It turned out that the vaccine antibodies decreased more quickly than had been expected, decreasing to less than 50% resistance (protection against hospitalization or death), so a 2nd jab was indicated, especially given the arrival of the more infectious and more virulent Delta variant. This boosted resistance back up to around 75%. Giving a booster of a different vaccine was found to take it back up to over 90%.I'm aware that Covid is deadly but one **ONE** vacccine shot would be enough already.
Boosters were considered fairly early, but initial moves were slow. As the Omicron variant started spreading rapidly, its virulence was uncertain, and health services were under extreme stress, a booster was considered to be the best way to minimize further serious illness or death, with the consequent additional pressure on health services.
Getting all three jabs within 6 months is probably not optimal - ISTR suggestions that longer gaps could be better. Here in the UK, it was typically 3-4 months between 1st & 2nd, and 6 months for the booster.
AIUI, it has since been shown that prior infection provides roughly as much protection against serious illness or death from further infection as the two jabs, and a booster increases that significantly.
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