Microbiology This virus is a single-stranded RNA orthomyxovirus. Annual vaccination is necessary because of antigenic drift and shift. A Measles virus B Influenza virus C Respiratory syncytial virus D Parainfluenza virus 💡 Explanation This virus is a single-stranded RNA orthomyxovirus. Annual vaccination is necessary because of antigenic drift and shift. ✓ Correct Answer: B. Influenza virus