Why are antibiotics only effective against bacteria? Because we have defined antibiotics to be compounds that act against bacteria.

Your phrasing reads kind of arrogant, just link up Antimicrobials to broaden their definition.

Small molecules that act against viruses are referred to as antivirals. They are less commonly used because they tend to be much more specific to the type of virus, so you can't just take one antiviral for any virus like you would take ampicillin for many different types of bacterial infection.

You're right but there should be a "for now" in here. DRACO)

Why are vaccines more effective against viruses? I'd rephrase that as "Why do we use more vaccines for viruses than for bacteria?". Some of that is because there aren't good small molecules to target viruses, so we have concentrated on vaccine development.

Huh? List of Anti-virals There are plenty and it's a huge market. Antivirals—an increasingly healthy investment

Partially, viruses are simpler entities with many fewer proteins than a bacterium, so it is easier to isolate the antigens that might provoke a productive immune response. Lastly, bacteria and viruses are seen differently by the immune system - viruses are assembled and disassembled during infection, while bacteria remain encased in a cell capsule, which can be fortified against the immune system. This difference might make a vaccine more likely to be successful in protecting against a virus than the bacterium.

These two points are the same, bacteria possess a larger repertoire to evade, attack, and redirect our immune system. This makes developing a vaccine particularly challenging because there may be redundant virulence mechanisms. But vaccines do exist with a lot in development: DNA Vaccines against Bacterial Pathogens