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 vaccines(The effectiveness of inactivated influenza vaccines* has been variable, and protection has been relatively brief. This has contributed) 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. 1972;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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