Sinovac Vaccine Falls Short of Expectations, But Options Limited

Also read: Indonesia to Start Vaccinating Children With the Covid-19 Vaccine

"Yes, you're getting some breakthrough infections with the delta variant, but they tend not to be serious infections," he said. "People aren't being hospitalized or losing their lives because of COVID-19."

"When you look at some of the data on the Sinovac vaccine," he added, "the levels of virus-neutralizing antibody, even after two doses, can still be quite low."

The Sinovac vaccine produced lower levels of these antibodies than seven other vaccines, including those from Pfizer, Moderna, University of Oxford-AstraZeneca and Johnson & Johnson, according to a study in the journal Nature Medicine.

The antibody response is even less effective against the delta variant, which has exploded in Indonesia and many parts of the world.

It's not clear, however, exactly what that decline means for patients. The vaccine still offers protection against the most serious forms of the disease, a Chinese official told state media.

In China's first delta outbreak, in Guangdong province earlier this month, "none of those vaccinated infections became severe cases, and none of the severe cases were vaccinated," said Feng Zijian, former deputy director at the Chinese Center for Disease Control and Prevention.

Meanwhile, supplies of other vaccines are arriving slowly in much of the world.

"Sometimes, that's all people have access to," Hotez said. "It's better than nothing."