I am no expert in HIV, but I know that ‘vertical transmission’ means something very specific:

Vertical transmission: the transmission of a disease from mother to child either during pregnancy, childbirth, or by breastfeeding.

Similarly, the definition of ‘horizontal transmission’ is well defined:

Horizontal transmission: the transfer of an infection from person to person.

So, when I read about ‘vertical’ and ‘horizontal’ interventions in a recent New England Journal of Medicine Editorial and the Controversies blog, I began to get a little confused. I have a PhD in epidemiology so don’t consider myself easy to confuse (in this particular domain), but I would have thought that a ‘horizontal intervention’ would be directed towards preventing horizontal spread of an infectious agent and a ‘vertical intervention’ would be directed towards preventing the vertical transmission of an infectious agent. But this is not how these terms are being applied. Instead, a ‘horizontal intervention’ is being used to describe an intervention applied to every patient (such as chlorhexidine bathing or hospital-wide hand hygiene interventions) whereas a ‘vertical intervention’ is being used to describe an intervention designed to reduce colonization or infection due to a specific pathogen (such as active screening and isolation to prevent the spread of MRSA). The use of the term ‘vertical intervention’ seems especially confusing, since it’s a ‘vertical intervention’ to prevent the horizontal transmission of a specific pathogen!

I fail to see how the terms ‘vertical’ or ‘horizontal’ intervention are useful when there are such well-established definitions for horizontal and vertical transmission. I think that ‘universal intervention’ (such as universal screening or decolonization) and ‘targeted intervention’ (such as active screening and isolation to prevent the spread of MRSA) make a lot more sense. These terms are already in common circulation, so I would urge those who favour the use of ‘vertical’ or ‘horizontal’ intervention to reconsider their terminology.