In order to investigate the shift in the rates and levels of antibody to hepatitis A virus, 567 children in 20 isolated groups of five day-care centers were observed over a period of 8 months during which the seasonal rise in hepatitis A morbidity occurs. Increases in the proportion of seropositive ranging from 5 to 37% were demonstrated in 6 groups, and were always associated with the occurrence of either overt or sub-clinical hepatitis A infection. High rates of seropositivity were also noted in the groups in which cases of hepatitis A had been registered prior to the period of observation. In some children with low and medium antibody levels, antibody titres showed further increases after reinfection. A substantial part of children retained low antibody titres during the entire period of observation, and eight previously sero-negative children developed low antibody levels after asymptomatic hepatitis A infections. In one group the spread of hepatitis A infection (clinical and asymptomatic) was prevented by the administration of commercially available immunoglobulin immediately after the discovery of an infected food handler. Passive antibodies were found in previously sero-negative children, and these antibodies dropped to undetectable levels two months after administration.
The titres of antibody to hepatitis A virus (anti-HAV) were found to fall rapidly during 3-4 years after hepatitis A and more slowly thereafter, though they never reached undetectable levels in at least 23 years. The levels of anti-HAV in convalescents after overt hepatitis A were found to be significantly much higher than in those who had anti-HAV as a result of asymptomatic infection.
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