The top public health official in Los Angeles County stood at a swine flu vaccination site in Compton, Calif., on Tuesday and gently told elderly residents that they really ought to go home. “I explained to people 65 or older, ‘The reason we are doing this is for children,’ ” said the official, Jonathan E. Fielding, the director of the county’s Department of Public Health. “I told them: ‘They are at very high risk for this flu, and you’re at low risk. I am sure you wouldn’t want to get a shot that left a kid who is at risk...