During which of the stages does an enveloped virus acquire its envelope? Prior to the spring of 2020 , the respiratory virus that most concerned public health officials was influenza (httos://wwwicdc,gov/flu/about/index,html). Read the information at this link (https://www.cdc.gov/flu/about/viruses/change.htm) then summarize the two ways that influenza can change. Antigenic drift Antigenic shift How are these two processes related to the fact that it is necessary for people to receive a flu vaccine every year?.