Meet the four frontrunners in the COVID-19 vaccine race

Like AZD1222, CanSino uses the viral vector-based vaccine based on a weakened adenovirus. However, the adenovirus in this vaccine—Ad5—is one that circulates in humans, not chimpanzees. This is problematic because past exposure to the human adenovirus appears to throw off immune responses to the bit of the vaccine that’s derived from SARS-CoV-2. In earlier published phase I trial data—previously reported by Ars here—researchers noted that those who had already been exposed to the adenovirus did not produce immune responses as robust as those who had not been exposed.

Nevertheless, CanSino forged on with a randomized phase II trial, involving 508 volunteers (aged 18 to 83) who received either a placebo or a single injection of Ad5-vectored COVID-19 at one of two dosage levels.

Mild side effects including fever, fatigue, headache, or pain at the site of injection were common. Though 24 participants in the high dose group and one in the low dose group had side effects rated as severe, there were no serious reactions.

Researchers found that more than 96 percent of participants who received Ad5-vectored COVID-19 developed antibodies against SARS-CoV-2. But researchers detected SARS-CoV-2 neutralizing antibodies in only 59 percent of the high dose group (148 out of 253 participants) and 47 percent of the low dose group (61 out of 129 participants).