Scientific American explores the potential risks and benefits of delaying vaccine doses. Two-shot vaccinations aim for maximum benefit: the first dose primes immunological memory, and the second dose solidifies it, says Thomas Denny, chief operating officer of the Duke Human Vaccine Institute. Though most trial participants received their second vaccine on day 21 or 28, some waited until day 42, or even longer. For example, of 15,208 trial participants who received the Moderna vaccine, only 81 (0.5 percent) received it outside the recommended window. Could a longer interval between first and second doses make a COVID vaccine more effective?