H1N1 vaccines. A report issued by the Centers for Disease Control and Prevention stated that among 120 adults who received an injection of a monovalent H1N1 influenza A vaccine, 116, or 97%, had an effective response by three weeks after the vaccination. They also reported that the rates of adverse events such as headaches were not significantly different from those of a control group.2 Is this an observational study or an experiment? Why?