Finding a safe, effective “mono” vaccine has proved devilishly complicated – although the need is obvious: Every year, a new crop of college students comes down with mono, which causes a terrible sore throat, enlarged spleen and debilitating fatigue that can last for weeks. Spread by saliva (kissing, sharing beer bottles, etc.), mono is so ubiquitous that by adulthood, 90 percent of American adults have had it.

In 80 percent of cases, mono is caused by the Epstein-Barr (EBV) virus, a member of the herpes family; most of the rest are caused by cytomegalovirus (CMV), also a herpes virus. To date, there has been only one successful vaccine against a herpes virus. It is the one that at low doses protects kids against chicken pox, and, at a 10 times higher dose, protects older adults against shingles. There are no vaccines against any other herpes viruses.

There are no vaccines against any other herpes viruses – and one reason is that all herpes viruses can, after an acute infection, hide in cells for decades. The Epstein-Barr virus not only hides, but can also trigger a rare kind of cancer, Burkitt’s lymphoma. That means that any vaccine based on live Epstein-Barr virus could theoretically also carry this cancer risk, said Dr. Mark Pasternack [cq], chief of pediatric infectious diseases at Massachusetts General Hospital. Researchers have trying to get around this problem, with mixed results.

And there’s another serious roadblock to a mono vaccine. As with AIDS (caused by an altogether different virus), a potential vaccine, to be fully protective, has to induce two separate types of immunity – antibodies, to keep the virus from infecting cells, and so-called T-cells, that would also attack virus-infected cells. So far, no one has been able to find a safe mono vaccine that could produce both antibody and cellular immunity, said Dr. Clyde Crumpacker [cq], an infectious disease specialist at Beth Israel Deaconess Medical Center.

Unfortunately, there’s not a lot you can do to protect yourself against mono. Even after the acute symptoms go away, people with mono can carry the virus in their saliva for months. “You don’t have to be symptomatic to be infectious,” said Pasternack. So to avoid being exposed, or exposing others, on a college campus would mean living a restricted lifestyle – something that is not likely to happen anytime soon.