This is true of all vaccines: Their protective effects take several days or weeks to kick in. It’s the reason we get our flu shots in the fall, well before the height of respiratory-virus season, and it’s why health officials often recommend that vaccines required for travel, such as those that ward off yellow fever, be administered about a month or more in advance. Vaccination, and the defenses it affords, is less a singular event than a series of steps on a shifting landscape.
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