A subvariant of the Omicron strain of the novel coronavirus, BA.2, now accounts for just over a third of daily reported COVID-19 cases in the United States, according to government data released on Tuesday. As overall infections continue to decline from January’s record highs, the subvariant is gaining ground across the country, data from the Centers for Disease Control and Prevention (CDC) show. Omicron BA.2 accounted for 34.9 percent of U.S. infections for the week ending March 19, the CDC said. That compares with 22.3 percent for the week ending March 12, which was revised down from 23.1 percent, according to a CDC model that estimates proportions of circulating variants. In the Northeast, including New Jersey, New York, and Massachusetts, Omicron BA.2 now makes up more than half the cases. The CDC and the World Health Organization (WHO) have classified the subvariant as a “variant of concern.” The WHO says …