When Pfizer was gearing up to launch Prevnar 13 in adults age 50 and over, it faced a few unique challenges.

First off, at the time, it wasn’t common for consumers to go into a doctor’s office and ask for a vaccine by name—let alone a brand name, Julie Post, the Prevnar 13 adult consumer team lead at Pfizer, told an audience at last week’s DTC National Conference in Boston.

Second, Merck’s Pneumovax had been on the market for years—and had already been given to just about one of every two adults eligible. Pfizer would have to persuade them they needed Prevnar, too.

And third? The adults Pfizer was targeting—baby boomers—felt "invincible," she said. "To them, age is a mindset, not a number. I think many of the adults in our cohort felt like they’re doing everything possible to stay healthy ... they’re not going to get pneumonia ... that’s for older adults."

So instead of trying to convince them otherwise, the pharma giant "leaned into that" mindset. "We knew in order to break through to this audience, we needed to empower them and not scare them," Christa Albeck, senior product manager at Pfizer Vaccines, explained.

The result was "One Step," a spot that featured healthy adults getting one Prevnar shot between doing a single yoga pose and eating a single blueberry. It emphasized "the notion of one and done" to people who were "otherwise primarily healthy," Post said. In terms of staying healthy, for Pfizer’s target consumers, "Prevnar 13 was an option that was relatively simple compared with the other things they were doing," Albeck added.

While the message proved effective, its success only made Pfizer’s job more difficult the second time around—and its top-line performance harder to replicate.

"The remaining pool was harder to move," Albeck said, noting that "their reasons for not getting vaccinated might be more difficult" for the company’s marketers to predict. For its second DTC effort, Pfizer went with "more of a message about protection," likening Prevnar to the safety gear active adults might bring along for activities such as ziplining, rollerblading and kayaking.

Within eight months of launch, Pfizer had improved its Prevnar awareness by over 1,500%, with more than half of the target customer base aware of the vaccine. That base was looking for more information, too: The company recorded an increase in onsite engagement of 8,000% over that same timeframe. Plus, 44% of targeted consumers were talking to their physicians about getting vaccinated specifically with Prevnar.

Once again, though, Pfizer is up against an even more difficult task as it tries to convert the remaining consumers in its target group. "We know that the remaining consumer population is really unique—they have unique challenges and unique barriers," Albeck said, acknowledging that "we have a lot of work ahead of us."

Going forward, it needs a campaign that’ll "resonate with this hard-to-reach, hard-to-activate population," she continued. But "we’re up for the task."

In the meantime, though, sales of the blockbuster shot are suffering against its previous numbers. In last year's fourth quarter, Prevnar fell significantly short of Wall Street estimates; its $1.41 billion haul came in 25% below 2015's $1.86 billion tally. In 2015, the vaccine enjoyed a "high initial capture rate of the eligible population," Pfizer said in a statement in January, leaving less "catch-up opportunity" for 2016 and beyond.