Pilot studies on recombinant cold-adapted live type A and B influenza virus vaccines.

Recombinant live attenuated type A and B influenza virus vaccines derived from standardized cold-adapted parent strains were given singly and in combination to volunteers. The vaccine viruses were well tolerated, functioned as good antigens, and failed to spread to intimate household contacts. Thirty-nine isolates that were recovered after a single passage in humans appeared genetically stable. The results of histopathologic studies in ferrets encourage development of an animal model for attenuation of the virus.

Interpretations of influenza antibody patterns of man.

The age distribution of antibodies to A2/Hong Kong/68 influenza virus was ascertained in sera collected before the pandemic of 1968 in order to determine whether Hong Kong-like viruses had previously prevailed in man, and, if so, at what period. The findings indicate that Hong Kong-like viruses were probably involved in outbreaks at or about the turn of the century. The data are interpreted to indicate that antigenic drifting among Asian influenza strains from the A2/Japan/305/57-like variant, believed to be responsible for the pandemic of 1889-90, to the Hong Kong-like variant of 1900, is a phenomenon that has been repeated in recent times. The findings constitute another example of antigenic recycling after a long period of absence, and support the view that the number of antigens of influenza A is finite.

Laboratory and clinical characteristics of attenuated strains of influenza virus.

Prior to 1967, attenuation of influenza virus was achieved by gradually lowering the incubation temperature until optimal growth at 25 degrees C was obtained. The process of attenuation of a Hong Kong strain was modified and considerably shortened. The temperature of incubation was changed abruptly from 35 degrees C to 25 degrees C and a cold variant was selected using the plaque-assay system.A set of genetic markers was developed for assessing the potential virulence of cold-passaged variants. The cold variant of the Hong Kong strain was temperature-sensitive, acid-labile and produced a small plaque in primary chick kidney cells incubated at 35 degrees C. Additional differentiating biological properties relating to the adaptation of the virus to growth at 25 degrees C and to loss of virulence in a susceptible host are presented.The cold-adapted variant was found to be relatively avirulent and highly antigenic for mice and ferrets, and virus was recovered from the nasopharynx of infected ferrets during the first 3 days. The virus recovered was still unable to grow well at 41 degrees C (rct/41-), was sensitive to acid pH and produced small plaques at 35 degrees C and larger ones at 25 degrees C.After a series of plaque purifications, the cold variant showed further loss of virulence to mice, more vigorous growth at 25 degrees C, complete failure to grow at 41 degrees C and good antigenic potency.The genetic markers were stable in the plaque-purified cold variant after at least 10 consecutive passages either in tissue culture at 35 degrees C, or in mice.Cold variants of type B influenza virus have a narrower range of temperature sensitivity compared with type A strains. Reduced plaquing efficiency and reproductive capacity occurred at 35 degrees C (rct/35-) with the attenuated type B strains instead of at 41 degrees C as with the type A strains.Clinical trials with the attenuated Hong Kong strain of influenza virus (A2/Aichi/2/68) have demonstrated the acceptability and immunogenicity of the strain in man.

Further observations on the significance of A-equine-2/63 antibodies in man.

The antibody pattern of equine-2/63 viruses has been more sharply defined using a large number of human sera collected in 1964. The birth dates of persons exhibiting the richest experience with equine-2/63-like viruses delineate a period of past prevalence in man of equine-2/63-like viruses. The period is believed to have begun in the mid-1870's and to have terminated in 1889-1890 at the time of the first Asian pandemic. The equine-2/63 antibodies found in human sera react specifically in the photometric test of Drescher. The equine-2/63 antibody pattern advances along the age scale in exact concordance with the passage of time. The homologous antibody response of the older subjects to equine-2/63 vaccine is more vigorous, reflecting the conditioning effects of prior exposures to equine-2/63 antigens. A "one-way cross" between equine-2/63 virus and A(2) and A(1) strains has been demonstrated. The antigenic ties between strains of influenza A isolated from humans, swine, horses, and birds is recognized and discussed. It is apparent that horses do not constitute an active reservoir for strains of human involvement. The epidemiologic significance of the antigenic linkages between strains isolated from different species remains obscure.