Vaccine and Infectious Disease Division

The positive effects of an imperfect vaccine

Herpes simplex virus 2 (HSV-2) is very common in some areas, with infection rates reaching nearly 50% in sub-Saharan Africa.  The virus causes lifelong infection, and is a major public health threat as it increases both acquisition and transmission of HIV.  Unfortunately, while antiviral medications can control herpes disease, they do not decrease HIV acquisition or transmission, so an effective HSV-2 vaccine represents the best chance at decreasing HSV-2-related incidences of HIV.  While no such vaccine yet exists, VIDI post-doctoral fellow Dr. Ramzi Alsallaq and colleagues have used mathematical models to predict the effects of different types of HSV-2 vaccines on the virus’ prevalence in Kisumu, Kenya.

Vaccines can prevent disease spread in three ways: by decreasing an uninfected person’s chances of catching the disease, termed their susceptibility; by reducing the amount of virus an infected person sheds, also known as infectivity; or by reducing an infected person’s shedding frequency.  Alsallaq and colleagues, including VIDI research associate Dr. Joshua Schiffer, VIDI member Dr. Ira Longini, VIDI co-director Dr. Larry Corey, and VIDI affiliate investigators Drs. Anna Wald and Laith Abu-Raddad, used mathematical modeling to predict the effects of “imperfect” HSV-2 vaccines, that is, vaccines with less than 100 percent efficacy in all of the three areas, on disease incidence and prevalence in Kenya.  They found that vaccines with high efficacy in decreasing susceptibility would be immediately effective at decreasing disease incidence on a population level, though prevalence took much longer to decrease.  They also found that vaccines with modest effects on susceptibility but high efficacy in decreasing viral shedding would have a significant effect on HSV-2 prevalence in Kenya, but this effect would also accrue slowly over several decades.  These findings point to the importance of evaluating not just susceptibility, but viral shedding, in future trials studying candidate HSV-2 vaccines.

Population level impact of an imperfect prophylactic vaccine for herpes simplex virus-2.  Alsallaq RA, Schiffer JT, Longini IM Jr, Wald A, Corey L, Abu-Raddad LJ.  Sex Transm Dis. 2010 May;37(5):290-7.