The Measurement by Serological Means of the Impact of the Hong Kong/68 Influenza Virus on a Population

The measurement by serological means of the impact of the Hong Kong/68 influenza virus on a population. SUMMARY: In order to obtain evidence on the size of the impact of the Hong Kong/68 variant of influenza A2 virus on the population of São Paulo, Brazil, serum samples taken in 1967 before this variant appeared and during successive years after it appeared were examined for their antibody content. Haemagglu-tination-inhibition tests performed on a total of 2726 serum samples from adults showed a sharp decrease in 1969 of the proportion of sera without antibody to the Hong Kong/68 variant and a corresponding mercase in the proportion with high titres. It was concluded that about three-quarters of the adult population became infected at some time after the variant appeared, the majority in the first year of prevalence. UNITERMS: Hong Kong/68 influenza variant *; Haemagglutination-inhibition test*; Influenza A2 virus*.


INTRODUCTION
The Hong Kong/68 variant of influenza A2 virus spread widely in the northern hemisphere in the winter of 1968-69 and was expected to appear in the southern hemisphere sometime during the winter months from March onwards.
As anticipated, strains of the virus were isolated from sporadic cases of respiratory infection in Rio de Janeiro in March 1969, from cases of influenza in Belém, Pará, where a sharp increase in absenteeism was noted in schools and factories associated with a high incidence of acute respiratory illnesses, and from cases of influenza in São Paulo ***.
Since information was scanty on possible indices which might indicate the size of an epidemic of influenza (such as increased mortality rates), it was thought possible that the examination of sufficient serum samples taken before and after the appearance of the Hong Kong/68 variant might allow some estimation to be made of the of the impact of this virus on the population.To this end a serological study was done by the technique of haemagglutination-inhibition and the proportion calculated of the people who acquired antibody during the period that the virus was known to be circulating in the population.Sera from 1967 and 1969 were tested simultaneously on every occasion to minimise variations due to differences in the other reagents in the test.

RESULTS
Over the 3 year period of the survey serum samples were taken from 2726 adults.
The 487 samples obtained in 1967 had been taken from adults aged 15 years to 35 years or over.These, taken before the Hong Kong/68 variant appeared, were used to produce a base line against which subsequent levels of antibody to this virus could be set.The 980, 790 and 469 samples collected, respectively, in 1969, 1970 and 1971 were obtained from adults within a similar age range.The distribution of haemagglutination-inhibitin antibody titres in all these samples to the Hong Kong/68 variant is shown in Table 1.The difference between the results with sera obtained in 1967 and those obtained in 1969, 1970 and 1971 is considerable, particularly in the proportions without any detectable antibody and in those with titres of 80 or higher.In Figure 1 the results are grouped and presented graphically to show these differences and to demonstrate the similarity in the proportion of sera with low titres between 10 and 40.The fall of approximately 42% in the proportion of negative sera between 1967 and 1969 and the rise of approximately 33% in the propor-tion with high titres, gives some indication that the probable incidence of infection by the Hong Kong variant among the population was between 30% and 40%.An analysis of the results with sera obtained in 1969 and with those obtained in 1970 and 1971 shows no significant differences.
There was little alteration in the proportion of sera showing low titres throughout the period between 1967 and 1971 and it could be that such antibody levels might be protective and thus the group remained fairly constant during the prevalence of the Hong Kong variant.
The age distribution of sera with antibody titres of 80 or higher in the serum samples obtained during 1967 and 1969-71 is shown in Figure 2. The overall proportion of sera with titres of 80 or higher in 1967 was 2.4% against 35.1%, 37.3% and 40.8% respectively in the years 1969, 1970 and 1971 (Table 1).

DISCUSSION AND CONCLUSION
The demonstration of the changing incidence of antibody to influenza over sucessive periods of time allows some estimation to be made of the impact of the virus in places where accurate epidemiological indices of influenza are difficult to obtain and measure.
The isolation of virus from patients with clinical illness is of importance in order to establish the identity of the circulating influenza variants but this will give little information on the scale of infection in the population as a whole.
The situation in São Paulo was such that it was clear that little evidence was available of the impact of the Hong Kong/68 variant other than that of a few scattered isolations of the virus which indicated the presence of the virus and reports of small outbreaks.Thus it was hoped that a serological study would enlarge considerably the epidemiological picture.
The results described here suggest that the A2/Hong Kong/68 virus had already spread quite extensively by the time the first strains were isolated early in 1969 as the proportion of sera without any antibody was considerably lower in February 1969 compared with samples collected in 1967.The increase in this period in the number of sera with high titres was equally striking.
The main rise in the proportion of sera with high antibody titres occurred in the first year of prevalence of the Hong Kong/68 virus.In the subsequent years the proportion increased only slightly and all age groups showed a similar small variation, with the age group 25-35 showing the highest proportion with high titres in each of the three years.
It is concluded that about three-quarters of the adult population appear to have been effectively immunised, the majority in the first year that the Hong Kong/68 virus appeared.