A vaccine that "provides long-lasting, protective immunity in mice against all influenza A virus strains" has been developed, according to a report by Belgian researchers (Nature Medicine 1999;5:1157).
The influenza A virus has two proteins on its surface - haemagglutinin and neuraminidase - that are highly immunogenic but also variable. It is because of this variability that a broad-spectrum influenza A vaccine has not been developed, say the authors, Dr Sabine Neirynck and colleagues (Flanders Interuniversity Institute for Biotechnology, Ghent, Belgium). They have evaluated a vaccine based on the M2 protein, which is an integral protein in the membrane of the virus known to be important in the infectious cycle of influenza A. There has been no amino acid change in the extracellular part of this protein from the first strain of human influenza A isolated (in 1933) to that most recently sequenced, they say.
Three 10mg immunisations were given to mice, each at an interval of three weeks. Three weeks after the last immunisation, the mice were given a potentially lethal influenza A challenge. Only two out of 11 unvaccinated mice survived, compared with 23 out of 24 of those vaccinated. The immunity was still present 26 weeks later.
In conclusion, Dr Neirynck et al say that, although M2 is a weak antigen, it is a promising candidate as a broad-spectrum vaccine against influenza A. They conclude: "Given the nearly invariable amino acid sequence of the extracellular part of the M2 protein, it is reasonable to assume that the immune protection would be effective against all influenza A strains."
Dr Edwin Kilbourne (department of microbiology and immunology, New York medical college, US) comments in an editorial (Ibid, p1119) that, encouraging as these results are, it may be premature to herald this vaccine as "universal". Only two influenza viruses were included in the study and the M2 protein was the same with each virus, he says. Dr Kilbourne concludes that, "although these findings clearly demonstrate that M2 can mediate immunity, they do not permit extrapolation to include ‘all influenza A strains'".