Serological and prophylactic trials of Asian influenza vaccines in Sweden, 1957

A serological trial of a formolized monovalent Asian influenza vaccine of conventional type, using a range of falling vaccine dilutions, showed a satisfactory antihaemagglutinin response in groups of volunteers vaccinated with relatively low numbers of haemagglutinating units (HU) of the vaccine.Booster injections, given 15-19 days after the primary vaccination, gave a further increase and more persistence of antibody response.In a field protection trial, performed in more than 1000 recruits who received a similar vaccine prepared from another line of the same virus strain, a good degree of protection (57%-78%) was obtained after a single injection of 2000 HU of aqueous vaccine. A lower but still statistically significant degree of protection (29%-45%) obtained with only 400 HU of the vaccine adsorbed on aluminium phosphate. Two injections or 2000 or 400 HU of vaccine (without AlPO(4)), given 9 days apart, protected, respectively, 69%-86% and 45%-51% of the vaccinated. The slightly better protective effect of the two-injection scheme appears to be due to the double amount of vaccine given rather than to a recall reaction.The numerical results of both trials were used for an evaluation of the quantitative relation between the antibody-stimulating and protective effects of influenza vaccines.Some pitfalls in the current method of expressing the potency of influenza vaccines in terms of their haemagglutinin content are discussed.