Republican Sen. Lindsey Graham of South Carolina said in a tweet on Monday that he had tested positive for COVID-19 even after being vaccinated. Graham said he has "mild" symptoms at the present time and will be quarantining for 10 days. "I am very glad I was vaccinated because without vaccination I am certain I would not feel as well as I do now," he added. There is growing worry about breakthrough infections in vaccinated people, even though such infections are rare.