The VSD monitoring system found that 130 people ages 65 and older had a stroke within 21 days of receiving the Pfizer Omicron booster among about 550 000 seniors who received the vaccine, a CDC spokesperson said. No deaths have been reported. No other surveillance system has detected a similar safety concern for the Pfizer booster so far, and investigators had not seen an increased risk of stroke from the booster after reviewing data from the Centre for Medicare and Medicaid Services, the Department of Veterans Affairs, the Vaccine Adverse Reporting System and Pfizer's global safety database. "Although the totality of the data suggests it is very unlikely that the signal in VSD represents a true clinical risk, we believe it is important to share this information," the CDC said. Investigators hope to have a clearer picture and more data in the coming.