Arlyn Calos is no longer a mother.

The 23-year-old lives in one of Manila’s slums. A few years ago, health workers went door-to-door, asking parents to get their children vaccinated against measles.

Calos refused.

Then, in 2019, as a measles epidemic gripped the Philippines, her three-year-old daughter Jennlyn developed a high fever and a rash. Calos took her to hospital, but she died just a few days later.

A week later her remaining child, eight-month-old John Paul, also fell ill from the disease and passed away.

As measles vaccination rates fell in the Philippines, cases of the deadly disease were rising.

Why were Calos and other parents refusing to get their children vaccinated?

“A lot of people said that it was dangerous... So I got scared,” Calos says.

At the root of this fear is a vaccine scandal that has plagued the Philippines for years.

It involved Dengvaxia - the world’s first vaccine for the mosquito-borne disease, dengue.

Developed by French pharmaceutical company, Sanofi Pasteur, clinical trials showed it reduced cases of severe dengue by about 93%.

It was rolled out in the Philippines in 2016, to much fanfare, part of a global push to reduce the incidence of dengue.

The hype around Dengvaxia was short-lived in the Philippines. In November 2017, Sanofi updated its advice on the vaccine, saying it could cause more severe dengue in children who hadn’t had the disease before.

But none of the more than 30,000 children in the clinical trial died of dengue, and all the cases of so-called “severe dengue” were relatively mild.

Sanofi’s definition of “severe dengue” also contained much milder symptoms than other international definitions.

Elsewhere, Dengvaxia’s warning labels were updated and its distribution continued.

But the damage to its reputation had been done.

The Philippines’ government pulled the plug on the vaccine.

Health officials involved in the vaccine’s roll-out were questioned.

Misinformation about Dengvaxia spread online and parents whose children had been vaccinated voiced their anger and fear.

Leading these angry parents was the Philippines’ Public Attorney’s Office, a legal service for poor Filipinos.

It investigated the deaths of 160 children, who they say died because of Dengvaxia.

No scientific link has ever been made between the vaccine and the deaths.

Since confidence in vaccines has plummeted in the Philippines, many preventable childhood diseases have returned.

Watching this with dismay were scientists and medical experts, such as clinical epidemiologist Professor Charles Yu.

“There was an explosion of hysteria, fuelled by the fact that no one was actually listening to the explanation of what happened,” he says.

Professor Yu is currently supervising several different COVID-19 vaccine trials in the Philippines.

Because of Dengvaxia, the Philippines has become a leading example of what not to do when introducing a new vaccine.

However experts like Dr Yu haven’t lost hope.

“I think that we can always rise from the tragedy,” he says. “Maybe we could be a good example of standing up and righting the wrongs.”

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