It didn’t seem that long ago that infectious disease experts spoke hopefully of a spring reprieve in COVID-19 cases. But that was before the latest, even more, transmissible Omicron variant, BA.2.12.1, exploded across Massachusetts in recent weeks and began to shove aside its predecessor, BA.2, in other regions, as well. Cases are climbing, edging the United States across the threshold of 1 million deaths. Hospitalizations are rising too, albeit more slowly.