The influence of antigenic variation on influenza A2 epidemics

Naturally acquired immunity to influenza type A: a further prospective study

During the 1976 influenza epidemic, the incidence and severity of attack by A/Victoria/3/75 strain were studied in 312 participants who were divided into two groups: in Group 1 were 216 participants who had a history of laboratory-proven infection by one of the earlier strains of Hong Kong subtype; in Group 2 were 96 participants who had no known history of such prior infection. No participant in either group received influenza vaccine. The efficacy of clinical immunity, acquired as a result of prior infection by an earlier Hong Kong subtype strain, was judged by comparing the findings in Group 1 and Group 2. Incidence of laboratory-proven A/Victoria/3/75 infection was found to be 27% in Group 2 (26 cases confirmed) and 10.6% in Group 1 (23 cases confirmed). Among Group 1 members, the incidence was highest in those with the longest interval since prior Hong Kong subtype infection. Severe influenzal illness was seen in 14 of the 26 proven cases in Group 2, and in only three of the 23 proven cases in Group 1. The prior infection in these three had been by A/Hong Kong/1/68 strain in 1969-1970. Thus in Group 1, severe influenzal illness was not observed until six years or more had passed since prior infection (and three episodes of antigenic drift had intervened). Results suggested that low serum antibody levels may be unreliable as indicators of clinical immunity. Preepidemic sera of 99 members of Group 1 and 82 members of Group 2 showed no HAI antibody to A/Victoria/3/75 (titre less than 8), but the incidence of laboratory-proven A/Victoria/3/75 infection among these individuals was lower in Group 1 (19%) than in Group 2 (30.5%).

Naturally acquired immunity to influenza type A: a clinical and laboratory study

After type A influenza virus had undergone major antigenic change in mid 1968, it was noted that individuals previously infected by strains of the old subtype (Asian), especially late strains, appeared to be unexpectedly resistant to clinical attack by the new subtype (Hong Kong). Prospective studies have since shown that, during the A/England/42/72 influenza epidemic of 1972, in which the incidence was approximately 7% in the community, clinical influenza due to this virus was not found in 229 subjects previously confirmed as having had A/Hong Kong/1/68 influenza, even though vaccine which had been effective against A/Hong Kong/1/68 was ineffective against A/England/42/72. During the A/Port Chalmers/1/73 influenza epidemic of 1974, clinical influenza resulting from Port Chalmers virus was not found in a closely monitored group of 176 unvaccinated subjects previously infected by A/Hong Kong/1/68 or A/England/42/72, although laboratory studies demonstrated Port Chalmers infection in five of these (2-8%). By contrast, among 99 subjects who had no such history of earlier infection, 22 developed laboratory-proven Port Chalmers influenza and most of them had typical illness.

Use of excess mortality from respiratory diseases in the study of influenza

Since 1970, WHO has conducted a collaborative study on the use of "excess mortality" from respiratory diseases (that is, the number of deaths actually recorded in excess of the number expected on the basis of past seasonal experience) in order to assess in 13 different countries the severity of influenza epidemics. The use of computer-produced seasonal expected and actual curves permits quick visual assessment of influenza activity in any one country, as well as comparisons between different countries. The study demonstrates that an excess in the observed over the expected number of total deaths does not necessarily indicate an excess in deaths from influenza, and it is therefore limited, at least for the present time, to deaths from respiratory disease. It provides a week-to-week record of deaths from acute respiratory disease in countries where weekly returns are available and a retrospective analysis of the disease pattern in the collaborating countries. This study will be continued for a number of years and, apart from its value to the individual countries, should provide useful comparisons between countries with different climates.

Epidemiology of the Hong Kong-68 variant of influenza A2 in Britain

Two influenza epidemics in Britain in 1968-9 and 1969-70, were due to the Hong Kong/68 variant of influenza A2 virus. The first epidemic was prolonged with low morbidity and mortality rates; the second was sharp with high rates. The difference between total morbidity and mortality in the two epidemics, however, was less than it appeared to be-the estimated excess morbidity and mortality due to all causes in 1969-70 was only about 50% greater than in 1968-9.Antibody studies showed that about one-quarter of two groups of adults investigated were infected in the first epidemic and about one-third in the second. After the two epidemics about one-third still had no antibody to the A2/Hong Kong/68 virus.

Changes in the antibody status of a population following epidemic infection by influenza virus A2-Hong Kong-1-68

The haemagglutinin of influenza virus A2/Hong Kong/1/68 was shown to be markedly different from that of previously isolated A2 virus strains. No haemagglutination-inhibiting (HI) antibody to A2/Hong Kong/1/68 virus was detected in serum specimens collected in 1966 from persons aged 60 years or less. In contrast, HI antibody tests with 270 sera collected in 1968 indicated that 9.6% had demonstrable HI antibody at low titres, and 35.2% of 454 postepidemic (1969) sera had demonstrable HI antibody at relatively high titres. Most sera from persons aged 80 years and more collected in 1968 and 1969 had demonstrable HI antibody to influenza virus A2/Hong Kong/1/68. No HI antibody to the Hong Kong virus was detected in pre-epidemic sera from children aged 6 months to 3 years, whereas 32% of postepidemic sera had HI antibody. The acquisition of HI antibody to A2/Hong Kong/1/68 was not accompanied by an increase in the incidence or titres of HI antibody to heterotypic A2 influenza viruses. For sera from children aged 4-11 years, an increase of HI titre to heterotypic A2 influenza was found.