Question: What, if anything, is known about vaccine efficacy for either vaccine if someone only gets 1 shot?

Answer: We talked about this issue in our Q&A of 1/6.  Since then (on 1/21), CDC updated its clinical guidance, stating [bold font is mine]: 

“The second dose should be administered as close to the recommended interval as possible [21 days after first dose for Pfizer / 28 days for Moderna]. However, if it is not feasible to adhere to the recommended interval, the second dose of Pfizer-BioNTech and Moderna COVID-19 vaccines may be scheduled for administration 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.”  

Based on currently available analyses of the Pfizer clinical trial data published last month in the New England Journal of Medicine (NEJM), we know that the Pfizer vaccine efficacy in the interval between first and second dose is 52% (range: 30% to 68%) (Figure 1).  As authors state, this finding indicates “early protection by the vaccine, starting as soon as 12 days after the first dose.”  For Moderna, researchers report in their analysis of clinical trial data published last month in NEJM, “results from a preliminary exploratory analysis suggest that some degree of prevention may be afforded after the first dose.” The small number of infections + short follow-up time + non-random sample was too limited to draw firm conclusions about the degree of protection afforded by just 1 dose. That said, in the document Moderna submitted to the FDA, they also include efficacy data on those who had only received one dose at the time of interim analysis, showing an efficacy rate of 80% (range: 55% to 92%) with the caveat that the “small, non-random sample and short median follow-up time limits the interpretation of these results.”  Both Pfizer and Moderna caution that the duration of protection afforded after just one shot is also unknown beyond the 21 day / 28 day interval.  Studies designed to answer this question are ongoing.  

If you’d like to read more, this article from The BMJ, Covid-19 vaccination: What’s the evidence for extending the dosing interval?, and this one from the BBC, How effective is a single vaccine dose against Covid-19? are both worth your time!

Figure 1. Efficacy of Pfizer-BioNTech Vaccine Over Time (upper box inset= after 1st dose, before 2nd dose) (from NEJM)