Tuberculosis incidence in sub-Saharan Africa is falling too
slowly to meet global targets of an 80% reduction by 2030, and where incidence
is falling, the decline is strongly associated with the extent of antiretroviral
coverage in people living with HIV, a study by former World Health Organization
modelling experts has found.

The findings, published in the Bulletin of the World Health Organization in June 2019, come from a
modelling study carried out by Christopher Dye and Brian Williams.

They looked at the relationship between TB incidence,
diagnosis, treatment, HIV burden and antiretroviral coverage in 12 countries in
sub-Saharan Africa with a high standard of TB and HIV surveillance data.

TB disproportionately affects people living with HIV, with immune
suppression leading to the reactivation of latent TB infection. Countries with
the highest prevalence of HIV also have amongst the highest TB incidence. Antiretroviral
therapy reduces the risk of developing TB, as does isoniazid preventive
therapy.

With little evidence that TB diagnosis and treatment are leading to an acceleration in TB reduction, increasing the coverage of ART is important.

Review of national surveillance data showed that TB
incidence fell in all countries in the region between 2003 and 2016 in
HIV-positive people and in all countries except Uganda in HIV-negative people.
The average annual reduction in incidence in people with HIV ranged from 8.1% a
year in Botswana, 8.7% in Zimbabwe and 9% in Namibia to only 0.5% in South
Africa. Incidence per 100,000 people with HIV was highest in
Lesotho (979 per 100,000) and lowest in Uganda (146 per 100,000) and Rwanda (46
per 100,000).

The burden of HIV among newly-diagnosed TB patients varied
considerably; whereas 47% of newly-diagnosed TB patients tested HIV-positive in
Tanzania, 70% in Botswana and Zambia, 78% in Zimbabwe and 84% in eSwatini
(Swaziland) did so.

The researchers modelled the impact of antiretroviral
treatment and isoniazid preventive therapy on TB incidence using data on
availability and uptake of both interventions. Both interventions have been
shown to reduce the incidence of TB in people with HIV They also looked at the
impact of TB case detection and TB treatment on TB incidence, assuming that
detection and treatment of TB would interrupt transmission chains and prevent
new cases.

TB incidence and prevalence rose in all countries from 1990
as a consequence of the rise in HIV prevalence but began to decline as case
detection and treatment outcomes improved.

Antiretroviral therapy began to be introduced from 2003,
although the rate of increase in coverage varied between countries. Whereas
coverage increased at a steady rate between 2003 and 2016 in Botswana, coverage
began to increase at a more rapid pace after 2008 in South Africa, eSwatini,
Namibia, and Zimbabwe.

Overall, modelling found an inverse correlation between ART
coverage and TB incidence, and regression analysis found that ART coverage was
associated with a higher proportion of TB cases prevented in people living with
HIV (r2=0.68, p 0.001).

Although approximately 500,000 TB cases were prevented in
South Africa between 2003 and 2016 due to antiretroviral treatment, the model
estimates, this only amounted to preventing 11.2% of TB cases, because of limited
ART coverage.

In the 12 countries studied, antiretroviral treatment
prevented between 1.88 million and 2.06 million TB cases in people living with
HIV, or 17.6% of the cases that would have occurred otherwise.

In contrast, isoniazid preventive therapy had a negligible
effect on TB incidence despite strong evidence of effectiveness and guidance
from the World Health Organization on the need to implement IPT. Isoniazid
preventive therapy coverage did not exceed 4.2% in South Africa, the country
most successful in implementing IPT, and three countries in the region (South
Africa, Malawi and Zimbabwe) accounted for 91% of all IPT coverage.

The researchers failed to find any correlation between the
estimated level of case detection and TB incidence across the 12 countries. If
case detection is higher, more TB cases should be diagnosed and treated,
preventing onward transmission.

In conclusion, the authors say that TB incidence is
declining too slowly to meet international targets. To meet the 2030 WHO End TB
strategy’s target of an 80% reduction in TB incidence, the incidence of TB
needs to fall by 10% a year. With little evidence that TB diagnosis and
treatment are leading to an acceleration in TB reduction, increasing the
coverage of ART will make an important contribution to reducing TB incidence
where the prevalence of HIV is high.