Brisbane/60/2008, wild type (a B/Brisbane/60/2008-like virus)".

Google the same document for Fluzone quadrivalent, and you should find, for the 2020/2021 flu season, the strains "A/Guangdong-Maonan/SWL1536/2019 CNIC-1909 (H1N1), A/Hong Kong/2671/2019 IVR-208 (H3N2), B/Phuket/3073/2013 (B Yamagata lineage), and B/Washington/02/2019 (B Victoria lineage)" on pg 21.

[schoen]

There's also a summary maintained at Wikipedia

https://en.wikipedia.org/wiki/Historical_annual_reformulatio...

although it's presumably taken from the same sources you suggest.