I would like to clarify. Technically, you are considered a non-resident for five years from 2015-2019. You are exempt from counting days for the F-visa and you did not fulfill the SPT Substantial Presence Test for the H-visa. You are exempt from paying the FICA taxes while on the F-visa. However, IRS requires you to start paying FICA after you change to your H-visa even though you might still be a non-resident for tax purposes.
To see the information from the IRS, click here: https://www.irs.gov/government-entities/employers-must-withhold-fica-taxes-for-aliens-who-change-vis...
You do have another option to file as a dual-status if you meet the rules. See post from @pk below.
[Edited 4/6/2020 | 10:19 AM]
@yashK
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