Influenza occurs in the United States every year, but the incidence and geographic extent vary widely. Periodically, it appears in epidemic form as a result of antigenic variation in prevalent viruses and the relative susceptibility of the population. Both type A and type B influenza viruses undergo antigen changes. Such changes usually occur slowly, but occasionally they are rapid and abrupt. Epidemics caused by type A influenza viruses occur more frequently and are generally more severe than those caused by type B.
The effectiveness of inactivated influenza vaccines1 has been variable, and protection has been relatively brief. This has contributed
Influenza Vaccine: Recommendation of the Public Health Service Advisory Committee on Immunization Practices. Ann Intern Med. ;77:425–426. doi: 10.7326/0003-4819-77-3-425
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Published: Ann Intern Med. 1972;77(3):425-426.
Infectious Disease, Influenza, Prevention/Screening, Vaccines/Immunization.
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