The study will enroll about 3800 cisgender men and transgender people who have sex with cisgender men and/or transgender people at clinics located in Argentina, Brazil, Italy, Mexico, Peru, Poland, Spain, and the United States. To join this study, participants must be healthy, HIV-negative, and between 18 and 60 years old. These participants must be willing to have medical check-ups and regular HIV counseling and testing. There are also other criteria that must be met for a person to be eligible to join the study. If you have chosen to use HIV pre-exposure prophylaxis (PrEP), you are not eligible for this study, as PrEP, when taken correctly, is a highly effective HIV prevention method. If you are interested in accessing PrEP, we can help link you to services.

We will answer any questions you have to make sure you understand what is involved in participating in this study.

• You will have physical exams that include blood tests. Blood samples will be used: to test for HIV infection; to check how your immune system responds to the study vaccines; and for limited genetic testing and other tests that will help researchers understand your health and immune system better.
• Over the course of the study, you will go to the clinic approximately 14 times over 3 years. At 4 of these visits you will get injections of the study vaccines or the placebo (sterile salt water). You will receive six total injections during the study.
• You will be asked to keep track of how you feel for the 7 days following each injection. After this time, you and the site staff will be in contact to see how you are feeling.
• The follow-up visits will include collecting blood samples so that we can measure how people's immune systems respond to the vaccine regimen, HIV counseling and testing, and answering questions from the clinic staff. These visits will be shorter than visits where you receive injections.
• The HIV counselors will advise you about the best ways to protect yourself from HIV infection during the study.

The study enrolled the first participant in November 2019, and will continue enrolling participants throughout 2020 and 2021. The study is taking place at multiple clinics in Argentina, Brazil, Italy, Mexico, Peru, Poland, Spain, and the United States.

Participants who receive study vaccines could have a reactive result or “test positive” on some types of HIV tests that are commonly used. If a participant gets an HIV study vaccine, their body may make antibodies to HIV. Standard HIV tests like the Rapid HIV test (fingerprick test) measure HIV antibodies as a sign of HIV infection. Because of this, participants who receive the study vaccines in Mosaico could have a reactive HIV test result even if they are not infected with HIV. This is called vaccine-induced sero-positivity (VISP), or you may see this called a vaccine-induced seroreactive (VISR) test result. No health problems are associated with VISP. However, if someone believes you are infected with HIV, you could face discrimination and/or other problems. For example, you could have problems obtaining medical or dental care, employment, insurance, a visa for traveling, or entry into the military. You might not be allowed to donate blood or other organs.

People with VISP need specific HIV tests to see if a reactive test result is due to VISP or a true HIV infection. Research clinics participating in this study have access to these specific tests that look for the virus itself, instead of looking for antibodies.