(CNN) - There's a "reasonable chance" the United States will lose its measles elimination status in October because of ongoing measles outbreaks in New York, according to Dr. Nancy Messonnier, director of the US Centers for Disease Control and Prevention's National Center for Immunization and Respiratory Diseases.
"It certainly is incredibly frustrating and upsetting to the public health community that we may lose measles elimination status, because we do have a safe and effective vaccine," Messonnier said.
When the World Health Organization declared in 2000 that the United States had eliminated measles, it was hailed as one of the biggest public health achievements in the nation's history. Losing that elimination status would be a black eye to the United States, public health experts said.
"We're embarrassed. We're chagrined," said Dr. William Schaffner, a longtime adviser to the CDC on vaccine issues.
WHO removes a county's elimination status when measles has been spreading continuously for one year. A measles outbreak in New York City started on September 30, 2018, and has caused more than 600 confirmed cases of measles. An outbreak in nearby Rockland County, New York, started the next day and has caused more than 300 cases.
Those two outbreaks have largely been among children in the ultra-Orthodox Jewish community whose parents have refused to vaccinate them.
Twenty-nine other states have had measles outbreaks in the past 12 months, but those were much more short-lived than the ones in New York.
CDC plans on releasing a detailed statement next week about the country's measles elimination status, according to Messonnier.
Schaffner, an infectious disease expert at Vanderbilt University, said he thinks it's highly unlikely the measles outbreaks will be over before September 30.
This week, the CDC announced 12 new cases of measles, most of them in New York. Schaffner said things will likely only get worse when children there go back to school early next month and begin congregating again in close quarters.
It also could have ramifications worldwide. Spiegel, a former senior official at the UN Agency for Refugees, said it could undermine longstanding US efforts to convince other countries to double down on vaccinating their citizens.
"If we are not able to take care of our own backyard, how can we tell others what to do?" he said.
When vaccine rates plummet, death and disability from measles increases. In 2017, there were 110,000 measles deaths globally, mostly among children under age 5, according to WHO. Measles can also cause blindness and encephalitis, or swelling of the brain.
It was just three years ago that WHO declared the Americas to be free of measles.
"This is a historic day for our region and indeed the world," Dr. Carissa Etienne, then director of the Pan American Health Organization, a part WHO, said at the time.
No one foresaw what would follow.
For those countries, economics and political upheaval were at play.
For the United States, it was Facebook and Twitter.
As anti-vaccine sentiment has grown on social media -- some of it propelled by Russian bots and trolls -- more and more parents have opted to not immunize their children.
These parents tend to live in clusters, such as the ultra-Orthodox Jewish community in New York, which helps encourage measles to spread.
While social media is a part of parents' everyday lives, the ravages of vaccine-preventable diseases are in the distant past.
Schaffner said when he's spoken to parent groups, there's been considerable ignorance about even basic facts about infectious diseases.
"This is a true story. At one of these meetings I was talking about polio, and a mother asked me -- and this is a complete quote -- 'Why are you suddenly talking about shirts?' It took me a minute, but I realized she thought I was talking about polo shirts," Schaffner said. "This was a college educated woman out in the business world and she hadn't come across the concept of polio. She may be a bit of an outlier -- but maybe not. I think she actually illuminates the problem."
Looking back, public health experts see lost opportunities for combating that problem.
While anti-vaxers were busy spreading their false propaganda, "the good guys," as Schaffner calls them, failed to effectively communicate how dangerous diseases like measles can be.
Those "good guys," he said, include groups such as the CDC as well as doctors' groups.
"I think this was not our finest hour," Schaffner said.
Dr. William Moss, an infectious disease pediatrician at Johns Hopkins, agrees.
While anti-vaxers circulated videos on social media of mothers citing false claims about vaccines, public health groups failed to tell the stories of the ravages of diseases like measles.
"We didn't do enough to get a mother on camera to say 'my child died of measles' or had brain damage from measles. I think we could have done better with our public health messaging," said Moss, executive director of the International Vaccine Access Center at Johns Hopkins.
Messonnier, the CDC doctor, said she and her colleagues have learned a lot of lessons about social media.
"I do think it caught us all a little flat-footed -- how quickly the myths and misinformation spread," she said. "Of course, I wish I had 20/20 hindsight and had figured this out a couple of years ago."
CNN's John Bonifield contributed to this story.