Why scientists are not able to discover vaccine of corona virus.

Viruses are simple entities that are able to survive only inside a host. They consists of a genetic material (either DNA or RNA, not both) enclosed inside a protein coat. 

It is difficult to make vaccines against certain viruses as they have a unique ability to undergo modifications in their protein structure when they come in contact with medicines or vaccines. In other words they can change their cell surface glycoproteins very frequently hence, the vaccine manufactured targeting these viruses might not stimulate any immune response.

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