One of the cases they are examining is a Virginia woman who died in March, health officials confirmed Tuesday.
All six cases occurred among women between the ages of 18 and 48, and symptoms occurred 6 to 13 days after vaccination, the CDC said.
To date, the rate of vaccinations has exceeded predictions made earlier in the year. More than a third of the country has received at least one dose, and average daily injections have topped 3 million. At the same time, average daily infections rose nearly 7 percent nationwide over the past week, driven by more-transmissible new variants and a loosening of restrictions.