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. 1969;41(3-4-5):589–594.

Laboratory and clinical characteristics of attenuated strains of influenza virus*

H F Maassab, T Francis Jr, F M Davenport, A V Hennessy, E Minuse, G Anderson
PMCID: PMC2427707  PMID: 5309481

Abstract

Prior to 1967, attenuation of influenza virus was achieved by gradually lowering the incubation temperature until optimal growth at 25°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°C to 25°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°C. Additional differentiating biological properties relating to the adaptation of the virus to growth at 25°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°C (rct/41—), was sensitive to acid pH and produced small plaques at 35°C and larger ones at 25°C.

After a series of plaque purifications, the cold variant showed further loss of virulence to mice, more vigorous growth at 25°C, complete failure to grow at 41°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°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°C (rct/35—) with the attenuated type B strains instead of at 41°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.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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