SEOUL, Dec 15 (Reuters) - South Korea reported 7,850 new coronavirus cases for Tuesday, its highest daily total, as breakthrough infections among those already vaccinated continue to spike, with the number of patients in serious condition also reaching a fresh high at 964. Daily tallies of infections shot past 7,000 for the first time last week, just days after passing the 5,000 mark, putting ever greater strains on the country"s medical capacity. Total infections in the pandemic so far have risen to 536,495, including 128 cases of the Omicron variant, with 4,456 deaths, according to the Korea Disease Control and Prevention Agency. Prime Minister Kim Boo-kyum warned on Wednesday that the government is considering reimposing strict distancing curbs including a ban on gatherings and a curfew on dining in eating establishments. An official announcement is expected on Friday. South Korea has fully vaccinated more than 94% of its adults so far, and is accelerating its ongoing campaign promoting booster shots by shortening intervals for all ages. But the number of new cases has been spiralling since authorities eased distancing rules last month as part of a "living with COVID-19" policy.