CanSino Bio Photo:VCG

Chinese vaccine producer CanSinoBIO said on Wednesday that it had not received any report of blood clots after vaccination of millions of people with its recombinant adenovirus vector COVID-19 vaccine, amid reports that inoculation has been halted in various countries for AstraZeneca and Johnson & Johnson's COVID-19 vaccines, which use the same technical method as CanSinoBIO's product.

CanSinoBIO said that, although its vaccine uses the same technique, it uses different adenovirus than the other two companies. CanSinoBio uses adenovirus type-5 as vector, a common adenovirus among humans, while AstraZeneca uses chimpanzee adenovirus and Johnson & Johnson uses adenovirus type-26.

The US Centers of Disease Control and Prevention (CDC) and Food and Drug Administration (FDA) on Tuesday issued a statement recommending a pause in the use of Johnson & Johnson vaccine out of an abundance of caution after they reviewed data of six cases, including one death, of a rare and severe type of blood clot reported in the US after receiving the shots.

Despite the blood clot issue, Chinese experts said it does not necessarily mean that the technique is to blame, noting that Russia and China have also administered such vaccines but no serious clot cases have been reported. 

The human body usually produces platelet-associated antibodies after accepting vaccines. Such antibodies can promote the coagulation of platelet, which possibly could lead to blood clots. But it is still unclear why such antibodies would occur after vaccination, a Beijing-based immunity expert who requested not to be named told the Global Times on Wednesday. 

The expert noted that blood clots are a very rare side effect of vaccines. Seemingly no similar report of blood clot related to any commercial vaccine had been reported before the COVID-19 pandemic. 

Cases of bleeding due to a decrease of platelets after vaccination have been reported, but it is also very rare, according to the expert.  

Another possible reason lies in the process of production. It could be a lacking of experience or the problem of materials, the Beijing-based anonymous expert said, referring to media reports in March that said workers at a US manufacturing subcontractor of both AstraZeneca and Johnson & Johnson mixed ingredients of the two's vaccines.

The problem is that first-class producers such as Merck or GSK are not involved in COVID-19 vaccine. So the serious side effects reported after vaccination in the West could be related to limited production conditions and inexperience of the producers, the experts said. 

The rollout of the Johnson & Johnson vaccine has been suspended in the US, EU and South Africa following the US CDC and FDA joint statement.

Western media have expressed concerns over delayed vaccination in areas affected by the suspension of the two vaccines. 

However, the Beijing-based expert warned that it is better to be cautious and called for further study on the occurrence of platelet-associated antibodies after vaccination. "It may be a very small incidence among a large population, but it would be a 100 percent incidence for a person once it happens to him or her."