Epidemiologists in New York and a few other cities that were awash in swine flu last spring are detecting very little evidence of a resurgence. Officials in New York, which was the nation's hardest-hit city, say that flu activity is no higher than it normally is at this time of year and that school attendance is normal. New York City public health officials are still conducting an extensive immunization campaign, and they agree that it is far too early to draw any final conclusions.