Long-term efficacy of oil-adjuvant influenza vaccine in an industrial population.

The mild influenza epidemic of mixed A and B strains early in 1966 provided an opportunity to assess the efficacy of an oil-adjuvant influenza vaccine which had been administered more than two years earlier to 1,341 volunteers at two U.K. Atomic Energy Authority establishments. At the 5% confidence level, a statistically significant reduction in sickness absence due to influenza was found in this vaccinated group when compared with a control group of 918 employees. This trend was observed at both establishments. Some confirmation was thus obtained of serological predictions that protection would last two to three years or longer after inoculation. There was some indication that the vaccine might be more effective in older workers. This trial suggests the need for more long-term evaluation of oil-adjuvant influenza vaccines.