NATURAL ACCUIRED IMMUNITY AGAINST INFLUENZA A AND B VIRUSES

In order to understand the nature of influenza epidemics, it is necessary to investigate the susceptibility to the infection on the part of individuals as well as the very nature of influenza viruses. The most important factor which determines one's susceptibility to influential affection is the neutralizing capacity of one's serum against influenza viruses. According to the literature available on this subject, it is probable that human adults always possess a certain level of neutralization antibodies against influenza viruses A and B, and that children, except for a year after birth when an adult's level of antibodies transferred from mother can be demonstrated in them, show the lowest level of the neutralizing capacity which, as a result of repeated influenzal affection, gradually increases with age and reaches the adult's level before 10 to 15 years of age. However, our existing knowledge is not yet sufficient to make this point clear, and it is desirable to prosecute the study on a large number of individuals in many different areas.So we investigated the distribution of neutralization antibodies against influenza viruses A and B in different age groups of population, particularly school-aged (6 to 15 years of age) children, in different areas of Western Japan. The areas where the investigation was made numbered nineteen, including cities, rural districts, remote mountain regions and solitary islands, and the number of people whose sera was examined totaled more than 1500. Neutralization antibodies of serum against influenza virusee A (PR8 strain) and B (Lee strain) were estimated by the hemagglutination inhibition test, and the results obtained were expressed in terms of Fukumi-Yamamoto's “hemagglutination inhibition index”.The results obtained were as follows:1. The hemagglutination inhibition indices of adults were between 3.0 to 4.0, and this range of neutralizing capacity is maintained throughout life.2. In all areas examined, school-aged children showed considerable or an adult's level of neutralization antibodies. The fact indicated that infection of influenza viruses A and B were very common in those areas.3. In children, the adult's level of the neutralizing capacity against A virus was reached before the ages of 6 to 13. But against influenza B virus, this level war not yet reached untill 14 or 15 in many areas.4. The mode of distribution of the neutralization antibodies in different age groups of children can be classified into two different types. In the first type, the antibodies are distributed in a narrow range at the adult's level, showing no differences in different age groups. In the second type, the antibodies as a whole are distributed in a wide range, the levels varying from a very low value to that of the adult; furthermore, they show definite age differences in their distribution.5. As the state of age distribution of neutralization antibodies against influenza viruses A and B in a certain population clearly reflects the state of its resistance to influenza epidemics, the knowledge of the changes in the age distribution soon after outbreak of the epidemic will enable us to diagnose whether the causative agent was virus A or B. Some illustrative epidemics were descibed.