Epidemiological Studies in Influenza

Animal models for influenza virus transmission studies: a historical perspective

Animal models are used to simulate, under experimental conditions, the complex interactions among host, virus, and environment that affect the person-to-person spread of influenza viruses. The three species that have been most frequently employed, both past and present, as influenza virus transmission models-ferrets, mice, and guinea pigs-have each provided unique insights into the factors governing the efficiency with which these viruses pass from an infected host to a susceptible one. This review will highlight a few of these noteworthy discoveries, with a particular focus on the historical contexts in which each model was developed and the advantages and disadvantages of each species with regard to the study of influenza virus transmission among mammals.

Animal models for evaluation of influenza vaccines

Influenza viruses are emerging and re-emerging viruses that cause worldwide epidemics and pandemics. Despite substantial knowledge of the mechanisms of infection and immunity, only modest progress has been made in human influenza vaccine development. The rational basis for influenza vaccine development originates in animal models that have helped us to understand influenza species barriers, virus-host interactions, factors that affect transmission, disease pathogenesis, and disease intervention strategies. As influenza evolution can surmount species barriers and disease intervention strategies that include vaccines, our need for appropriate animal models and potentially new host species will evolve to meet these adaptive challenges. This chapter discusses animal models for evaluating vaccines and discusses the challenges and strengths of these models.

[Influenza]

Influenza is the last great uncontrolled plague of mankind. Pandemics and epidemics occur at regular time intervals. The influenza viruses are divided into the types A, B and C and show unique variability of their surface antigens (hemagglutinin and neuraminidase). Influenza viruses of type A show the largest degree of antigenic variation which, in turn, resulted in the definition of a number of subtypes, each comprising many strains. By comparison, influenza viruses of types B and C exhibit much less variation of their surface antigens. As a consequence, no subtypes but many different strains have been recognized. The degree of antigenic variation correlates with the epidemiologic significance of the virus types, type A being the most and type C the least important. Two different kinds of antigenic variation have been recognized: In the case of minor variation of one or both surface antigens, the term "antigenic drift" is employed. Antigenic drift occurs with all three types of virus, it is caused by point mutations which increase the chance of survival of mutants in the diseased host. In addition, influenza A viruses show sudden and complete changes of their surface antigens in regular time intervals, resulting in the appearance of new subtypes. This event is called "antigenic shift". The mechanisms responsible for antigenic shift are poorly understood, only. In addition to the recycling of preceding subtypes, reassortment resulting from double infection of cells with strains of human and animal origin are considered possible explanations. By use of modern DNA recombinant technology, the base sequences of a series of virus genes and, as a consequence, the amino acid sequence of the corresponding antigens have been determined. By means of monoclonal antibodies, the antigenic structure of many influenza antigens has been further elucidated. It can be expected that further research on the molecular basis of antigenic variation could finally result in an understanding of the causal mechanisms. It is an outstanding feature of the epidemiology of influenza A viruses that a family of related strains prevails for a certain period of time and disappears abruptly as a new subtype emerges.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunity in Influenza: The Bearing of Recent Research Work

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:—

Whether, on general epidemiological grounds, an attempt to vaccinate against influenza virus is likely to be profitable. Whether the production of a rise in antibodies in man will be a good guide to the immunity induced by a vaccine. Whether we are right in using killed virus and in fearing a live vaccine. What strains we ought to use in making a vaccine. When and how often we should vaccinate.

Immunity in Influenza: The Bearing of Recent Research Work: (Section of Epidemiology and State Medicine)

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 aetiology.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.