The Centers for Disease Control says that a preliminary COVID-19 vaccine “safety signal” has been identified and is investigating whether the Bivalent Pfizer-BioNTech vaccine creates an increased risk of ischemic stroke in people 65 and older.

In the statement, the CDC says that the preliminary signal hasn’t been identified with Bivalent Moderna COVID-19 vaccine.

“Following the availability and use of the updated (bivalent) COVID-19 vaccines, CDC’s Vaccine Safety Datalink (VSD), a near real-time surveillance system, met the statistical criteria to prompt additional investigation into whether there was a safety concern for ischemic stroke in people ages 65 and older who received the Pfizer-BioNTech COVID-19 Vaccine, Bivalent.”