Their shots just aren’t as good as mRNA vaccines.
slate.com
“Even without an mRNA vaccine, there is progress China could make on vaccination. A large
study conducted by looking at case rates in Hong Kong’s population of 7.4 million also observed a lower efficacy of two doses of CoronaVac compared with two doses of Pfizer’s mRNA vaccine. However, three doses of CoronaVac almost matched the efficacy of three doses of Pfizer’s vaccine, at least in the short term. The study’s authors therefore recommend vaccinating the older population with a third dose of either CoronaVac or the Pfizer vaccine—though, again, the latter isn’t available in China.