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The duration of immunity to influenza in man is difficult to assess from clinical data because of the difficulty of diagnosing the disease with certainty; two influenza-like attacks suffered by a patient within a short period may not have the same ætiology.
Serological relationships amongst strains of influenza virus are complicated. It seems probable that strains cannot be rigidly classified into types but that several antigens are present, distributed amongst strains in varying proportions.
The relationship of pandemic (1918-19) influenza to that of recent lesser epidemics is obscure. The supposed origin of swine-influenza in the U.S.A. in 1918 and the presence of antibodies to swine-influenza in the sera of most adult human beings have led to a suggestion that swine-influenza is a survival in the pig of 1918-'flu. The serological evidence for this view is now seen to be capable of other interpretations.
Factors concerned in the immunity of experimental animals to influenza are discussed— degrees of immunity in the ferret; immunity of the nasal passages to big doses of virus; immunity of the lungs; immunity to contact infection. Active immunity runs parallel with titre of neutralizing antibodies so long as one is dealing with one strain of virus. Cross-tests amongst different strains complicate the picture.
In planning vaccination of human beings we wonder:—
(1) Whether, on general epidemiological grounds, an attempt to vaccinate against influenza virus is likely to be profitable.
(2) Whether the production of a rise in antibodies in man will be a good guide to the immunity induced by a vaccine.
(3) Whether we are right in using killed virus and in fearing a live vaccine.
(4) What strains we ought to use in making a vaccine.
(5) When and how often we should vaccinate.