Question: My wife’s school district is trying to get her to go back 3 weeks after her first Pfizer dose, 1 week before her second dose. What is the protection she will have first going back?  

Answer: In good news, she will have a good deal of protection 3+ weeks after her first dose; her risk of infection will be reduced by an estimated 60%.  Of course, the protection afforded 7+ days after the second dose will be even stronger; her risk of infection will be reduced by an estimated 92%.  Read on for the details. 

We talked about Pfizer vaccine efficacy after the first dose in our Q&A of 1/23, but since then, we have more real-world evidence.  As I wrote in late January, the Pfizer clinical trial data published in the New England Journal of Medicine (NEJM), shows that the Pfizer vaccine efficacy in the interval between first and second dose is 52% (range: 30% to 68%).  More recently, NEJM published an analysis of real-world vaccine effectiveness from Israel, BNT162b2 mRNA Covid-19 Vaccine in a Nationwide Mass Vaccination Setting.  Researchers compared infection and disease outcomes between vaccinated and unvaccinated individuals. Using data from a study group of nearly 1.2 million individuals, researchers calculated vaccine effectiveness 14 – 20 days and 21-27 days after the first dose and 7+ days after the second dose (Table 1).  As you can see in Table 1, protection afforded after the first dose as documented in real world settings is quite similar to that documented in the clinical trial, and is relatively high (though not nearly as high as effectiveness after the 2nd dose).  And in case you want a reminder of what the terms “vaccine efficacy” and “vaccine effectiveness” mean, check out our Q&A of 2/19 and/or this recent NY Times article, What Do Vaccine Efficacy Numbers Actually Mean?, which also offers some great visualizations.    

Table 1. Pfizer Vaccine Effectiveness (from NEJM paper)