Donald G. McNeil, Jr. reports in The New York Times that the flu season is “moderately severe” this year but an imperfect vaccine and bitter cold temperatures could make it worse, according to flu experts and public health officials. The H3N2 strain, which tends to kill more of the very young and very old, is making up 80 percent of the cases this year. Even this late in the season, experts still recommend getting flu shots to prevent pneumonia and death. “It’s far from a perfect vaccine, but we can still do a lot of good with a pretty good one,” said Dr. William Schaffner, head of preventive medicine at Vanderbilt University’s medical school.