WASHINGTON—U.S. services industry activity accelerated in December, but sky-rocketing COVID-19 infections depressed employment, heightening the risk that the economy shed jobs for the first time since the labor market recovery from the pandemic started. The Institute for Supply Management (ISM) said on Thursday its non-manufacturing activity index increased to a reading of 57.2 last month from 55.9 in November. That lifted the index to within striking distance of its 57.3 level in February, before the coronavirus pandemic reached the United States. A reading above 50 indicates growth in the services sector, which accounts for more than two-thirds of U.S. economic activity. Economists polled by Reuters had forecast the index falling to 54.6 in December. The surprise pick-up in services sector activity mirrors a similar rise in production at factories last month despite the country being engulfed by a fresh wave of coronavirus cases. As with the manufacturing data, the improvement …