Measles cases are continuing to spread throughout the U.S. with outbreaks in at least six states.

Public health experts have previously said lagging vaccination rates are to blame for the rise in cases, at least partly due to vaccine hesitancy and vaccine fatigue left over from the COVID-19 pandemic.

However, even a small uptick in MMR (measles, mumps and rubella) vaccination could prevent millions of infections, according to new research.

The study, published in JAMA last week, used a model to simulate the spread of vaccine-preventable infectious diseases across the U.S., evaluating different scenarios with different vaccination rates over a 25-year period.

The Centers for Disease Control and Prevention (CDC) currently recommends that people receive two vaccine doses — the first at 12 to 15 months and the second between 4 and 6 years old. One dose is 93% effective and two doses are 97% effective against measles, the CDC says. Most vaccinated adults don’t need an additional dose.

At current vaccination rates, the model predicts measles could once again become endemic — constantly present — in the U.S. with an estimated 851,300 cases over 25 years. If vaccination rates decline by just 10%, the model estimates 11.1 million cases of measles over 25 years.

CDC data show vaccination rates have been lagging in recent years. During the 2023 to 2024 school year, 92.7% of kindergartners received the MMR vaccine, according to data. This is lower than the 93.1% seen the previous school year and the 95.2% seen in the 2019 to 2020 school year, prior to the COVID-19 pandemic.

“That result — that we’re already at this tipping point for measles in the U.S. — was a really striking finding that somewhat surprised us as well,” Dr. Nathan Lo, study co-author and an infectious diseases physician at Stanford Medicine, told ABC News.

“We continue to see measles outbreaks, but, by doing the study … you start to crystallize that result that even under the current levels of vaccine decline, there is a very feasible scenario where, over time, our country has measles return where it’s a common household disease again.”

Dr. William Schaffner, a professor of preventive medicine at Vanderbilt University Medical Center in Nashville, who was not involved in the study, said the findings are a “well-thought-out worst-case scenario.”

“I’m not entirely sure that would actually happen because … the general level of vaccination remains pretty high, but there are pockets of unvaccinated children, and what you’re seeing now is the fact that this virus has been reintroduced several times from other parts of the world,” he told ABC News. “I think it’s a bit of a worst-case scenario but, as a worst-case scenario, it’s scary and, in that sense, reasonable.”

However, the model estimated that just a 5% increase in vaccination would lead to only 5,800 cases over the same 25-year period.

This is because when more than 95% of people in a community are vaccinated, most are protected from measles through community immunity, also known as herd immunity, according to the CDC.

“My hope is that this study can provide that data to parents to say, ‘This is the benefit of continuing to vaccinate your child. This is the kind of alternate reality that we’re preventing. And yes, the risks aren’t here right now, but perhaps not too far off,'” Lo said.

Schaffner said there are pockets of the U.S. where MMR vaccination rates are 80% or less and vaccination rates would need to increase substantially in those areas to reduce the number of cases long-term.

However, “if we increase the level of vaccination by a small amount that would reduce the risk of other small outbreaks here and there,” Schaffner said.

The study also found that a 50% drop in vaccinations would lead to an estimated 51.2 million cases over 25 years, but Lo thinks that would only occur if there were a large-scale policy change by the Department of Health and Human Services or the CDC to reduce or remove the childhood vaccine recommendation.

As of Friday, the CDC has confirmed nearly 900 measles cases in at least 29 states. That number is likely an undercount due to delays in states reporting cases to the federal health agency.

In western Texas, an outbreak has been spreading with 663 reported cases of measles, according to new data published Tuesday by the state Department of State Health Services. At least 87 people have been hospitalized over the course of the outbreak.

Measles was declared eliminated from the U.S. in 2000 due to the highly effective vaccination program, according to the CDC. However, an outbreak lasting 12 months or more would threaten to end measles elimination status in the U.S. The Texas outbreak began in January of this year.

To drive vaccination rates up, Lo and Schaffner recommend that parents who haven’t vaccinated their children yet speak to their pediatrician to address their concerns.

“We have to provide recommendations and let people know the facts that these vaccines are very effective and that they are safe,” Schaffner said. “But we have to do more to try to reach out and provide reassurance, and this has to be done very much on a local basis.”

For example, when it comes to the community affected in Texas, Schaffner said they need local influencers they trust to turn to.

“Speak with your local physician, health care provider, the person who cares for your children,” Schaffner said. “Have a conversation with them, and that’s our best hope for regaining some of this trust which public health had and which has slipped away.”