The HAV is an enterovirus about 28 nm in diameter (McCollum, 1982). It contains a single-stranded RNA and four polypeptides. Comparative studies of HAV from different geographic areas have been limited, but it appears that only one serotype exists. This simplifies potential vaccine development.

Fecal excretion of infected virus begins about 25 days after oral infection with HAV. Peak infectivity probably occurs before the onset of symptoms in the fourth week after exposure.

The host immune response to HAV infection involves both IgM and IgG (McCollum, 1982). Anti-HAV IgM appears as virus excretion begins to subside. Shortly thereafter, IgG levels begin to rise. IgG persists while IgM levels fall over the next three to six months. Cell-mediated immunity to HAV infection has not been reported.

Estimates of the numbers of cases of illness, hospitalization, and death due to HAV are based on information supplied by Alter (personal communication, 1983), in which hepatitis figures reported to the Centers for Disease Control (Centers for Disease Control, 1983; Francis et al., 1984) were adjusted on the basis of more precise data gathered in the Sentinel County Study. This system facilitates estimation of underreporting, age-specific incidence, and the true distribution of cases between hepatitis A and hepatitis B.

Estimates given below include only icteric cases. (Estimates in Appendix H for hepatitis B include other symptomatic cases.) Calculations based on the sources cited above indicate that there are 35,000 non-hospitalized and 13,000 hospitalized cases of HAV illness and 152 deaths each year.

The distribution of cases among age groups in Table G.1 is based on calculations using data reported to the CDC. Application of these percentages to the estimated total number of cases, hospitalizations, and deaths produces the figures shown in Table G.2. The number of episodes of illness included in Category B is comprised of both typical cases and cases convalescing from hospitalization. Durations of typical cases are based on advice from a number of sources; durations of hospitalizations are from the CDC data (Alter, personal communication, 1983).

Estimates included in Table G.2 may be low because anicteric cases (with symptoms other than jaundice) have not been included. No data could be found on which to base estimates of the numbers of these cases.