Serious allergic reactions after  COVID-19 vaccines are likely more common than serious reactions following flu vaccines, but are still very rare, according to a new report.

The report authors, from the Centers for Disease Control and Prevention (CDC), analyzed data from the first 1.9 million doses of COVID-19 vaccine administered in the U.S. from Dec. 14 through Dec. 23. For most of this time, only the Pfizer-BioNTech COVID-19 vaccine was available in the U.S.