Background: The combination of PD-(L)1 inhibitors and chemotherapy (IO+Chemo) has become the standard first-line treatment for multiple types of cancer. In contrast to PD-(L)1 monotherapy, there is a lack of established biomarkers to distinguish the outcomes of IO+Chemo. The paucity of high-quality omics data in this setting has not allowed for deep biomarker exploration. We utilized RNA-seq data from two in-house randomized controlled trials (RCTs), RATIONALE-309 and ORIENT-11, with the goal to find transcriptomic biomarkers to identify patients who may not derive benefit from IO+Chemo. Methods: RATIONALE-309 (NCT03924986) was a phase 3 study conducted at 42 sites in Asia. Treatment-naive recurrent/metastatic nasopharyngeal cancer patients were randomly allocated to either PD-1 inhibitor tislelizumab or placebo plus gemcitabine and cisplatin. ORIENT-11 (NCT03607539) was a phase 3 study that enrolled patients from 47 centers in China who had previously untreated locally advanced/metastatic nonsquamous non-small cell lung cancer. Patients were randomized to PD-1 inhibitor sintilimab or placebo plus pemetrexed and platinum. We compared several immune-related biomarkers, including IFN-γ signature, PD-L1, cytolytic score (CYT), T-cell inflamed gene expression profile, B cell infiltration, and ESTIMATE immune score, and explored the optimal predictive threshold. Results: In the RATIONALE-309 and ORIENT-11 trials, 247 and 171 patients with RNA-seq data were analyzed, respectively. Although the addition of Chemo to IO could theoretically benefit tumors with low immune activation, we hypothesized that patients might not benefit from IO+Chemo if their activation level is extremely low. We first tested the predictiveness of several thresholds (20-50^th quantiles) for each selected biomarker in the two RCTs. We found that low CYT and B cell are the two best indicators for identifying a lack of survival benefits from IO+Chemo versus Chemo, especially when using 20-30^th quantile as threshold. To incorporate their distinct biological information and predictive value, we combined their genes into a single gene signature to represent immune activation. We found that patients in the lower quantile (<20^th) showed no benefit of IO+Chemo over Chemo in terms of overall survival (OS) (RATIONALE-309: HR=1.01, P=0.99; ORIENT-11: HR=1.54, P=0.31) and progression-free survival (PFS) (RATIONALE-309: HR=0.76, P=0.42; ORIENT-11: HR=1.30, P=0.52). Moreover, this lack of advantage was maintained in key subgroups such as PD-L1-positive patients and former/current smokers across the two RCTs. Conclusions: Based on the two large-scale RCTs, our findings have implications for the selection of biomarkers when identifying suitable candidates for IO+Chemo treatment. B cells and CYT are crucial components required for the efficacy of IO+Chemo.