Here's a good overview of the various types of Covid vaccines and their mechanisms -
The AstraZeneca and J+J Covid vaccines are DNA/viral vector type, while the Pfizer and Moderna vaccines are mRNA type.
One problem with the DNA type vaccines is the risk that the viral DNA will integrate into/change the human recipient's genome. The AstraZeneca vaccine uses a chimpanzee adenovirus as the vector to deliver the DNA into the nucleus of the human recipient's cells. mRNA vaccines don't use the viral vector, and the mRNA stays in the cytoplasm of the cell, and (supposedly) doesn't enter the nucleus.
Most of the other Covid vaccines (China, Russia, India) are of the older type that inject inactivated virus or viral subunits/fragments. One problem with these is that they only induce antibody mediated immunity, and not cell mediated immunity, and often have to use an adjuvant to boost the body's response (like the infamous squalene adjuvant that can trigger autoimmunity)
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