To comprehend the co-dependence and rivalry between China and India and their global implications, this chapter invites and enables readers to think from and with the two countries by pointing to the often ignored leftist science populism that underlines Global South societies’ management of the dual-task of modernisation and globalisation. This helps to identify the latent effects in the two countries’ selective global outreach and to understand their limits in leading South-South collaboration. The chapter first elucidates the concept of leftist science populism and its political logic. This helps to contextualise the gap between the two countries’ official views and actual practices in R&D exchanges and the latent effect of the two countries’ global expansion, which is discussed in the second section. Finally, the COVID vaccine diplomacy exhibits the two countries’ latest struggle to gain a better position in the global epistemic hierarchy. Whereas China’s vaccine diplomacy can be summarised as ‘contrast, collaborate and calumniate’, India adopted an approach that resembled ‘contest, convert and control’. Yet they both experienced some setbacks due to a deficiency in soft power, which is necessary to bring quality change in how science is applied and evaluated.