By Ahmed Aboulenein and Jeff Mason WASHINGTON (Reuters) – Health officials are considering extending the eligibility for a second COVID-19 vaccine booster dose to people under 50 amid a steady rise in cases, with the United States seeing a threefold increase over the past month. The U.S. Food and Drug Administration had in late March authorized a second booster dose of the Moderna and Pfizer/BioNTech SE vaccines for people aged 50 and older, citing data showing waning immunity and the risks posed by Omicron variants of the virus. “With regard to a fourth dose for those under the age of 50, tha…