Epstein–Barr virus
As of 2021, a vaccine against Epstein–Barr virus was not yet available. The virus establishes latent infection and causes infectious mononucleosis. It is a dual-tropic virus, causing infection of both B cells and epithelial cells. One challenge is that the Epstein–Barr virus expresses very different proteins during its lytic and its latent phases. Antiviral agents act by inhibiting viral DNA replication, but as of 2016 there was little evidence that they are effective against Epstein–Barr virus, they are expensive, they risk causing resistance to antiviral agents, and (in 1% to 10% of cases) can cause unpleasant side effects.