Abstract

The topic of this paper is the demonstration of the combined effectiveness of condom and spermicide use. It is reasoned that 2 methods, which separately provide only moderate efficacy, act together independently and the probability of both failing is the product of the 2 probabilities of failure. Spermicides have a typical user failure rate of 21% and condoms 12%; combined, the 1st year probability of failure among typical users is 2.5%. Under perfect use, where there is correct use for every act of intercourse, the assumption of independence is very likely, and efficacy would be a high as steroidal implants. In addition to highly efficacious protection from pregnancy, there is protection form HIV and other sexually transmitted infections (STD). Spermicides referred to are rubber compatible, not oil based, and conform to the IPPF Agreed Test for Total Spermicidal Power. Condoms are defined as good quality which resist breakage and conform to the International Condom Standard, 1990. It is also advised that, in the event of condom breakage without spermicide, genitalia be washed immediately with soap and water to minimize risk. Also, proper douching that directs liquid sideways, not toward the cervix, should further reduce risk, It is underscored that contraceptive efficacy is not an effectiveness rate, but a failure rate. Effectiveness is the proportionate reduction in the risk of conception per cycle or infection per coitus caused by use of a method. The per cycle probability of conception is fecundability. The model described for combined use and STD use; it is based on the assumption that 1) the probability of conception and effectiveness and thus the per cycle probability of failure for the method is constant over time, and 2) there are 13 cycles per year. What is ignored is that failure rates decline among typical users with duration of use, so that life table procedures are not necessary. The annual probability of failure during perfect use of condoms and spermicides would by .00045, and if failure doubled, the probability would still be only .00186. Any lubricant also helps to reduce breakage of condoms, thereby enhancing efficacy. The probability of failure for spermicidally lubricated condoms during perfect use would be .005. If the annual probability of condom failure were doubled to 4% and spermicide effectiveness were the lowest at 73%, then the failure rate would be only 1%.