In some surprising news, the newly licensed dengue virus vaccine, trade name Dengvaxia, could do more harm than good if not implemented correctly, a new study warns.
Sanofi Pasteur, the pharmaceutical company behind the dengue vaccine, spent 20 years developing the vaccine and published a report on its promising results in 2015. Drug trials showed that Dengvaxia was 59.2% effective against dengue when results were compiled across age groups and various populations. Some differences appeared when the type of dengue, vaccination age, and previous dengue infections were factored in.
Neil Ferguson, director of the MRC Center for Outbreak Analysis and Modeling at the Imperial College London, says,
Its effectiveness depends on the local epidemiology of dengue and how intense the transmission is.
He says vaccinating people who are at high risk for contracting the virus and are in the right age group maximizes the vaccine’s benefits.
The vaccine is licensed for use in six countries as of now. The Philippines became the first country to launch it in April this year, with Brazil, Paraguay, Singapore, El Salvador and Mexico announcing that they will roll it out soon. All these countries have high incidences of dengue.
In the new study, Ferguson examined data from the vaccine’s clinical trials to evaluate Dengvaxia’s impact in various settings. He and his team found that using it in areas with low risks of the disease, where people are not likely to become infected, could lead to an increase in complications for those who do have the virus, because of the complex way dengue interacts with the human immune system.
Ferguson points out that contrary to other diseases, dengue is more dangerous the second time a person gets it. In individuals who have never contracted dengue, the vaccine may act as a silent infection of sorts, raising the risks for a more severe dengue should it occur.
Derek Cummings, a co-author on the study and a professor of biology at the University of Florida, says that “The immunity we develop both protects us and places us at risk. It can have the potential to make things worse if it’s misused.”
It all comes down to how the dengue virus works on the immune system. When a person first gets dengue, symptoms are mild and flu-like as the human body develops antibodies. Upon a second infection, the antibodies will recognize the virus but will be unable to neutralize the threat, instead helping it cause more severe symptoms — a phenomenon called antibody-dependent enhancement.
The study found that the long-term impact of Dengvaxia, even in high-risk environments, is likely to reduce dengue cases by only 10 to 10% because of this seesaw of reducing cases in some and increasing it in others.
Ferguson and Cummings say this complication could be addressed with the development of serological tests to check if people have already been infected before administering the vaccine. Ideally, only those who have previously been infected would be vaccinated, and only one dose would be needed.
Cummings says the vaccine is “fantastic” for previous dengue patients and would be highly effective in them. This screening would mean that people who have not contracted dengue will have to go without the vaccine, but will also lower their risks for severe forms of the virus.
This study likewise found that vaccinating prior dengue patients would reduce their hospitalization risks by over 90% — evidence that Dengvaxia needs to be correctly used on the right people, at the right time.
Ferguson says,”This vaccine is not a silver bullet for dengue.” In fact, it is best for health care providers and lawmakers in high-risk countries to educate people on the benefits and risks of the vaccine, and set proper expectations so that they understand how the vaccine can help them.