both AstraZeneca and Johnson & John has about 67% - 76% effectiveness depending on whether it's against asymptomatic or mild cases of the disease so China's Sinovac should be the same and is shown to be the same according to various tests conducted by various countries. But this is against the original unmutated virus, according to numerous studies and particularly this one:
"Lab tests have suggested so far that the major approved vaccines will work against P1, but with reduced efficacy. A study from Oxford University, published on 30 March in the peer reviewed journal Cell, looked at the antibody response in blood samples from people with P1 elicited by the AstraZeneca and also the Pfizer/BioNTech vaccines. They showed nearly a threefold reduction in neutralisation, so efficacy is reduced".
https://www.theguardian.com/world/2021/mar/01/brazil-covid-variant-p1-britain
In other words, all vaccines had neutralization reduced by 1/3, if AstraZeneca only had on average 76% efficacy rate, then after 1/3 reduction against the virus strain with the mutated variants, it would only have (76 * (1 -0.33333333333)) = 50% efficacy. So if China's Sinovac had about the same efficacy rate as AstaZeneca against the original virus, then it makes sense that its real efficacy rate is 50 something % against the virus with the mutated variant strains.