Reports on HIV/AIDS: 1990

NOVEMBER 30, 1990, MMWR, Vol. 39, RR-16 [inclusive page numbers]
estimate the potential need for antiretroviral and other therapies, the workshop group
evaluated the need for surveillance of immune-status indicators to augment data on
HIV/AIDS incidence and prevalence in projecting the impact of the epidemic.
The distribution of measures of immune status among HIV-infected persons can be
estimated from data obtained in surveys, from ongoing cohort studies, and from such
statistical methods as back-calculation. Surveys can directly measure both the
number of infected persons in a sample of the population and the distribution of
CD4+ cell counts among those infected. Cohort studies of selected risk groups
provide historical data regarding changes in CD4+ cell-count distributions over time
that can be used in conjunction with estimates of rates of decline of CD4+ cell counts,
HIV incidence, and cause-specific mortality rates to project current and future CD4+
cell-count distributions.
Workshop participants reviewed data from immunologic studies of active-duty
military personnel with HIV infection (20; National Naval Medical Center, unpublished data) and in cohorts of homosexual and bisexual men (21,22). Using the Army
data (20) and projection method, it was estimated that by 1989 approximately 17% of
HIV-infected persons evaluated between 1985 and 1989 had <200 CD4+ cells/mm3
and that an additional 41% had 200-500 CD4+ cells/mm'. The Navy data and
projection method yielded corresponding estimates of 19% and 45%, respectively.
Thus, 58%-64% of persons with HIV infection may have CD4+ counts of <500/mm3.
These estimates are consistent with those reported from cohort studies among
homosexual and bisexual men (21,22).
To estimate CD4+ cell-count distributions, modeling methods such as backcalculation use data on the incidence of end-stage disease (e.g., AIDS case reports,
HIV-associated deaths), together with estimates of the distribution of times from
infection to various stages of immune suppression. An extension of the backcalculation method permits estimation of the number of infected persons in various
stages of disease. Using adjusted data on AIDS incidence and a median of 3.6 years
from low CD4+ cell counts to AIDS estimated from cohort studies of males with
hemophilia (23), Brookmeyer estimated that as of July 1987, there were approximately 150,000 adults infected with HIV who did not yet meet the CDC AIDS case
definition, but who had <200 CD4+ cells/mm3; these persons accounted for approximately 15% of his estimate of 987,000 HIV-infected adults who had not yet progressed to AIDS (24).
The back-calculation method is sensitive to estimates of median time from low
CD4+ cell counts (<200 cells/mm3) to AIDS. Various cohort studies estimate the
median time to range from 1 to 3 years (e.g., studies of males with hemophilia and
studies of homosexual and bisexual males). Therefore, if the incubation period differs
by risk group or is affected by therapy, estimates of the distribution of immune-status
indicators based on back-calculation may have to be adjusted. Presently, however,
estimates of the proportion of HIV-infected adults with <200 CD4+ cells/mm3 are
consistently 15%-20%, regardless of the projection method used.
When CD4+ cell counts for HIV-infected persons can be obtained, the workshop
participants judged such counts to be the most useful predictors of stage of disease
currently available. Although CD4 + cell counts are used clinically to indicate the need
for and timing of therapeutic interventions, additional research is needed to evaluate
the usefulness of changes in CD4+ cell counts as markers of therapeutic efficacy and
to correlate such changes with the prevention or remission of clinical symptoms.

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