Preliminary CDC Data Finds Side Effects from COVID Booster Similar to Second Shot Interview with:

Dr. Hause

Anne Hause PhD
Centers for Disease Control and Prevention What is the background for this study?

Response: On August 12, 2021, the Food and Drug Administration (FDA) amended Emergency Use Authorizations (EUA) for the Pfizer-BioNTech and Moderna COVID-19 vaccines to authorize administration of an additional dose following completion of a primary vaccination series to eligible persons with moderate to severe immunocompromise. What are the main findings? 

Response: Preliminary data on the safety of an additional dose of mRNA COVID-19 vaccine found that side effects, which are expected with vaccination, were similar to those from second vaccine doses, and were mostly mild, moderate, and short-lived.

The most commonly reported side effects included arm pain at the shot location, fatigue, and headache. These initial findings indicate no unexpected patterns following an additional dose of COVID-19 vaccine. What should readers take away from your report?

Response: As of September 19, 2021, approximately 2.21 million persons have received additional doses of COVID-19 vaccines. During August 12–September 19, 2021, no unexpected patterns of adverse events were found among more than 21,935 registrants who received an additional dose of COVID-19 vaccine. Most reported local and systemic reactions were mild to moderate, transient, and most frequently reported the day following vaccination. Most registrants who received an additional dose reported a primary mRNA vaccination series followed by third dose from the same manufacturer. What recommendations do you have for future research as a result of this work?

Response: CDC and FDA continue to monitor COVID-19 vaccine safety, including the safety of additional doses of COVID-19 vaccine, through various surveillance systems. These data are used to guide vaccine recommendations and protect public health. One of the surveillance systems CDC is using is v-safe, a smartphone-based safety surveillance tool that uses text messaging and web surveys to give personalized health check-ins after you receive a COVID-19 vaccine.

CDC will continue to monitor the safety of additional doses of COVID-19 vaccine. Additional data on adverse reactions associated with different combinations of vaccines and of time since completion of primary series will be important to guide public health recommendations. Is there anything else you would like to add?

Response: The findings in this report are subject to at least four limitations.

First, enrollment in v-safe is voluntary and likely not representative of the immunized U.S. population. Second, during this study period additional dose recommendations were limited to immunocompromised persons who completed a primary series of mRNA COVID-19 vaccine; however, v-safe does not include specific information about immune status. Additional dose recipients likely include persons with and without immunocompromise. Third, a causal relationship between a vaccine and clinically serious adverse event reported following vaccination cannot be established with v-safe. Finally, insufficient data were available to determine patterns of adverse events after receipt of an additional dose from a manufacturer different from the primary series or for the Janssen vaccine.


Self WH, Tenforde MW, Rhoads JP, et al. Comparative Effectiveness of Moderna, Pfizer-BioNTech, and Janssen (Johnson & Johnson) Vaccines in Preventing COVID-19 Hospitalizations Among Adults Without Immunocompromising Conditions — United States, March–August 2021. MMWR Morb Mortal Wkly Rep 2021;70:1337–1343. DOI: icon.



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Last Updated on September 30, 2021 by Marie Benz MD FAAD