SOUTH AFRICA: Questions about new prevalence survey
9 September 2008
JOHANNESBURG, 9 September 2008 (PlusNews) - Several prominent
demographers and scientists have vigorously refuted Health
Minister Manto Tshabalala-Msimang's claim that South Africa's
HIV epidemic is declining and that the country "may be making
some real progress in its response to the HIV epidemic".
Tshabalala-Msimang's statement was based on a national survey of
HIV prevalence among pregnant women, which researchers are
describing as deeply flawed.
The report, posted on the health department's website on 29
August, states that prevalence among pregnant women fell from
29.1 percent in 2006 to 28 percent in 2007.
Prof Rob Dorrington, a demographer at the University of Cape
Town, and his colleague, Prof David Bourne, point out in a
letter published in the South African Medical Journal (SAMJ)
this week that the 2007 survey employed a methodology not only
radically different from that used in 2006, but also "manifestly
The authors detected a problem when they noticed that changes in
prevalence by age group did not tally with the change in overall
prevalence, and that district figures were inconsistent with
They deduced that in 2006 the results from district antenatal
clinics were simply totalled to derive prevalence estimates for
the country's nine provinces, but in 2007 the health department
began weighting provincial figures according to age groups,
based on general population estimates for age distribution.
Dorrington and Bourne describe the new methodology as "clearly
problematic" because the age distribution of women attending
antenatal clinics is very different from that of the female
population as a whole.
"Since the prevalence of HIV also has a
distinct age pattern, and prevalence is lower in the youngest
and oldest age groups, using the population of all women to
reweight the data will inevitably underestimate the prevalence
of women attending public antenatal clinics," they wrote.
After recalculating the 2007 figures, using the same method
applied to the 2006 data, the authors estimated HIV prevalence
among pregnant women at 29.4 percent. Antenatal prevalence
figures are used in combination with other surveys and
mathematical models to determine HIV prevalence in the overall
population, but the revised figure suggests that the number of
South Africans living with HIV has probably not declined.
Dorrington and Bourne also recalculated the estimates for each
of the provinces and found that the age weighting had given rise
to "absurd results", particularly for the Western Cape, where
prevalence dropped from 15.1 percent in 2006 to 12.6 percent in
2007, according to the survey.
In reality, only two districts in the province showed small
declines and, after totalling data from all the districts, the
authors estimated a prevalence rate of 15.3 percent.
The Treatment Action Campaign (TAC), a national AIDS lobby
group, released a statement on Monday saying that the health
department had "made a mistake or misled the public" by failing
to state the change of methodology in the 2007 report.
TAC spokesperson Lesley Odendal recommended that the health
minister's claims of a drop in HIV prevalence "be treated with
She also pointed out that even a slight increase in prevalence
would not necessarily indicate that the HIV epidemic had
worsened. "It has become extremely difficult to interpret the
meaning of prevalence in recent years for two reasons: the HIV
epidemic has matured into an AIDS epidemic [with more people
dying], and antiretroviral treatment is helping people with HIV
to live much longer."
In their letter to the SAMJ, Dorrington and Bourne note that the
survey was "surprisingly silent" on the impact of
life-prolonging antiretroviral (ARV) treatment on prevalence.
About 500,000 HIV-positive South Africans have benefited from
the roll-out of ARV treatment, which could actually cause
prevalence to increase, depending on mortality rates, but the
2007 survey revised the total number of people living with HIV
down from 5.41 million in 2006 to 5.27 million in 2007.
Dorrington and Bourne conclude that "analysis of these data
appears to be becoming increasingly beyond the skills of the
Department of Health" and recommend that the government enlist
the help of the broader scientific community to help interpret
future figures on prevalence.
The Department of Health said it will
release a statement responding to the criticisms.