American troops are being encouraged but not required to get COVID-19 vaccine booster shots, according to a newly published memorandum. Troops were told earlier this month in the memo, published by the military on Tuesday, that “receipt of a COVID-19 vaccine booster dose is not mandatory.” The memo, from Gilbert Cisneros, undersecretary of defense for personnel and readiness, also said that a booster shot is not required for a person to be considered fully vaccinated. Cisneros noted that the Centers for Disease Control and Prevention recommended last month that all adults 18 and older get a booster dose because of the waning effectiveness against both infection and severe disease seen among those who received a primary regimen. People can get a Pfizer or Moderna booster six months after the completion of the primary series or a Johnson & Johnson booster two months following the single-shot vaccine. Teenagers aged 16 or 17 …