In 2013, Facebook was collaborating with researchers on a major campaign to encourage its users to get flu shots, but scrapped the plan for unknown reasons in 2014 after months of work had been done, according to a person with direct knowledge.

The idea was to pick a public health issue to determine whether targeting users on a social messaging on a site like Facebook could change behavior. It would’ve been a big first step for the social networking giant into the $3 trillion health sector.

Among the ideas floated were a Facebook page to provide information about flu vaccines, a badge for users to share that they’d been vaccinated and new forums for people to share testimonials, according to project proposals viewed by CNBC.

There were also plans for a more detailed study of individual Facebook users in Indiana.

But the company shut down the project, which was headed up by former Facebook CIO Timothy Campos, after months of time and investment. Its partners included Harvard Medical School and at least one major U.S. drug maker, the person said.

The people involved did not get a clear reason why Facebook shut down the project. One possibility is the growing controversy around vaccines, which had come up in conversations around the project. Another is a lack of comfort around nudging users towards medical procedures.

Facebook did not immediately respond to a request for comment.

Facebook has seen mixed success in using its platform for health research.

In 2013, Facebook had success with a feature for users to share their organ donor status. A study found that the simple update gave a 21-fold boost to the number of people registering themselves as an organ donor in a single day.

But two years later, the company found itself in hot water in the wake of reports that its researchers had adjusted users’ news feeds to study how emotions can be spread on social media.

Also in 2015, an editorial in Time called for Facebook and other social media sites to do more to block pages and groups associated with the anti-vaccination movement.

A year later in 2016, Facebook CEO Mark Zuckerberg sparked outrage from the so-called “anti-vaxxer” community for posting a photo of his daughter on his Facebook page with the caption: “Doctor’s visit — time for vaccines!”

Public health experts say that social media companies could do a lot more to spread positive health messages, if it takes pains to protect users’ privacy.

“Facebook is still trying to understand how social media sites can support public health,” said Marquesa Finch, managing director at public health and technology fund P2 Health Ventures, who has been closely following Facebook’s steps into the sector.

“This kind of a project would have been really valuable for them, as a new way to understand its users,” she added.

Facebook is currently focusing its health efforts on the pharmaceutical sector. In June, it hosted a summit in New York to discuss tweaks to the product that make it easier for drug makers to advertise on the platform.