SEROLOGICAL VARIATION 261 



A-prime strains developed good titers to both A-prime and PR8 strains 

 (Francis et al., 1947). However, a significant proportion of children vaccinated 

 with PR8 virus in 1948 developed antibodies to A-prime strains or even to 

 swine virus (Quilligan et al., 1948). Thus, while serological studies with animal 

 sera were describing strain changes in different years, the studies with human 

 sera continued to emphasize the lack of strain specificity, especially in the 

 response to infection (Horsfall and Richard, 1941; Anderson, 1947; Magill and 

 Sugg, 1944). Nevertheless, the initial and convalescent titers were frequently 

 found to be much lower when tested with strains from a current epidemic than 

 with older stock strains. This is illustrated in data from patients in 1947 

 (Table II). 



b. Age Distribution of Antibody to Different Strains. That the major antigens 

 representative of the A-prime strains were not new to the population was 

 shown in the fact that gamma globulin prepared in 1943 and 1944 had 

 demonstrable antibodies to strains isolated in the years from 1947 to 1951; 

 material prepared after 1947 showed fourfold increases in titer to these 

 strains and to the PR8 strain, while the already high titers to WS and swine 

 strains were unchanged (Davenport et al., 1953). 



A systematic examination of antibody to strains of different years through- 

 out the age span of the general population in 1952 revealed three distinctive 

 patterns (Davenport et al, 1953; Francis et al, 1953). The sera of children 

 contained antibodies essentially oriented to the A-prime strains, which had 

 been and were currently prevalent. This had been noted also by van der Veen 

 (1951) and Hilleman (1954). They were highest in the five to twelve year old 

 groups, but after the age of seventeen to eighteen the titers to these strains 

 were low. Antibody to the strains of 1933 to 1943 appeared about the twelfth 

 year of age, but was at its highest between seventeen and twenty years, and 

 then leveled off for the older ages. Antibody to swine influenza was first seen 

 at twenty-nine years, reached its peak in the age group thirty-five to thirty- 

 eight and then settled at a lower level. The first two patterns correspond in 

 time with established prevalence of known serological groups of type A virus 

 and hence permit the inference that virus antigenically similar to swine 



