Question: Thinking about getting my booster dose (I meet the criteria), but curious to know whether I should expect any side effects.  What are the side effects associated with a 3rd/booster dose?

Answer: Based on recent analyses (described herein), you should expect to have a reaction similar to what you experienced after the second dose.  We have data from 306 participants 18-55 years old in Pfizer’s clinical trial who received a 3rd dose.  The data show that the frequency and type of side effects (reterogenicity) are similar to those experienced after the 2nd dose.  The most common side effects experienced within 7 days of booster were “injection site pain, fatigue, headache, muscle and joint pain, and chills.”  Of course, we’d like to see data from even more people, including people of older ages.  Here’s where a report published in CDC’s MMWR just yesterday(!) comes in.  The report, “Safety Monitoring of an Additional Dose of COVID-19 Vaccine — United States, August 12–September 19, 2021,” presents analysis of >22,000 v-safe (a voluntary, smartphone-based U.S. safety surveillance system) participants who received a 3rd mRNA dose, 12,591 of whom completed a health check-in survey after each of 3 doses of an mRNA vaccine received.  The data show that “local reactions are slightly increased and systemic reactions are slightly decreased after dose 3 of an mRNA than after dose 2;” and “Local (16,615; 74.9%) and systemic (15,503; 69.9%) reactions were frequently reported during the week after an additional dose of COVID-19 vaccine, most commonly on the day after vaccination. Frequently reported reactions were injection site pain (15,761; 71.0%), fatigue (12,429; 56.0%), and headache (9,636; 43.4%).” (Figure 1)   

Figure 1. Reactions and health impacts reported by dose and mRNA vaccine type, among persons who received 3 doses (from CDC’s MMWR)