Dr. William Schaffner, an infectious disease expert at Vanderbilt University, said, “It’s hard to think of another circumstance when a major public health program was introduced with this much controversy.” The problems with the vaccine rollout have “cast a pall” on other efforts to develop dengue vaccines, he said, and public health experts are worried that the distrust could spill over to other vaccination programs.
“Vaccines have plenty of controversy,” Dr. Schaffner said. “We don’t need to make more.”
Sanofi has said that the risk to people who get the vaccine is still extremely small. Moreover, in those who have previously been infected, the vaccine works well: In people older than 9, it is about 66 percent effective.
But that has not allayed fears in the Philippines, where Sanofi has said 10 percent of the children who were vaccinated probably had no prior exposure to the virus.
Dr. Su-Peing Ng, the head of medical affairs of Sanofi Pasteur, the company’s vaccine division, said in an interview that “there is no cause for alarm,” adding, “those vaccinated in high-exposure settings are much more protected than those who were not.”
The World Health Organization, which published guidelines on how to use the vaccine in 2016, has distanced itself.
“We did not give a blanket recommendation that the vaccine should be introduced,” said WHO’s representative in the Philippines, Dr. Gundo Weiler. “This is a decision for governments and governments alone.”
Dengvaxia was first approved in 2015, and Brazil and the Philippines initiated government-sponsored vaccination campaigns. Brazil’s government said it would continue its program but that it would follow Sanofi’s recommendation to avoid vaccinating people who have never had dengue.
The company has told investors that it expects to lose 100 million euros, or about $117 million, as a result of diminished sales. Sanofi is one of the world’s biggest pharmaceutical companies, reporting sales of nearly 34 billion euros in 2016, or nearly $40 billion.
Two other dengue vaccines are in late-stage development and could threaten future sales of Dengvaxia if they show better results.
“It’s a huge disappointment and it’s a big lesson in humility,” said Michel De Wilde, the former senior vice president of research and development at Sanofi Pasteur, who oversaw the vaccine’s development until he left the company in 2013.
The illness, also called breakbone fever, can be excruciating, with high fevers, headaches, muscle and joint pains and lingering weakness. Sometimes the disease causes hemorrhage or shock, which can be fatal.
Some deadly cases have been linked to the fact that there are actually four different types of dengue virus. Research has found that severe illness can occur in people who had one type and later become infected by another. The body’s immune response to the first virus is thought to make the second illness worse, a discovery credited to Dr. Scott B. Halstead, who has been studying dengue since the 1960s.
In February 2016, as the Philippine program was getting underway, Dr. Halstead warned in a scientific article that the vaccine could put people at risk if they had not previously had dengue. He said the issue was well known. “We’ve been talking about this for years,” he said recently. “It isn’t any hidden secret.”
He and others pointed to a trial of Dengvaxia in children, published in 2015, that seemed to confirm fears that the vaccine could be harmful to those with no previous exposure. The potential danger is not from the vaccine itself, but from the immune response to it. The researchers found that in children under 9 years old, those who were given Dengvaxia and later caught dengue were more likely to be hospitalized for severe illness than those who had not been vaccinated.
The finding originally led Sanofi to restrict Dengvaxia to children 9 and older, although the company did not concede that the higher hospitalization rate among younger children was due to their lack of prior dengue exposure. A panel set up by the World Health Organization recommended that the vaccine be used only in places where the incidence of dengue was high.
Brazil decided to limit its government program to people who are over age 15.
Even as Sanofi pushed back against researchers’ warnings about the vaccine’s potential hazards, it ordered another analysis of the data.
Dr. Ng, the Sanofi official, said that before the additional analysis, “there wasn’t a robust way in which we could answer that question. It’s only this new analysis that has given us this insight.”
The new analysis found that lack of past infection was tied to an increased risk of severe dengue. But the risk was small — two extra cases per 1,000 previously uninfected people vaccinated, over five years of follow-up. And there were no deaths reported.
But how to tell if someone has had dengue? Doctors and patients cannot always tell because symptoms can be vague.
No rapid test exists. Current tests take a few days to produce results, and may have trouble distinguishing dengue from Zika, a related virus. Trying to use them would further complicate a vaccination process that is already cumbersome, because it requires three visits. Sanofi has said it will invest in efforts to develop a better test.
At a Philippine Senate hearing on Dec. 11, Thomas Triomphe, a regional Sanofi official, emphasized the vaccine’s benefits and said that to permanently remove it would leave 90 percent of the population “at the mercy of an epidemic which has been found to be preventable.”
But parents feel they have traded one risk for another that might be worse. Maria Brenal Bernal said her daughter, Reyna Rose, had been sick on and off since receiving her first shot of Dengvaxia in August.