Offering Sinovac Biotech Covid-19 vaccines to the public in Singapore for the first time since Friday, several private clinics reported overwhelming demand for the Chinese-made shot, despite already available rival vaccines having far higher efficacy.

Singapore has vaccinated almost half its 5.7 million population with at least one dose of the vaccines from Pfizer-BioNTech and Moderna. Both have shown efficacy rates of well over 90 percent against symptomatic disease in clinical trials, compared with Sinovac's 51 percent...