Friday, July 09, 2004

Cutting AIDS transmission

I have been running around claiming that getting every HIV/AIDS sufferer into treatment would cut the transmission rate and thus the rate of new cases drastically, based on the notion that people with no virus circulating in the blood cannot transmit it. This would mean that treating everyone who is HIV+ today would be the cheapest strategy, starting at about $10 billion annually. The reason is that costs would start to go down once transmission is limited, through normal mortality and further medical advances. Only a cure would cut costs rapidly, of course, but we aren't talking about that here.

Some people have challenged this idea, saying that people on anti-retroviral treatment are still infectious. This is true in part, but even if some people would be somewhat infectious after starting treatment, it doesn't contradict what I have been saying. Any reduction in transmission rate is good. Anyway, we need to understand this as well as we can, so I went and looked for more detailed analyses. We don't have the full answer, largely because it would be unethical for anybody to do controlled experiments on infecting people with HIV, but here is what I have found out so far.

Yes, some patients are still infectious during treatment. No, this is not most patients. It means the sickest patients, those who visibly have AIDS, and are not merely HIV+ (HIV positive).

I have quoted some sources below, and of course there are many more. My conclusions from them are as follows.

People with advanced AIDS, that is T-cell count below 200, can continue to have a high viral load until their immune systems start to recover, but patients who start treatment earlier typically have undetectable levels of virus in their blood. Both have high virus levels in lymph tissue, where it does not make them infectious.

Patients who are HIV+ but have not progressed to AIDS commonly have undetectable viral load in their blood and high levels in lymph tissue, and have been found to be almost always non-infectious in studies of couples where one is HIV+ and the other initially isn't.

Almost all of the transmission appears to occur during the initial infection before immune suppression of the virus, and much less during the later infectious AIDS stage, when health drops off dramatically.

I expect that most patients bedridden with full-blown AIDS will not be having sex before their viral load drops and their immune system kicks in again.

Clearly medication is not sufficient by itself. We need education, condom distribution, and testing as well. But my point about reducing the spread of HIV/AIDS with treatment appears to be valid.

HIV Sexual Transmission Factors
The viral load is the chief predictor of the risk of heterosexual transmission of HIV-1, and transmission is rare among persons with levels of less than 1500 copies of HIV-1 RNA per milliliter.

HIV, bank accounts, and evolution
The virus infects different tissues, but if one looks at the amount of virus present in the blood, it follows a common pattern (figure 5.A.1). Within a few weeks of being infected, the virus multiplies to high levels and is easily detectable in blood (the "primary infection"). The immune system -- which is ultimately destroyed by the virus -- responds at this time and brings circulating viral levels down to the point that they are undetectable. This immune response includes antibodies against the virus, so at this point, the infected person will test positive by the standard tests (such a person is said to be HIV+ but does not yet have AIDS). The infection persists however, with low levels of virus in the blood (but high levels in lymph nodes). This "asymptomatic" phase lasts several years. Eventually (typically 8 years into the infection), the person develops AIDS, which is fatal unless successfully treated.
[To model the known rate of spread of HIV/AIDS in different transmission modes]
Some crude estimates suggest numbers approximately as follows:

if during the primary infection: 1.1 new new transmissions per infection

if during the AIDS stage: 70 new new transmissions per infection

Thus the worldwide spread of HIV can be explained if transmission occurs during the primary phase to infect slightly more than 1 new individual (on average). The alternative, transmission during the late stages of infection, requires that everyone infected must be infecting 50-100 people. The available data implicate early transmission.

So anyway, I'll come back to all this soon. HIV/AIDS is the second biggest single issue facing developing countries in terms of estimated expense to deal with it, after debt service, and by far the biggest in terms of degradation and death.

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