WASHINGTON — The tone of some Republican leaders on COVID-19 vaccines has shifted in response to the highly infectious delta variant, but a continued emphasis on values like personal liberty and privacy could muddle the message. The delta variant is proliferating, comprising 83 percent of cases in the United States and an even higher percentage in places with lower vaccination rates, according to the Centers for Disease Control and Prevention, with repercussions for the country’s most vulnerable people as well as the economy’s fragile recovery. At the same time, nearly half of the House Republican caucus has yet to say publicly if they are themselves vaccinated, according to CNN.