Two previous trials of an HIV preventive vaccine, referred to as the MRK rAd5 vaccine, were halted because people receiving the vaccine were at greater risk of HIV infection. The MRK rAd5 vaccine delivered parts of HIV inside a recombinant Ad5 (rAd5) virus vector, which is a modified version of a common virus that does not usually cause serious disease. Analyses of these trials found that the increased risk of HIV infection was seen only in male participants who already had detectable antibodies to Adenovirus type 5 (Ad5).