AFRO-NETS> RFI: Size of a water molecule (3)

[Christian Labadie <CLabadie@t-online.de>]

RFI: Size of a water molecule (3)
---------------------------------


> To show that HIV cannot pass through a condom I usually use the 
> water test. My argument being a water molecule is smaller than a
> virus, if it cannot pass through the condom there is no way HIV 
> could pass through. I know the virus is 125 nanometers but I cannot
> find the size of a water molecule.

What would you answer to a person asking you to do the same test us-
ing cooking oil? A fat molecule is larger than water and smaller than 
the HIV virus, but it is recommended not to use them as a lubricant 
with condoms.

Such water test may make people feel over-confident about using con-
doms. It is important to encourage the use of condoms, but it is im-
portant to give an ESTIMATION of the RISK of using them.

Thus I would like to give you the answer to the failure rate of using 
condoms over ten years, which I asked in my previous message. Of 
course free and easy condom-supply is a good thing, and the purpose 
of my demonstration is not to diminish the value of free access to 
condoms.

To find out the rate of protection of condoms against AIDS, one needs 
the rate of HIV prevalence (estimated to ca. 35% in Botswana) and the 
rate of successful use of condoms during one year.

This rate can be deduced from the failure of condoms as a contracep-
tive method. The published failure rate (see reprint in my previous 
post) is 10% per year, which would give a 90% rate of successful con-
traception. It should be noted that a woman is fertile only a frac-
tion of her cycle. Let us admit that this fraction of time is one 
third (this may be overestimated but it will correct for the possible 
change in sexual-drive over the cycle). This would mean that during 
two thirds of the cycle, there are no chances of becoming pregnant, 
and that one third of the cycle accounts for the 90% rate of success 
(1 x 1 x 0.9 = 0.9). However in the case of AIDS/HIV, the virus 
transmission may occur during the whole cycle, the 90% rate for one 
third of the cycle must then be multiplied to itself 3 times to ac-
count for a whole year (0.9 x 0.9 x 0.9 = 0.729). The apparent condom 
effectiveness in the real situation for one year would be 72.9% (or a 
failure rate of 27.1%).

But not all sexual partners would be infected, thus the failure rate 
of using condoms during one year as a mean of protection against 
HIV/AIDS in a population 35% sero-positive, would be the product of 
the prevalence with the condom failure rate (0.35 x 0.271 = 0.09485). 
This product returns 9.48 % or a success rate of 90.52 %.

Using this approximation one can compute a table giving the predicted 
efficacy of condoms over ten years as a mean of protection against 
HIV/AIDS in an environment of 35% HIV prevalence:

1 year  -> 90.5 % success rate
2 years -> 81.9 % success rate (0.9052 x 0.9052 = 0.9052 ^ 2 = 0.819)
3 years -> 74.2 % success rate (0.9052 ^ 3 = 0.742)
4 years -> 67.1 % success rate
5 years -> 60.7 % 
6 years -> 55.0 %
7 years -> 49.8 %
8 years -> 45.1 %
9 years -> 40.8 %
10 years -> 36.9 %

This computation may be over simplistic, and I would appreciate any 
corrections or improvements. However it may explain why prevention 
programmes based only on condoms are less likely to function in coun-
tries with a high HIV prevalence than in countries with a low preva-
lence.

The advertisement of condom products as "safe sex" may be compared in 
some respects with the advertisement of "light cigarettes" as "cancer 
safe".

This is no reason to drop condoms... quite the contrary. But it is a 
reason to show that the users have the responsibility to provide ad-
ditional methods to improve and to *multiply* chances of success.

Thus I would present the condom as a parachute. Do you want to jump 
with only one? We should suggest other ways to *complement* condoms. 
One way would be to say "use condoms AND don't ejaculate". The "AND" 
multiplies chances. Or better "use condoms AND don't ejaculate AND 
check-up with a medical doctor before changing partner" etc..

Of course, during the first year of a "safe sex" campaign, condoms 
may be sufficient. But in the following years it is important to in-
troduce additional methods, and to further educate persons with more 
detailed explanations.

These are rates assuming male condoms. It would be important to com-
pare such rates using female versus male condoms.

Christian Labadie, M.S.
mailto:CLabadie@t-online.de

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