Note: Happy President’s Day Weekend.  I’m pausing daily updates for the long weekend until Tuesday, 2/16.  In the meantime, please keep sending your questions and I’ll add them to the queue.  

Question: I am going on a cross country RV trip in May for 6 weeks. If I get the opportunity to take the first vaccine before I leave but have to wait 6 to 8 weeks for the booster will it still be effective?

Answer: I’m going to focus this answer on the two vaccines currently authorized for use in the US– Pfizer and Moderna.  I imagine that by the time May is here and you’re off on your amazing RV adventure, we’ll have a much wider variety of vaccines in distribution and the guidance on dose timing could very well differ by vaccine.  For example, AstraZeneca reports improved efficacy with a longer interval between doses (12+ weeks).  So back to what we know of Pfizer and Moderna… Ideally, you would be vaccinated according to the clinical trial protocols, which means 21 days after the first dose for Pfizer and 28 days after for Moderna. Of course, we can’t all achieve the ideal.  In that case, an interval of 6 weeks should still be okay.  I’d recommend against waiting longer than 6 weeks as we have no data on the efficacy of vaccines administered beyond that 6 week window (see CDC clinical guidelines, relevant portion copied below).  

We talked about vaccine efficacy after 1 dose in our Q&A of 1/23.  Since then, the UK’s vaccination program is reporting that real-world data suggest the Pfizer vaccine is 65% effective after the first dose (findings not yet peer reviewed).  This to say, you will have some important protection after the first dose and you should aim to get your second dose as soon as you can within the 3-6 week window.    

CDC clinical guidelines
The second dose should be administered as close to the recommended interval as possible. However, if it is not feasible to adhere to the recommended interval and a delay in vaccination is unavoidable, the second dose of Pfizer-BioNTech and Moderna COVID-19 vaccines may be administered up to 6 weeks (42 days) after the first dose. There are currently limited data on efficacy of mRNA COVID-19 vaccines administered beyond this window. If the second dose is administered beyond these intervals, there is no need to restart the series.