Why only 3 doses, flu vaccine is every year and still thousands die
The flu virus mutates much more - it passes from humans through biirds and other animals back to us each year, so last year's vaccine is not very good this year and the same applies to your immune memory.
Covid 19 doesnt mutate much - it appears to have a checking system at replication that discards mutation. Mutation has been at a small area on the virus, and so the same vaccine will likely work in the future.
The third dose produces 7-11 TIMES as many neutralising antibodies as the second dose and while not proof it suggests the third dose MAY provide much longer immunity as does repeated hepatitis B vaccine. At the end of the day we wait for the evidence but the world is already making enougth vaccine for the entire planet every year (there is a subject for another thread) so if we do need another top up it isnt that difficult at least in the developed world
what's the difference between droplets and aerosol
Size. Droplets are bigger so fall onto the floor and surfaces. Aerosols are smaller and float around. I am very suspicious abouyt the hand washing industry - London Transport had a good study undertaken where stairs, trains and buses were repeatedly swabbed and they effectively failed to find any Covid 19 virus on anything.....people got infected from virus particles floating in the air