In a welcome shift, President Trump on Friday unambiguously urged people to get their measles shots. Now, he should drive the message home and take on the fake news of the anti-vaccination community that is truly the enemy of the people.

"They have to get the shot," Trump told White House reporters on Friday morning when asked about the measles outbreak. "Vaccinations are so important. This is really going around now. They have to get their shots."

That was a sea change from his previous irresponsible flirtation with the lie that vaccines are linked to autism, one that he promoted on Twitter, and also on the debate stage, while arguing he believed vaccines should be given but more spread out.

[Related: Anti-vax organization sues New York over measles vaccine mandate]

His unqualified declaration on measles shots is important at a time when we're facing a completely unnecessary outbreak of a potentially deadly virus that science has given us the tools to safely prevent.

But a one-off statement is not going to fight all the fearmongering that has been put out about vaccination, which has helped drive measles cases to a 25-year high.

When Trump wants to, he can get out a message like nobody else. Think of "Low Energy Jeb" or "Lyin' Ted" or "no collusion" — ideas he promoted over and over again, on Twitter, at rallies, and in interviews, so that everybody was aware of them.

That's what needs to happen when it comes to vaccination.

The creation of the vaccine was one of the true miracles of modern medicine. Oftentimes when we discuss medical innovations, it's in terms of treatments. In this case, there is vast empirical evidence to demonstrate both the safety of vaccines and their effectiveness.

"In the decade before 1963 when a vaccine became available, nearly all children got measles by the time they were 15 years of age," according to the Centers for Disease Control and Prevention. "It is estimated 3 to 4 million people in the United States were infected each year. Also each year, among reported cases, an estimated 400 to 500 people died, 48,000 were hospitalized, and 1,000 suffered encephalitis (swelling of the brain) from measles."

After the introduction of the vaccine, the number of cases decreased dramatically, and the CDC eventually declared the disease to be eradicated in 2000. In 2004, there were only 37 cases in the entire U.S., and deaths have been rare.

But more recently, as a result of misinformation being spread, too many people have avoided vaccinating their kids, putting not only themselves at risk, but also others.

The result has been more outbreaks. In 2014, the number of measles cases skyrocketed to 667, and after retreating for several years, it's back with a vengeance this year. April is not even over yet, and we're already at 695 cases, marking the highest number since 1994, when there were 958. If the spread keeps up at the current pace, it would exceed that in a few months and could approach the level of 1992 by the end of the year, when there were 2,200 cases.

This is not, to be clear, a libertarian matter in which people choosing to go unvaccinated or to avoid vaccinating their kids are only putting themselves at risk. The measles vaccine, while largely effective, is not 100% effective: It is 93% effective for those receiving one dose, and 97% effective for those receiving two doses. Also, babies cannot be given a full measles shot until they are one year old, and some people may have legitimate medical reasons for being unable to get them.

So, what scientists say is that it's important to establish herd immunity — that is, if around 92% to 95% of people receive the vaccine, then enough of the population will be immune so that it will be difficult to spread, and we will be able to protect those who are most vulnerable, such as newborns too young to get vaccinated.

The genesis of the discredited claim linking autism and vaccines was a fraud perpetrated by a British doctor named Andrew Wakefield, who received hundreds of thousands of dollars from trial lawyers hoping to sue manufacturers by proving vaccines were unsafe. Wakefield and his team deliberately fabricated information. The study has since been retracted, and Wakefield has been stripped of his license to practice medicine.

However, the damage was done as celebrities and prominent publications gave oxygen to the lie — most notably, Robert F. Kennedy Jr. advanced the idea in a conspiratorial article in Rolling Stone that took years to retract, and Jenny McCarthy.

The public policy tools to encourage vaccination have their limits. Religious and philosophical exemptions have long been used as a loophole to allow parents to send unvaccinated kids to school, and even states that have gone further encounter problems. When California banned nonmedical exemptions in schools, for instance, medical exemptions soared 250%, with irresponsible doctors willing to write letters to schools for a fee. "Anti-vaxxer" parents could also choose to home-school so their children would remain unvaccinated.

But Trump could help tremendously by using his platform to amplify the message that people need to be vaccinated, and emphasize that those who choose not to are putting others at risk. To be clear, it's very unlikely that a sustained campaign by Trump to argue for vaccination and to debunk the lies would convince the hardcore "anti-vaxxers." Conspiracy theories are self-sustaining, and any effort to debunk them often gets adopted into the conspiracy theory, demonstrating a cover-up and an effort to silence the real truth, and only hardening their position. But aggressively driving home the message to get vaccinated could help with people who are on the fence, or if nothing else, it would serve as a reminder to those who may not have a strong problem with vaccinations but have just been less diligent in keeping up to date.

The more people that can get vaccinated, the more likely it is that we'll be able to establish and sustain herd immunity, allowing the rest of us to protect against the hopefully small group of committed "anti-vaxxers" who will likely never go away.