Reports on HIV/AIDS: 1990

NOVEMBER 30, 1990, MMWR, Vol. 39, RR-16 [inclusive page numbers]
CD4 + cell counts of <500/mm3 has been estimated to be approximately one-third the
risk for untreated patients (A 11 ); this estimate is consistent with data presented at the
workshop. Brookmeyer obtained good fits to recent data on AIDS incidence under the
assumption that 10% of those in stage one and 50% of those in stage two began
receiving treatment in July 1987.
3. Statistical methods used to make AIDS case projections
AIDS case projections were derived both from extrapolation from recent data on
AIDS incidence and from back-calculation. Projections based on consistent cases (see
Section 1) were increased by 18% because analyses of surveillance data show that
approximately 15% of cases diagnosed during the last year are not consistent with the
pre-1987 definition (1/0.85 = 1.18). Projections based on reported AIDS cases were
also increased by 18%, corresponding to CDC's estimate that nationally approximately 8,5% of all diagnosed cases are ultimately reported to the surveillance system.
Projections derived from extrapolation. Two extrapolation models were fit to
adjusted AIDS incidence. Hyman fit a quadratic spline function of time, with a knot
during 1987. The model was nearly linear after 1987. T. Green (CDC) fit a function of
time to adjusted monthly incidence for July 1987 through June 1989 using weighted
least squares (Al). The weights incorporated uncertainties in the estimates of
reporting delays; the variability in the number of cases reported was assumed to be
proportional to this number of cases. A linear function of time gave an adequate fit to
the data, as suggested by the smoothed trend in Figure 1. Unweighted least squares
gave results nearly identical to those from the weighted analyses. The predictions
from the consistent case series were nearly the same as the projections from all cases
after the former were inflated by 18%.
Projections derived from back-calculation. With the exception of Hyman, the
analysts listed in Table B1 also predicted future AIDS cases from back-calculation. In
addition, Gail made predictions by using a modification of back-calculation (Gail,
written communication). He first estimated HIV incidence through June 1987 by
applying standard back-calculation to AIDS cases diagnosed through that date. He
then calculated future AIDS cases under the assumption of a distribution of incubation periods reflecting increasing proportions of infected persons without AIDS who
were receiving therapy, with an effect starting in July 1987 (implying secular changes
in the distribution of incubation periods beginning then). Gail assumed, *as did
Brookmeyer, that improved medical care can reduce the risk of progression to AIDS
by 65%. From fitting adjusted AIDS incidence data, Gail estimated that improved care
gradually phased in to cover 60% of those in groups with good access to health care
(e.g, homosexual men, transfusion recipients, persons with hemophilia) and 20% of
those in other groups. Improved care does not require treating all infected persons in
these groups. It may require only more effective patient management (e.g., close
follow-up of patients until they are at high risk of developing AIDS before giving
treatments such as zidovudine or prophylactic pentamidine).
Hay's back-calculation results in Table B1 imply that approximately 10,000 HIV
infections occurred among adults and adolescents from 1986 through 1988. In an
alternative analysis, Hay assumed that there were 15,000 new infections in both 1986
and 1987; 20,000, in 1988; 25,000, in 1989; and 30,000, in 1990. Because data from
active-duty U.S. military personnel indicate that many more than 10,000 HIV infec
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