Next to ICH E6 (R2) with reference to (serious) adverse event reporting, as previously indicated (the trial protocol should define exact reporting requirements in the particular trial), you may refer to "CT-3", listed in EudraLex Volume 10. See: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:52011XC0611(01)&from=EN
. CT-3 provides more detailed (EU) guidance on case reporting in clinical trials.
I don't believe it says (explicitly) that (S)AE reporting should always start immediately after informed consent is obtained. As in ICH E6, there is (among other things) reference to (S)AE reporting requirements to be stated in the trial protocol. Section 3 of CT-3, indicates that "The sponsor should continuously weigh anticipated benefits and risks of the clinical trial, which includes ongoing safety evaluation of IMPs."
Particular interventions in a clinical trial which may start immediately after consent has been obtained, may result in risks (events) to the trial subject and as such I can imagine that a Sponsor defines that (S)AE collection should start when informed consent is obtained. And it might also help the Sponsor to make a distinction between events occurred due to non-IMP-related interventions and due to IMP-related interventions.
I can also imagine that (S)AE reporting requirements may depend on e.g. the study phase (knowledge of the product & disease), trial indication and design.
In any case, the Sponsor should be able to justify the (S)AE reporting requirements defined in the trial protocol, taking into consideration not only legal requirements, but also appropriate protection of the trial subject as well as scientific considerations.
Any additional thoughts?