Sinovac ineffective to Delta Variant

The efficacy of the Sinovac vaccine was always in question with only 50.4% against the first incarnation of the Covid-19 virus.

Now that the Covid-19 has varied many times and the delta variant is spreading like wildfire in China. With >90% inoculation rate in China (with the Chinese vaccine) why is the covid-19 delta variant able to spread so quickly in China.

It seems that Chinese Covid-19 vaccine is completely ineffective against the delta-variant. Every other vaccine producer has lowered their efficacy rate for their vaccine against the delta-variant except Sinovac. Actually Sinovac has yet to release data on the phase-3 trials; if any was even conducted.