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November 12, 1973

A/Hong Kong Influenza Immunity Three Years After Immunization

Author Affiliations

From the departments of epidemiology and international health (Dr. Foy and Ms. McMahan) and pathobiology (Dr. Cooney), the School of Public Health and Community Medicine, University of Washington, Seattle.

JAMA. 1973;226(7):758-761. doi:10.1001/jama.1973.03230070024006

We studied influenza infection and disease in school children vaccinated with either 400 chick cell agglutination units of A/Hong Kong influenza virus vaccine or monovalent influenza virus vaccine type B (controls) in 1968, through the third A/Hong Kong influenza epidemic in January 1972, in Seattle.

During the three A/Hong Kong influenza epidemics (1968 to 1969, 1970, and 1972), the serologically determined infection rates among controls were 14%, 23%, and 26%. Only one of the 156 control subjects had serologic titer rises in two epidemics, suggesting that repeated A/Hong Kong influenza was rare. Based on serologic data and illness reports from 303 children observed during the 1972 epidemic, the A/Hong Kong influenza virus vaccine given three years earlier was still approximately 60% effective in preventing influenza. Considering that many control children had become naturally immune in the interim, true efficacy may have been higher. Vaccine-induced immunity was also reflected in lower rates of school absenteeism among the 1,516 students so studied.