Do foreign patents need to mirror the US patent that they…

hello. if you want to claim the priority date of an earlier-filed US patent application then everything in the foreign patent must be supported by (i.e. found in) the US patent....and any material in the foreign patent that is not supported will not be entitled to that earlier priority date. So for the improvement, that is new matter and cannot get the earlier filing date....and the rules governing how the new matter is handled will depend on the specific country (e.g. China).

Referring to Articles 19, 28, and 34(2)(b) of PCT:-

1. After receiving the ISR and Written Opinion, the applicant is entitled under Chapter I to amend the claims, and only the claims, once. The time limit for providing such amendments is the latter of two months from the transmittal of the ISR or 16 months from the earliest priority date. Amendments to claims are to be accompanied by statements explaining the amendments, any impact it may have on the description and drawings. The amendments should not contain subject matter that go beyond the disclosure in the PCT application as filed.

2. Also, the applicants are entitled to make amendments to the claims, the description, and the drawings, before the start of international preliminary examination, under chapter II. Again the amendments should not contain subject matter that go beyond the disclosure in the international application as filed.

3. If the ISA issues one or more written opinion, then the applicants are given one more opportunity to make any amendments. These amendments can be submitted to avoid possible objections as to lack of novelty or lack of inventive step in view of the citations listed in the ISR; to meet any objections noted by ISA.

Generally, no amendments can contain subject matter that is not present in the disclosure of the original application as filed. Further, the amendments introduced itself should contain any obscurity, and objectionable under PCT.

I am not sure how well the original application of your matter is supporting the 'new activity'. If the 'new experimental data' falls within the subject matter of the original application as filed, you may be able to file amendments stating that the amendments are within the scope of the disclosure originally filed.