Social and Medicare deduction after H1B visa approval or from Oct 1 ?

[dipjyoti]

My employer has started deducting the Social and Medicare taxes, after the H1B visa approval. But the change of status from F1 to H1B happens only from Oct 1 onwards (LCA date). So, won’t the Social and Medicare deductions be done from Oct 1 onwards ? or is it done after the visa approval notice from USCIS ? Can anyone please confirm this ?