werner@aecom.UUCP (Craig Werner) (01/16/85)
The Epidemiology of AIDS The following is from the Jan 11, 1985 issue of JAMA (253/2), "The Incidence Rate of Acquired Immune Deficiency Syndrome in Selected Populations" Incidence is defined as the number of new cases reported per a given time, usually (and in this case) in years for a given population, the higher the incidence, the higher the risk of getting a disease. For chronic diseases, it is usually steady. For epidemics, it tends to rise dramatically (as in AIDS) and then fall (something that AIDS has yet to do) Some of the data: Annual Incidence: Year cases Rate/100,000 1978 4 0.002 (in US, total pop. 1979 9 0.004 age 5 and above) 1980 44 0.02 1981 239 0.11 1982 961 0.44 1983 2501 1.20 ->5/84 3096 1.43 Note the nearly 1000 fold increase in incidence. This is clearly a disease in the exponential stage. 1.5/100,000 means 1 in 66,000 Americans has AIDS, or if they were spread uniformly, that would be one for every sellout football game. (That last example is of course, facetious, but illustrative) However, they are not spread uniformly, as seen below: Interpretation follows the data: Group # cases Incidence Single Men (US) 3,752 8.9 SM in Manhattan 1,358 175.5 SM in other NY boroughs 413 37.6 SM in SF 473 204.5 SM in LA 308 44.2 IV Drug Users (US) 1,381 113.3 IVDU in NY 675 203.2 IVDU in New Jersey 188 269.0 Haitians (US before 1978) 27 1.8 (US after 1/1/78) 192 82.0 Hemophiliacs (severe) 33 298.5 (mild) 2 43.2 In female contacts of male IV drug users 26 3.1 Persons receiving blood Adult (> 10 units) 25 4.8 Children(> 10 units) 5 22.5 Children (10 or less) 5 0.8 Comments: Most incidence data have trouble with the reporting of cases. In this case, however, the number of cases is probably more accurate than the population estimate. This reason however, is the reason the incidence is for Single Men, and not for Sexually Active Gay Men (which has been the usual phrasing of the risk group) because it is very easy to tell if a man is single, but much harder (and more controversial) to find out if he is gay, or if he is sexually active (and how would you define that) In fact, 87% of AIDS victims who are single men are gay or bisexual. (Same Source) For IV drug users, NJ may be higher because the estimate for NJ only included heroin users, and for NY and USA included heroine and cocaine. Also, IV drug users are usually less mobile than gays, so pockets may develope Note the numbers for Haitians. The very low number before '78 probably shows that the disease came to Haiti at approximately the same time it came to the US. It might have even spread there from the US. Either way, it means that the island itself is not to blame. For hemophiliacs and blood donors, the data clearly suggests that the disease is in fact spread by blood (though possibly not exclusively) since the risk is related to the amount of blood that the patient receives. There is one other thing not visible from the above selected numbers. Transmission from males to females is much more common than from females to males. It may have something to do with females (tending to have less partners, perhaps?) or it may actually say something about the mode of transmission of the disease. Robert Gallo of the NCI suggested that not just the virus but also infected tissue was necessary for the most efficient transfer of the disease, and sperm contains cellular matter, so it might follow. Of course, the disease can be spread without cells. Factor VIII (for hemophiliacs) is cell-free and has 40 total cases. OK, this article is a little too long. Next time, more on the soon-to-be released blood test. Oh, somebody wanted sources for my first article: Here they are, from Nature, Dec 20-27 1984 The CD4 (T4) antigen is an essential component of the receptor for the AIDS retrovirus. p. 763. T-lymphocyte T4 molecule behaves as the receptor for human retrovirus LAV. p.767. I didn't cite them the first time because they hadn't been published yet. -- Craig Werner !philabs!aecom!werner What do you expect? Watermelons are out of season!