The return of the historic influenza A H1N1 virus and its impact on the population of the United Kingdom

Origin of the H1N1 (Russian influenza) pandemic of 1977-A risk assessment using the modified Grunow-Finke tool (mGFT)

In 1977, the Soviet Union (Union of Soviet Socialist Republics [USSR]) notified the World Health Organization (WHO) about an outbreak of H1N1 influenza, which later spread to many countries. The H1N1 strain of 1977 reappeared after being absent from the world for over 20 years. This pandemic simultaneously spread to several cities in the USSR and China. Many theories have been postulated to account for the emergence of this pandemic, including natural and unnatural origins. The purpose of this study was to use the modified Grunow-Finke risk assessment tool (modified Grunow-Finke tool [mGFT]) to investigate the origin of the 1977 H1N1 pandemic. Data was collected from WHO archives and published documents. The assessment of the pandemic's origin involved the utilization of a modified version of the original Grunow-Finke risk assessment tool (GFT). Using the mGFT, the final score was 37 out of 60 points (probability: 62%), indicating a high likelihood that the Russian influenza pandemic of 1977 was of unnatural origin. Several variables supported this finding, including the sudden re-emergence of a previously extinct strain, a genetic signature of laboratory modification for vaccine development, and unusual epidemiology. Inter-rater reliability was moderate to high. By applying the mGFT to the 1977 Russian influenza pandemic, we established a high probability that this pandemic was of unnatural origin. Although this is not definitive, it is consistent with the possibility that it originated from an incompletely attenuated live influenza vaccine. The mGFT is a useful risk analysis tool to evaluate the origin of epidemics.

Integrating influenza antigenic dynamics with molecular evolution

Influenza viruses undergo continual antigenic evolution allowing mutant viruses to evade host immunity acquired to previous virus strains. Antigenic phenotype is often assessed through pairwise measurement of cross-reactivity between influenza strains using the hemagglutination inhibition (HI) assay. Here, we extend previous approaches to antigenic cartography, and simultaneously characterize antigenic and genetic evolution by modeling the diffusion of antigenic phenotype over a shared virus phylogeny. Using HI data from influenza lineages A/H3N2, A/H1N1, B/Victoria and B/Yamagata, we determine patterns of antigenic drift across viral lineages, showing that A/H3N2 evolves faster and in a more punctuated fashion than other influenza lineages. We also show that year-to-year antigenic drift appears to drive incidence patterns within each influenza lineage. This work makes possible substantial future advances in investigating the dynamics of influenza and other antigenically-variable pathogens by providing a model that intimately combines molecular and antigenic evolution.

DOI:http://dx.doi.org/10.7554/eLife.01914.001

Every year, seasonal influenza, commonly called flu, infects up to one in five people around the world, and causes up to half a million deaths. Even though the human immune system can detect and destroy the virus that causes influenza, people can catch flu many times throughout their lifetimes because the virus keeps evolving in an effort to avoid the immune system. This antigenic drift—so-called because the antigens displayed by the virus keep changing—also explains why influenza vaccines become less effective over time and need to be reformulated every year.

It is possible to determine which antigens are displayed by a new strain of the virus by observing how blood samples that respond to known strains respond to the new strain. This information about the “antigenic phenotype” of the virus can be plotted on an antigenic map in which strains with similar antigens cluster together. Gene sequencing has shown that there are four subtypes of the flu virus that commonly infect people; but the relationship between changes in antigenic phenotype and changes in gene sequences of the influenza virus is poorly understood.

Bedford et al. have now developed an approach to combine antigenic maps with genetic information about the four subtypes of the human flu virus. This revealed that the antigenic phenotype of H3N2—a subtype that is becoming increasingly common—evolved faster than the other three subtypes. Further, a correlation was observed between antigenic drift and the number of new influenza cases per year for each flu strain. This suggests that knowing which antigenic phenotypes are present at the start of flu season could help predict which strains of the virus will predominate later on.

The work of Bedford et al. provides a useful framework to study influenza, and could help to pinpoint which changes in viral genes cause the changes in antigens. This information could potentially speed up the development of new flu vaccines for each flu season.

DOI:http://dx.doi.org/10.7554/eLife.01914.002

Influenza A(H1N1): a widening spectrum?

OBJECTIVE

To study the incidence of H1N1 influenza from 1977 to 1988 in unvaccinated volunteers and the effects of continuing minor antigenic change (antigenic drift) in the virus.

DESIGN

Prospective study by a group of general practitioners, backed up by virological findings.

PARTICIPANTS

Mainly patients of the general practitioner group, also some doctors and members of staff. There were 287 participants during 1977-1981, and 207 at the end of 1988.

INTERVENTION

Any participant deemed to be "at risk" was encouraged to be vaccinated and to withdraw from the study.

BACKGROUND

In 1957, H1N1 subtype influenza had been displaced by H2N2 (Asian) subtype. In 1968, H2N2 was displaced by H3N2 (Hong Kong) subtype. During 1977, H1N1 influenza unexpectedly reappeared in Asia, and spread widely. The resurgent strain, designated A/USSR/90/77(H1N1), caused world pandemics, attacking (almost exclusively) persons who had been born since the 1950-1951 northern winter and causing negligible mortality. It did not displace the current H3N2 strain, and strains of both subtypes have continued to emerge independently.

HYPOTHESIS

Antigens of A/USSR resembled closely those of the 1950-1951 H1N1 strain, which apparently was rendered antigenically inert between 1951 and 1976 (possibly frozen) and was reactivated during 1977. Antigenic drift was then resumed.

MAIN OUTCOME MEASURE

The A/USSR/90/77 strain and its close successor, A/Brazil/11/78, attacked mainly the young, whose previous exposure to H1N1 antigens had been minimal or zero. Mortality during the A/USSR pandemics was negligible because death from influenza in people aged less than 30 years is rare. Would continuing antigenic drift ultimately widen the H1N1 spectrum of attack?

RESULT

During the epidemic of 1988, A/Taiwan/1/86(H1N1) attacked a wider range of age groups than had A/USSR or A/Brazil.

CONCLUSION

Assuming that H1N1 viruses continue to undergo further antigenic drift, an ever widening age spectrum of H1N1 attack may be expected.

Infection with influenza A H1N1. 1. Production and persistence of antibody

Three outbreaks of influenza caused by influenza A H1N1 occurred in a boys' boarding school in 1978, 1979 and 1983. The serological response to infection with variants of the H1N1 virus was studied by radial haemolysis and haemagglutination inhibition after primary infection and reinfection. The persistence of this antibody was also studied. Infection in 1978 resulted in the production of persistent antibody to both the haemagglutinin and neuraminidase of the homotypic strain. Antibody which cross-reacted with later variants of the virus was less frequently produced, the peak response was delayed and such antibody persisted less well. Infection in 1979 resulted in a similar response to that observed in 1978 after primary infection. Reinfection resulted in a broad response in all cases. In 1983 all infected boys produced antibody which reacted with the homotypic strain but only approximately one-third of primary infections produced antibody which reacted with the A/USSR/92/77 strain. The neuraminidase of the A/USSR strain failed to detect one third of the primary infections. Reinfection again resulted in a broad response.

Antibody status to influenza A/Singapore/1/57(H2N2) in Finland during a period of outbreaks caused by H3N2 and H1N1 subtype viruses

The incidence of haemagglutination inhibition (HI) antibody (titre greater than or equal to 12) to influenza A/Singapore/1/57(H2N2) in sera collected from a Finnish population in the summer of 1981 was 58%. Subjects born after 1968 were essentially seronegative, and a comparable low HI antibody status was also recorded among the elderly, the lowest being in people born during the period 1901-10. A small increase in antibody titre to the H2N2 virus was observed in the different age groups after infections with the H3N2, but not the H1N1, subtype influenza A viruses. The heterotypic response, which could be due to HI or NA antibodies, was restricted almost exclusively to subjects already exhibiting this antibody in acute phase sera. Moreover, the anamnestic boosting was not as strong as that described in earlier studies from samples collected at the beginning of the present era of H3N2 viruses. At population level, the HI antibody status to H2N2 was about the same at the beginning and end of the follow-up period which covered eight epidemic seasons. The results are discussed with respect to the doctrine of 'original antigenic sin' and to the threat of re-emergence of the H2N2 viruses.

Naturally acquired immunity to influenza type A. Lessons from two coexisting subtypes

When the H1N1 subtype of influenza reappeared in the Northern Hemisphere during 1977, after a 20-year absence, it behaved very differently from the H3N2 subtype still in circulation. In Sydney, we studied the incidence of both subtypes of laboratory-proven influenza type A in 287 unvaccinated volunteers whose serum antibody titres were measured before and after each winter, to facilitate the detection of subclinical as well as clinical infection. During a 1977 epidemic, the A/Victoria/3/75 strain of the H3N2 subtype attacked participants of all age groups, whereas during epidemics of 1979 and 1981, the A/USSR/90/77 and A/Brazil/11/78 strains of the H1N1 subtype attacked only subjects born after 1950. The older participants apparently possessed homologous protection, acquired as a result of exposure to H1N1 more than 20 years earlier and not dependent upon strain-specific haemagglutination inhibition antibody.

The use of monoclonal antibodies in virology

Antibodies have been used for the last five decades in the laboratory diagnosis of a wide range of diseases caused by viruses and in detailed investigations of virus structure. However, immunological and serological assays have always had problems of interpretation, reproducibility and standardization, resulting partly from the unavoidable heterogeneity of the antibodies in the test. When a mouse, for example, is immunized with a virus, the animal may easily recognise 10–20 different antigenic determinants. As many as five distinct antibodies can be produced by the mouse against each determinant and these will often differ from antibodies made by another mouse against the same antigenic determinant. The method of lymphocyte fusion and the subsequent generation of monoclonal antibodies (Kohler & Milstein, 1975; Kohler, 1980) overcomes these limitations.