Pfizer and Moderna vaccines use messenger RNA that encodes the spike glycoprotein. The biology through which the mRNA vaccines induce their protection from COVID-19 is fundamentally different from that with other vaccines. What the immune system does between the first and second dose But the good news is that even while supplies remain limited, the science suggests that there’s nothing bad about getting a second dose as late as 42 days after the first. Since protection was approximately 95% for everyone who was vaccinated within this time “window,” there is little reason not to allow some flexibility in the timing of the second dose 2.Īs more vaccine becomes available, the timing of the second dose should be close to four weeks for the Pfizer and Moderna vaccines. In the clinical trial, the second dose of the Pfizer vaccine was administered as early as day 19 and as late as day 42 to 93% of the subjects. Why the CDC decided receiving the second dose within 42 days is OK In the case of the Pfizer and Moderna COVID-19 vaccines, the second dose increases the protection afforded by the vaccine from 60% to approximately 95%. The second dose, or booster, provides the opportunity for the immune system to ramp up the quality and quantity of the antibodies used to fight the virus. This allows the immune system to prepare its defense. The first dose primes the immune system and introduces the body to the germ of interest.
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