AsianScientist (Oct. 26, 2016) – Dengvaxia, the dengue vaccine developed by Sanofi, has now been approved for use in ten countries, most recently Singapore. Despite its limitations, the vaccine—which is the only one available against the mosquito-borne virus—is now being rolled out in large-scale public immunization programs in Brazil and the Philippines.
But such a strategy would not suit every country or region, say the authors of a recent study published in the journal Science. Their work suggests that in certain settings, routine vaccination may actually increase the incidence of severe dengue, and thus the numbers hospitalized due to the disease.
Modeling vaccine impact
While a first infection with one of the four dengue virus serotypes is typically mild, a second infection with a different serotype can result in severe disease. It’s thought that the body’s immune response to the first infection enhances the second. Similarly, researchers have long worried that a vaccine which stimulates uneven or inadequate immune responses could enhance subsequent natural infections.
Using publicly-available data from the Dengvaxia clinical trials, the authors of the Science study developed mathematical models to examine the impact of a dengue vaccine roll out. They found that in areas with high dengue transmission—where most people would already have experienced a prior dengue infection—vaccination could reduce dengue disease and hospitalizations by 20 to 30 percent. In areas with lower transmission levels, however, the vaccine could actually increase dengue illness and hospitalizations.
The authors propose that in previously infected people, vaccination acts like a silent secondary infection—it boosts pre-existing immunity, without causing disease, to levels that protect against subsequent infections. In people who have never had dengue, however, the vaccine mimics a naturally-acquired first infection, thus setting vaccine recipients up for severe disease when they do get infected naturally.
Sanofi, in a three-year follow-up study, indeed found that the vaccine was associated with an increased risk of hospitalization for young children—the age group least likely to have had a previous infection. The vaccine is thus not approved for use in children under the age of nine. NEXT PAGE>>>