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.
Foy HM, Cooney MK, McMahan R. A/Hong Kong Influenza Immunity Three Years After Immunization. JAMA. 1973;226(7):758–761. doi:10.1001/jama.1973.03230070024006
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