One suggested treatment for patients with COVID-19 is the administration of blood plasma from people who have recovered from the disease. Why would this treatment be effective? Would this treatment have the same effect as COVID-19 vaccination?

People who have been infected with the disease will have antibodies present in their blood. These circulating antibodies can combine with virus circulating in the bloodstream and neutralise it before it has the chance to infect cells and cause disease. Because the immune system is not directly contributing to protection in this scenario, this is known as passive immunity. Because the immune system hasn't been actively involved no immune memory will be generated. When a vaccine is used the adaptive immune system is activated and memory should occur meaning that longer term protection can be achieved.