Sean Gardner/Getty Images(NEW YORK) Early in the pandemic, scientists and public health experts leaned on their experience with other viruses to make predictions about COVID-19, hopeful that when enough people developed immunity, the virus would be stopped in its tracks. But in the years that followed, and even after the introduction of highly effective vaccines, vaccine scientists and public health experts interviewed by ABC News realized COVID-19 is unlikely to completely disappear. Although herd immunity through widespread vaccination can be a successful strategy for certain viruses, such as those that cause smallpox and polio, scientists no longer consider it an appropriate management strategy for the virus that causes COVID-19, these experts said. Herd immunity refers to a situation where a virus can't spread because it keeps encountering people who are resistant to it. As a result, a small number of people who lack resistance can still be protected by the "herd"