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Transmissibility of HIV infection: what we know in 1993.

When the HIV epidemic began just over a decade ago, it was a mystery where the virus had come from and how it was transmitted. Because HIV infection is a fatal illness, transmissibility remains an area of serious concern. Fortunately, it is not an easy disease to contract, and there are only certain, clearly identifiable ways in which it is transmitted. HIV infection is difficult to contract because of two factors. First, in only a limited number of ways can the virus be passed from one person to another: a) injection of infected blood directly into the blood stream or through the integument, or b) passage of the virus by blood or other infected secretions through mucous membranes. Second, even with the defined mechanisms of transmission, the disease is not passed every time these events occur.


Blood to Blood (or blood through skin). Table 1 presents accepted modes of blood and blood product transmission of HIV and estimates risks of an individual contracting HIV through each type of exposure. Extreme rates (greater than 50% risk) are associated with transfusions and transplants where the dose of virus is likely to be high. Sharing needles and treatment with blood products carry a 5%-10% estimated risk. Risk from a needle stick is low (0.4%) based on actual data. Finally, nonpercutaneous exposure of blood to skin and mucous membrances carries a very low risk. In CDC studies of health care workers, no cases studied prospectively existed where transmission occurred without a break in the skin or a break in some surface.

Transmission from Other Body Fluids. The virus also can be passed between two people who have close enough contact to mix body fluids, often with some contact with the vasculature of the recipient. Work in hospital settings helped clarify which fluids (in addition to blood) can contain HIV in sufficient amounts to pose a threat of infection: semen; vaginal secretions; cerebro-spinal fluid; amniotic fluid; synovial fluid; pleural, peritoneal, and pericardial fluids; and others visibly contaminated with blood.

Table 1 also demonstrates how the virus can be transmitted from one individual to another by body fluids. Sexual transmission represents a major mode of transmission. Male-to-male transmission poses a moderate risk (2%-5%). Since semen carries the virus, and since homosexual activity may lead to abraded mucous membranes and perhaps some bleeding, opportunity exists for infection. Male-to-female transmission carries a 0.1% to 2% risk and is substantially higher than female-to-male transmission. This difference probably occurs because semen can harbor a larger dose of virus than vaginal secretions do, and because the exposed area of mucous membrane is larger in the woman.

Vertical transmission from mother to fetus and newborn carries a high transmission rate. During the intra-uterine period the rate probably approaches 10%. With all the bleeding and sharing of fluids during the birth process, it is likely that a 15% risk of transmission exists when the baby passes through the birth canal. While controversial, an analysis of multiple studies from Lancet suggests breast feeding by HIV-infected women carries with it as high as a 14% risk of transmission to the breast-fed baby.


Throughout the AIDS epidemic, periodic fears emerge about how the HIV virus might be spread. Some fears simply involve theories by lay hypotheses. Others derive from case reports in medical journals. Such sporadic infections need substantiation that the secondary instance was caused by the same virus. Lacking such proof, individual reports should be considered "undocumented." Some modes of transmission, while theoretically possible, are now not accepted as real risks. Worldwide experience suggests they are unlikely to represent bona fide means of infection. Undocumented concerns include biting; kissing; sports injuries; close contact in school rooms or day care; normal family life; insect bites; contact of healthy intact skin with infected blood; contact with "oozing" nonbloody skin lesions; and contact with urine, feces, saliva, sweat, tears, and other nonbloody body fluids.
Table 1
Accepted Modes of Transmission of HIV

Percutaneous                                        Estimated Risk*

Needle stick from HIV-infected patient              Low (0.4%)
Sharing needles                                     High
Infected blood product (Factor VIII, before 1981)   High
Infected blood, plasma, platelet transfusion        Extreme
Infected organ transplant                           Extreme

Mucous Membrane

Exposure to blood                                   Very low


Male to female                                      Moderate
Female to male                                      Low

Male                                                Moderate

Intrauterine                                        Very high (10%)
Perinatal                                           Very high (15%)
Breast feeding                                      ?Very high (?14%)

* Estimated rates (single event): Extreme |is greater than~ 50%; very high 10%
to 50%; high 5% to 10%; moderate 2% to 5%; low 0.5% to 2%; very low 0.1%-0.5%.
Table 2
Heterosexual Transmission of HIV

                                     N     N HIV+   %

Female partners of infected males    307   61       20
Males partners of infected females    72    1        1

Odds Ratio 17.5

From Padian et al. JAMA. 1991; 366:1664-1667.


The real concern for schools involves proper handling of blood in significant amounts. If schools follow the CDC recommendations for Universal Precautions, they can report to school boards, PTAs, teachers, and local news media that they have taken every prudent measure. HIV is hard to spread. Fear of HIV transmission should not stand as a barrier to school admission of children with HIV infection.

Kenneth McIntosh, MD, Chief, Division of Infectious Diseases, Children's Hospital, 300 Longwood Ave., Boston, MA 02115.
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Title Annotation:human immunodeficiency virus
Author:McIntosh, Kenneth
Publication:Journal of School Health
Date:Jan 1, 1994
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