Genital herpes is a very common sexually transmitted infection (STI) that is estimated to infect 776,000 people in the United States each year. In fact, it so widespread that, according to the Centers for Disease Control and Prevention, as many as one in six adults in the United States may already be infected. There is no cure for herpes, which makes prevention that much more important. Researchers have been working for decades on a vaccine to prevent infection, and recently there has been some progress made on this front, with three vaccines in some stage of testing.

One of two herpes simplex viruses—HSV-1 or HSV-2—causes genital herpes. It used to be believed that HSV-1 caused herpes infections of the mouth (often thought of as cold sores) and HSV-2 was responsible for those infections of the genitals, but researchers now understand that either virus can infect the genitals or the mouth. Still, the vaccines in development are focused on HSV-2, as it remains the cause of most genital infections.

Last week, the pharmaceutical company Agenus reported positive mid-stage findings in the clinical trial of a new vaccine, HerpV, which contains a pharmacological agent called QS-21 Stimulon that is designed to boost the body’s own immune reaction. For this trial, the company enrolled 80 patients who were already infected with HSV-2 and had experienced between one and nine outbreaks within the last 12 months. Seventy participants received the vaccine, while the other ten were given a placebo.

Researchers measured how much virus each participant was releasing 45 days before the vaccines were given and then again after three doses of HerpV or the placebo were administrated. The results revealed a 15 percent reduction in the release of the virus by participants who received the vaccine. No reduction was seen in those who received the placebo. The less virus a person releases, the less likely he or she is to transmit the virus to a sexual partner. The vaccine also reduced the severity of infection by 34 percent.

The trial is continuing, and most participants were given booster shots six months after the initial three doses of the vaccine. The company says that it will have more data next year on the efficacy of this approach to preventing herpes.

Another company that has been working on a herpes vaccine, SmithKline Beecham, released results of its clinical trial earlier this fall, and the results were somewhat promising, though less so than had been hoped. This vaccine uses a protein from the surface of the virus combined with an immune booster. Researchers conducted two large-scale trials of couples in which one partner had genital herpes and the other did not.; to qualify for the first study, one partner had to be free of both HSV-1 and HSV-2, while the second study included couples in which the “uninfected” partner could have HSV-1 but not HSV-2. The uninfected partner was given either the vaccine or a placebo and was followed for 19 months.

The vaccine had some limited success. Fewer participants who received the vaccine contracted genital herpes during follow-up than those who got the placebo (3 percent versus 11 percent). Another 3 percent who had received the vaccine got infected but never had sores. In fact, the vaccine was about 75 percent effective in preventing sores in women who had never been infected with either HSV-1 or HSV-2. However, it provided no protection for men or for women who had already been infected with HSV-1.

Though this was not quite what researchers had hoped for, they noted that such a vaccine could still make a difference if it was given to girls before they become sexually active.

Finally, the National Institute of Health announced last week that it was starting enrollment on clinical trials of yet another vaccine to prevent HSV-2, this one developed by David Knipe, a professor of microbiology and immunobiology at Harvard Medical School, and manufactured by Sanofi Pasteur. This one is referred to as a replication-defective vaccine, which means it uses the virus itself, but scientists have removed two key proteins from it so that it cannot multiply to cause genital herpes. The hope is it will boost the immune system’s response if and when it comes in contact with the complete virus.

The clinical trial is enrolling now, and results will likely not be available until 2016.

Currently, there are vaccines on the market that can prevent infection with two STIs: human papillomavirus (HPV) and hepatitis B. Another vaccine would be great news, especially given how widespread herpes is in the United States. These vaccines, however, won’t be available for a number of years. In the meantime, condoms remain the best choice for sexually active individuals to prevent, or at least reduce, the spread of infection.