On May 6, 2016, Promoli put her toddlers Jude and his twin brother Thomas, down for an afternoon nap in their home. Jude had a low-grade fever, but he was laughing and singing when he went down for his nap.
When his mother went to check on him two hours later, he was dead. Promoli said the next few weeks were "a living hell."
"Having to go in and plan a funeral and find the ability somehow to even take steps to walk into a funeral home, to make plans and decide whether to bury or cremate your child -- it was just all so horrifying," she said.
When an autopsy came back showing Jude had died of the flu, Promoli started her flu prevention campaign.
That's when the online attacks began.
Some anti-vaxers told her she'd murdered Jude and made up a story about the flu to cover up her crime. Others said vaccines had killed her son. Some called her the c-word.
The worst ones -- the ones that would sometimes make her cry -- were the posts that said she was advocating for flu shots so that other children would die from the shots and their parents would be miserable like she was.
"The first time it made me feel really sick because I couldn't fathom how anybody could even come up with such a terrible claim," Promoli said. "It caught me off guard in its cruelty. What kind of a person does this?"
Twisted logic that relies on scientific lies doesn't bother Promoli so much anymore. She's continued with her flu shot campaign, persuading Canadian Prime Minister Justin Trudeau to publicize his flu shot.
"I've had to grow some very thick skin," she said.
She said no matter how many nasty messages she's received -- and she says she's received hundreds -- she'll continue her campaign.
"The work that we're doing might mean that somebody else doesn't have to go plan a funeral for their toddler, and that is everything," she said.
Other mothers have also persevered despite attacks from anti-vaxers.
Serese Marotta lost her 5-year-old son, Joseph, to the flu in 2009, and is now chief operating officer of Families Fighting Flu, a group that encourages flu awareness and prevention, including vaccination.
In 2017, she posted a video on the eighth anniversary of her son's death to reinforce the importance of getting the flu vaccine.
"SLUT," one person commented. "PHARMA WHORE."
"May you rot in hell for all the damages you do!" a Facebook user wrote on another one of her posts.
She says a Facebook user in Australia sent her a death threat.
"She called me a lot of names I won't repeat and used the go-to conspiracy theories about government and big pharma, and I responded, 'I lost a child,' and questioned where she was coming from, and she continued to attack me," said Marotta, who lives in Syracuse, New York.
Catherine and Greg Hughes, an Australian couple who lost their 1-month old son, Riley, to whooping cough, have also received online abuse. Too young to be vaccinated, Riley relied on herd immunity -- the vaccinations of others -- to protect him.
But herd immunity didn't protect him, since the area where the Hughes family lived in Perth has some of the lowest vaccination rates in Australia.
"Riley's death was a very inconvenient truth for anti-vaccine activists," Catherine said. "The nasty messages started 24 hours after he died. They called us baby killers and said we would have the blood of other babies on our hands. We've been told to kill ourselves."
The couple started a vaccination campaign, Light for Riley.
Catherine said they still receive vile comments years after Riley's death.
"[F**k] you, Hughes family," one Facebook user wrote on the Light for Riley page.
"What a [f**king] evil whore you really are," another user wrote to them in a private Facebook message.
Another Facebook user was more succinct.
"Please die," the user wrote in a private message.
"A lot of them come from the position that they have children that were vaccine-injured," Catherine said. "But a fair chunk of them are just haters."