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Why is it essential to get my second dose of the vaccine in the right time frame?

The first dose of the vaccine primes or prepares your immune system by teaching it to recognize and build an immune response against the spike protein from the virus that causes COVID-19 infections. The second dose is a booster, functioning as an immune reminder. After the waiting period, the second dose elicits a stronger response, providing more protection because the immune system has prepared itself in the event it sees that virus again. This second dose makes us more confident that you are fully protected since it boosts the immunity to a high level, conferring a 95% (Pfizer-BioNTech) or 94% (Moderna) protective efficacy in preventing COVID-19.

The time between the two vaccine doses allows the immune system to rest and prepare for a second response. Experts say that receiving the two doses too close together can shorten the total length of time the spike protein is in the body, reducing the time the immune system has to prepare a strong response. The timing is also based on the clinical trial design for both vaccines.There is limited data on the effectiveness of these vaccines when administered beyond the recommended window.

A study conducted by the Centers for Disease Control and Prevention found that vaccine effectiveness was 80% for frontline and essential workers, who were partially immunized. Partially immunized workers were defined as having received their first dose at least 2 weeks prior to being tested for COVID-19 but had yet to receive their second dose.

While it is recommended to receive the second dose as close to the recommended interval as possible (3 weeks for Pfizer-BioNTech and 1 month for Moderna), if it is not feasible, the second dose may be administered up to 6 weeks after the first dose.

Updated 6 April 2021

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