The difference is that this vaccine prevents people from getting "symptomatic COVID", not from becoming contagious (pre-symptomatic)
That's because the mechanism for COVID is different than the other diseases you mentioned. Which don't become contagious until "AFTER" the onset of symptoms, which the vaccine is meant to stop.
And the booster argument is a red herring. As many vaccines require periodic boosters, from a four shot regiment for Polio.
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Poliovirus vaccine:
The immunization schedule table says the child should get four vaccine shots starting from 2 months of age. Experts recommend a second shot of the vaccine when the child turns four months old. At the same time, the third dose is given to 6 – 17 months old infants. The last amount is given to kids aging between 4 and 6 years.
Diphtheria, tetanus, and acellular pertussis vaccine:
Popularly known as the DTaP vaccine is given to children aging less than seven years. As per the updated immunization schedule, kids are to give five DTaP doses.