LOS ANGELES (NYTIMES) - Ms Ellie Zeiler, 17, a TikTok creator with over 10 million followers, received an email in June from Village Marketing, an influencer marketing agency. It said it was reaching out on behalf of another party: the White House.

Would Ms Zeiler, a high school senior who usually posts short fashion and lifestyle videos, be willing, the agency wondered, to participate in a White House-backed campaign encouraging her audience to get vaccinated against the coronavirus?

"There is a massive need to grow awareness within the 12-18 age range," Village Marketing wrote to Ms Zeiler's business email. "We're moving fast and have only a few available slots to fill, so please let us know ASAP."

Ms Zeiler quickly agreed, joining a broad, personality-driven campaign to confront an increasingly urgent challenge in the fight against the pandemic: vaccinating the youthful masses, who have the lowest inoculation rates of any eligible age group in the United States.

Fewer than half of all Americans ages 18-39 are fully vaccinated, compared with more than two-thirds of those older than 50, according to the Centres for Disease Control and Prevention. And about 58 per cent of those ages 12-17 have yet to receive a shot at all.

To reach these young people, the White House has enlisted an eclectic army of more than 50 Twitch streamers, YouTubers, TikTokers and 18-year-old pop star Olivia Rodrigo, all of them with enormous online audiences.

State and local governments have begun similar campaigns, in some cases paying "local micro influencers" - those with 5,000 to 100,000 followers - up to US$1,000 (S$1,354) a month to promote Covid-19 vaccines to their fans.

The efforts are, in part, a counterattack against a rising tide of vaccine misinformation that has flooded the Internet, where anti-vaccine activists can be so vociferous that some young creators say they have chosen to remain silent on vaccines to avoid a politicised backlash.

"The anti-vaccine side of the Internet is still set on all this vaccine news," said Mr Samir Mezrahi, administrator of several "meme pages" such as Kale Salad, which has nearly 4 million followers on Instagram and posts viral videos and other content. "We're posting about J. Lo and Ben Affleck."

Ms Renee DiResta, a researcher who studies misinformation at the Stanford Internet Observatory, said that although influencer campaigns can be useful, they may be no match for mass, organic online movements. She noted the contrast between creators who have been asked to spread pro-vaccine messaging versus vaccine skeptics, who have made it a personal mission to question the injections.

"That's the asymmetric passion," she said. "People who believe it's going to hurt you are out there talking about it every day. They're driving hashtags and pushing content and doing everything they can do."

But even if the influencer campaigns amount to a sprinkler in a wildfire, some creators said, they felt compelled to join in.

"I didn't worry about the backlash," said Ms Christina Najjar, 30, a TikTok star known online as Tinx. "Helping spread the word about the importance of getting vaccinated was the right thing to do."

Ms Najjar said she was thrilled when the White House reached out to her through her manager in June. She soon posted a question-and-answer video about the vaccines with Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, on Instagram.

Their banter was light. Discussing what she called a "happy vaxx girl summer," Ms Najjar peppered Dr Fauci with questions: Was it safe to go out for a drink? Should we be concerned about getting pregnant after getting the vaccine? Do I look 26?

"You have an ageless look to you," he replied.

"I'll tell my Botox doctor that," she said.

Ms Najjar called the session "a great time," adding, "I think I flirted with Dr Fauci, but in a respectful way." A White House official said Dr Fauci was not available for comment.

State and local governments, too, are taking the influencers route, though on a smaller scale and sometimes with financial incentives.

In February, Colorado awarded a contract worth up to US$16.4 million to Denver-based Idea Marketing, which includes a programme to pay creators in the state US$400 to US$1,000 a month to promote the vaccines.

Ms Jessica Bralish, communications director at Colorado's public health department, said influencers were being paid because "all too often, diverse communities are asked to reach out to their communities for free. And to be equitable, we know we must compensate people for their work."

As part of the effort, influencers have shown where on their arms they were injected, using emoji and selfies to punctuate the achievement.

"I joined the Pfizer club," Ms Ashley Cummins, a fashion and style influencer in Boulder, Colorado, recently announced in a smiling selfie while holding her vaccine card. She added a mask emoji and an applause emoji.

"Woohoo! This is so exciting!" one fan commented.

Posts by creators in the campaign carry a disclosure that reads "paid partnership with Colorado Department of Public Health and Environment."

Governments' interest in the campaigns has spiked sharply in the past week, said Mr Rob Perry, CEO of Xomad, as concerns have grown about the spread of the Delta variant of the virus. He added that "when large numbers of influencers post in the same time period, vaccination rates go up."

For Ms Zeiler, the TikTok star, things moved quickly after she signed on to the White House-backed vaccination campaign. In June, she held an online conversation with Dr Fauci, using the time to squash the false rumour that vaccines cause infertility. It was a conspiracy theory that she had heard from friends and that she had seen videos of on her TikTok "For You" page.

"When I saw that I was like, OK, I need to ask him about it," she said. "It was kind of sad to see him be like, no, that's not true."

Ms Zeiler has since used her footage with Dr Fauci for other platforms, including Instagram, and created original content for YouTube promoting the vaccines. In a 47-second video, she spoke directly into the camera, ticking through the reasons she had gotten vaccinated and why others should too.

"Reason one," she declared, was "you can go wherever you want." Ms Zeiler said in an interview that her work was not done. "I know I won't stop until all my followers are safe and vaccinated," she said.