Drugstore chains Walgreens Boots Alliance and Rite Aid said on Wednesday they would start offering second booster doses of mRNA COVID-19 vaccines, after the shots were cleared in the United States. U.S. health officials on Tuesday authorized second booster shots of Pfizer and Moderna’s vaccines for people aged 50 and older, citing data showing waning immunity and the risks posed by Omicron variants of the virus. The BA.2 sub-variant of Omicron has triggered a surge in infections in many countries across Europe and Asia, and raised concerns over a potential new wave in the United States. More than half of the COVID-19 cases in the United States are now caused by the BA.2 sub-variant, according to data from the U.S. Centers for Disease Control and Prevention (CDC). The U.S. Food and Drug Administration (FDA) also authorized a second booster dose of the vaccines for younger people with compromised immune systems. …