Viral load, treatment and mother-to-child transmission

  • The link between viral load and mother-to-child transmission is very well established.
  • In the UK, the transmission risk from a pregnant woman on antiretroviral treatment with an undetectable viral load, to her child is 0.1% (1 in 1000).

The association between HIV viral load and the risk of any kind of HIV transmission was first established in studies examining how to prevent mother-to-child transmission (MTCT).

In 1994, two important studies were published which showed for the first time a correlation between maternal viral load and infant HIV infection,1 and that treatment with an antiretroviral drug, AZT (zidovudine, Retrovir), significantly reduced the risk of MTCT across all levels of maternal viral load.2

Three studies published in 1999 firmly established that the risk of MTCT correlates with maternal viral load, including among women receiving short-course AZT treatment.3 4 5 

However, although the risk is greatest for those pregnant women with high viral loads, transmission can occur even when maternal viral loads are very low or undetectable. Nevertheless a European/US study found that treatment-related low viral load resulted in a ten-fold decrease in the MTCT rate compared with naturally occurring low viral loads.6

In this study, 44 cases of MTCT were recorded among 1202 women with viral loads below 1000 copies/ml at delivery (or at the measurement closest to delivery), eight of whom were receiving antiretroviral treatment during pregnancy or at the time of delivery (or both). This resulted in a 1.0% transmission rate on treatment compared with a 9.8% transmission for untreated mothers with naturally low viral loads. It should be noted, however, that the lowest limit of detection for viral load assays used at the time was 500 copies/ml.

More recent data from the UK and Ireland7 have found that transmission is possible, although extremely unlikely, when the maternal viral load is below 50 copies/ml. 

Between 2000 and 2006, 5930 infants were born to HIV-positive mothers. Of the 2309 mothers with an undetectable viral load at or near the time of delivery, three MTCT were reported, resulting in a transmission rate of 0.1%.  This compares with a transmission rate of 0.8% for women who received antiretrovirals for at least the last 14 days of pregnancy, regardless of type of antiretroviral treatment or prophylaxis or mode of delivery. Not receiving antiretrovirals increased the risk of transmission nine-fold.

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References

  1. Weiser B et al. Quantitation of human immunodeficiency virus type 1 during pregnancy: relationship of viral titer to mother-to-child transmission and stability of viral load. Proc Natl Acad Sci U S A 91(17): 8037-41, 1994
  2. Connor EM et al. Reduction of maternal-infant transmission of human immunodeficiency virus type 1 with zidovudine treatment. N Engl J Med 331: 1173-1180, 1994
  3. Garcia PM et al. Maternal levels of plasma human immunodeficiency virus type 1 RNA and the risk of perinatal transmission. N Engl J Med 341: 394-402, 1999
  4. Shaffer N et al. Short-course zidovudine for perinatal HIV-1 transmission in Bangkok, Thailand: a randomised controlled trial. Lancet 353: 773-780, 1999
  5. European Collaborative Study Maternal viral load and vertical transmission of HIV-1: an important factor but not the only one. AIDS 13: 1377-1385, 1999
  6. Ioannidis JP et al. Perinatal transmission of human immunodeficiency virus type 1 by pregnant women with RNA virus loads <1000 copies/ml. J Infect Dis 183: 539-545, 2001
  7. Townsend C et al. Low rates of mother-to-child transmission of HIV following effective pregnancy interventions in the United Kingdom and Ireland, 2000-2006. AIDS 22: 973-981, 2008
This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.