by Sayer Li 
A new study published in Lancet reveals that an experimental vaccine manufactured by Merck using a genetically modified adenovirus actually increased the risk of contracting HIV infection in recipients.
Titled, “Recombinant adenovirus type 5 HIV gag/pol/nef vaccine in South Africa: unblinded, long-term follow-up of the phase 2b HVTN 503/Phambili study,” researchers randomly assigned a total of 801 HIV-1 uninfected, sexually active adults aged 18—35 years from five sites in South Africa to receive either the vaccine (400) or a placebo (401). 216 (27%) received only one injection, 529 (66%) received only two injections, and 56 (7%) received three injections.
The results were reported as follows:
At a median follow-up of 42 months (IQR 31—42), 63 vaccine recipients (16%) had HIV-1 infection compared with 37 placebo recipients (9%; adjusted HR 1·70, 95% CI 1·13—2·55; p=0·01). Risk for HIV-1 infection did not differ according to the number of vaccinations received, sex, circumcision, or adenovirus type 5 (Ad5) serostatus.
Differences in risk behaviour at baseline or during the study, or annualised dropout rate (7·7% [95% CI 6·2—9·5] for vaccine recipients vs 8·8% [7·1—10·7] for placebo recipients; p=0·40) are unlikely explanations for the increased rate of HIV-1 infections seen in vaccine recipients.
In other words, those receiving vaccines had a 70% increased risk [HR 1·70] of contracting HIV versus those receiving the placebo. The researchers concluded:
The increased risk of HIV-1 acquisition in vaccine recipients, irrespective of number of doses received, warrants further investigation to understand the biological mechanism. We caution against further use of the Ad5 vector for HIV vaccines.
