The CanSino vaccine is based on a modified cold-causing virus that can't replicate and carries the instructions for human cells to make a key SARS-CoV-2 protein. This is turn stimulates an immune response. Essentially the vaccine acts as a delivery truck carrying genetic cargo. https://twitter.com/Benazir_Shah/status/1327163458362740736

This is called a "viral vector" and is the same approach as the Oxford, J&J vaccines etc.
They mimic natural infection, causing the immune system to make not just antibodies, which happens with traditional vaccines, but also T cells that find and destroy infected cells

A key advantage over the Pfizer vaccine, which uses experimental mRNA technology, is it doesn't need to be stored in ultra cold freezers that can complicate supply and distribution, especially in less developed countries

One drawback of the Ad5 vector used by CanSino is how common this adenovirus can be in populations, which means people might have antibodies that attack the vaccine itself and reduce immune response. This was the case for roughly half of participants in Phase 2 results

Those results, published in The Lancet, also found people aged 55 had lower levels of antibodies, probably because they're more likely to have been pre- exposed to Ad5. Other companies are trying different viral vectors -- time will tell which, if any, works out best.