There currently is no human vaccine available. Several vaccines are under development by private and public researchers.[29] Developing a vaccine against the disease is challenging. With four different serotypes of the dengue virus that can cause the disease, the vaccine must immunize against all four types to be effective.[3] Vaccination against only one serotype could possibly lead to severe dengue hemorrhagic shock (DHS) when infected with another serotype due to antibody-dependent enhancement. When infected with dengue virus, the immune system produces cross-reactive antibodies that provide immunity to that particular serotype. However, these antibodies are incapable of neutralizing other serotypes upon reinfection and actually increase viral replication. When macrophages consume the ‘neutralized’ virus, the virus is able replicate within the macrophage, causing disease. These cross-reactive, ineffective antibodies ease access of virus into macrophages, which induces more severe disease (dengue hemorrhagic fever, dengue shock syndrome). A common problem faced in dengue-endemic regions is when mothers become infected with dengue; after giving birth, offspring carry the immunity from their mother and are susceptible to hemorrhagic fever if infected with any of the other three serotypes.[30] One vaccine was in phase III trials in 2012 and planning for vaccine usage and effectiveness surveillance had started