Make HIV vaccine tests gauge effect on infection AND contagion

November 13, 1995

ANN ARBOR—HIV vaccine trials currently on the planning boards should be redesigned so that they test not only for protection from infection but for a critical secondary benefit as well: the vaccine’s power to reduce contagiousness and arrest the spread of the disease to others, according to a University of Michigan epidemiologist.

” It is possible that a vaccine that protects people from an HIV infection only 25 percent of the time could still reduce an infected person’s contagiousness to others by 90 percent. If scientists fail to measure the vaccine’s impact on contagiousness, they might inadvertently overlook a potent weapon in the battle against HIV and AIDS,” says Dr. James S. Koopman of the U-M School of Public Health. His research findings will appear in the Nov. 15 issue of the American Journal of Epidemiology.

Vaccines can do three things. They can reduce our susceptibility to infection or they can protect us from developing a disease, with all its symptoms, even if we do become infected. ” However,” Koopman explains, ” it is less well known that vaccines also can reduce contagiousness so that, if vaccinated people do become infected, they are less likely to spread the disease to others.

” For instance, a vaccine given to monkeys that fails to protect them from a disease similar to HIV still greatly reduces their level of viremia during early infection, so they are much less contagious to other animals.

” There is good reason to believe that an HIV vaccine for humans would reduce contagiousness to a similar level. Indeed, since our recent research indicates that people are 100 to 1,000 times more contagious in the first two months of the HIV infection, we feel it is probable that a vaccine that reduces infectiousness, and thus contagiousness, during this stage alone could prevent or stop HIV epidemics,” Koopman says.

Under the current HIV vaccine field trial design, scientists will vaccinate currently uninfected individuals and compare their rates of HIV infection over time to the rates of uninfected individuals who were not vaccinated. Koopman urges, however, that scientists vaccinate uninfected, non-monogamous couples instead, and compare their rates of infection over time to uninfected couples who were not vaccinated.

Such a design will help scientists assess not only the degree to which vaccines protect individuals from infection but the degree to which they reduce contagiousness, and thus protect partners. ” Putting it very simply, if large numbers of individuals in the vaccinated couple group are infected with HIV but their partners remain free of the disease, it is likely that the vaccine reduced contagiousness,” Koopman explains.

” Ignoring the power of an HIV vaccine to make people less contagious could be catastrophic. If we can bring the epidemic to the point where, statistically, infected individuals infect less than one other person who can carry on the chain of infection, the epidemic will die out.”

A couples field trial also would be less time consuming and more cost effective,” Koopman points out. ” To be statistically valid, field trials with individuals will require about 5,000 vaccinated and 5,000 unvaccinated people. Since we would be measuring two outcomes in a couples trial, we would require only about 615 couples in each group—a significant savings in cost and time.”