WASHINGTON — Vice President Kamala Harris stood by the pharmacy counter of a Giant grocery store in a largely Black neighborhood here Thursday, talking an older customer through her booster dose of the COVID-19 vaccine. “It will feel, when you get the shot, the same way the first one felt,” Harris told her, recounting her own experience. “The next day, I realized I needed to take it a little slow.” This latest example of Harris’ retail effort to promote the vaccine, as the vice president stood between the frozen pizza bin and bags of quinoa, underscored her central role for the Biden administration in persuading Black Americans to take the shot. With many of them wary based on a history of mistreatment by the medical system, Harris’ participation is seen as crucial, especially as some other high-profile Black influencers, including NBA stars, have declined to take on a public role.