Here's the problem with Dengvaxia.
Typically, a vaccine works by triggering the immune system to make antibodies against the virus. These antibodies then fight off the virus during an infection.
But dengue is a tricky virus. Dengue antibodies don't always protect a person. In fact, these antibodies can make an infection worse. The dengue virus actually uses the antibodies to help it spread through the body. So a second infection with dengue — when your blood already has antibodies in it — can actually be worse than the first; a person is at a higher risk of severe complications like plasma leakage syndrome.
In its follow-up study, Sanofi found evidence that Dengvaxia acts like the first infection for a person who has not been previously infected. The body produces antibodies against the vaccine, which have a similar potential for harm.
The increased risk seems small. The vaccine raises the risk of hospitalization after a dengue infection from about 1.1% to 1.6%, the follow-up study from Sanofi found. So out of 1 million kids in the Philippines, the vaccine would cause about 1,000 to be hospitalized over five years, Sanofi estimated. (On the other hand, the vaccine would prevent about 12,000 hospitalizations for a new dengue infection in children who have had a prior dengue infection during this same time period.)
But in the world of vaccines, that's not an acceptable risk. A risk needs to be exceedingly small to be tolerated. For example, with the measles vaccine, the risk of encephalitis is about 1 in 1 million, or 1,000 times less than the risk from a measles infection, WHO says.
WHO eventually changed its recommendation. The agency now says the vaccine is safe only for children who have had a prior dengue infection.
By the time Sanofi acknowledged this problem with the vaccine, about 800,000 Philippine children had been vaccinated. The Sanofi study estimated that more than 100,000 of them had never been infected with dengue and should not have received the shot, according to WHO's revised recommendation.