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Impact of viral load and the duration of primary infection on HIV transmission: systematic review and meta-analysis

Blaser, Nelloa; Wettstein, Celinaa; Estill, Jannea; Vizcaya, Luisa Salazara; Wandeler, Gillesa,b,c; Egger, Matthiasa,d; Keiser, Oliviaa

doi: 10.1097/QAD.0000000000000135
Epidemiology and Social

Objectives: HIV ‘treatment as prevention’ (TasP) describes early treatment of HIV-infected patients intended to reduce viral load and transmission. Crucial assumptions for estimating TasP's effectiveness are the underlying estimates of transmission risk. We aimed to determine transmission risk during primary infection, and describe the relation of HIV transmission risk to viral load.

Design: A systematic review and meta-analysis.

Methods: We searched PubMed and Embase databases for studies that established a relationship between viral load and transmission risk, or primary infection and transmission risk, in serodiscordant couples. We analysed assumptions about the relationship between viral load and transmission risk, and between duration of primary infection and transmission risk.

Results: We found 36 eligible articles, based on six different study populations. Studies consistently found that higher viral loads lead to higher HIV transmission rates, but assumptions about the shape of this increase varied from exponential increase to saturation. The assumed duration of primary infection ranged from 1.5 to 12 months; for each additional month, the log10 transmission rate ratio between primary and asymptomatic infection decreased by 0.40.

Conclusion: Assumptions and estimates of the relationship between viral load and transmission risk, and the relationship between primary infection and transmission risk, vary substantially and predictions of TasP's effectiveness should take this uncertainty into account.

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aInstitute of Social and Preventive Medicine (ISPM), University of Bern

bDepartment of Infectious Diseases, University Hospital Bern, Bern, Switzerland

cDepartment of Infectious Diseases, University of Dakar, Dakar, Senegal

dSchool of Public Health and Family Medicine, University of Cape Town, Cape Town, South Africa.

Correspondence to Nello Blaser, Institute of Social and Preventive Medicine, University of Bern, Finkenhubelweg 11, CH-3012 Bern, Switzerland. Tel: +41 31 631 35 15; fax: +41 31 631 35 20; e-mail:

Received 5 July, 2013

Revised 1 November, 2013

Accepted 1 November, 2013

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© 2014 Lippincott Williams & Wilkins, Inc.