I work in a laboratory at CDC where I need to take showers and wash my hair every time I leave the lab (BSL3 with risk-based enhancements, you can read more about it at https://www.cdc.gov/cpr/infographics/biosafety.htm ), which means I walk around a lot with wet hair, so I have thought about this question too. 😊 Because flu is caused by a respiratory virus (and not wet hair!), the most important thing I do to protect myself from influenza by getting a flu vaccine every year.