The Centers for Disease Control and Prevention is urging health care providers to be “on alert” for patients with symptoms of measles — a virus declared eliminated in the U.S. in 2000 — after nearly two dozen cases have been reported across the country in the past month.

Between Dec. 1 and Jan. 23, the CDC has been notified of 23 measles cases, including seven “direct importations” by international travelers and two outbreaks with more than five cases each, according to a letter the federal agency sent to clinicians last week.

Most of the cases were among children and teenagers who had not been vaccinated against the virus, and nine of them were reported by the health department in Philadelphia — a city that is a two-hour drive from Baltimore. Additionally, the Virginia Department of Health notified people earlier this month that they may have been exposed to measles if they were at Dulles International Airport on Jan. 3 or at Ronald Reagan Washington National Airport on Jan. 4.

The outbreak may be scary, said Dr. Theresa Nguyen — chair of pediatrics at the Greater Baltimore Medical Center in Towson — but it’s important for parents not to panic.

Instead, she said, “the message is, ‘Get your children vaccinated.’”

The measles vaccine used today, MMR — which also protects people against mumps and rubella — was approved by the Food and Drug Administration more than 50 years ago. But the nationwide MMR vaccination rate has fallen by two percentage points over the past two school years, after sitting firmly at 95% for 10 years. In Maryland, the child vaccine exemption rate increased by 0.4 percentage points from the 2021-22 school year to the 2022-23 school year.

Measles, a very contagious viral illness characterized by a high fever, rash, cough, runny nose and pinkeye, hasn’t been endemic in the U.S. in more than 20 years. But in 1998, the progress made in convincing parents to vaccinate their children was jeopardized by the publication of a flimsy study that suggested getting the MMR vaccine may predispose a child to developing autism.

The journal that published the study has since retracted it, and Andrew Wakefield — the paper’s lead author — is now barred from practicing as a physician in the United Kingdom. But the vaccine hesitancy seeded by his research persists today, Nguyen said. And since the coronavirus pandemic, vaccine hesitancy has only grown, she added.

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Nguyen recently sent a letter of her own to GBMC’s pediatricians, asking them to be aware of the increase in cases and to add measles to the list of viruses that may explain a child’s fever or cough.

It also needs to be easy for families to vaccinate their children, said Dr. Esther Liu, chair of the department of pediatrics at the University of Maryland Baltimore Washington Medical Center in Glen Burnie.

The hospital’s community outreach program is working with a local elementary school to help parents ensure that their children are caught up on their vaccines before an outbreak begins, Liu said.

The CDC recommends that all children get two doses of MMR vaccine. Children should receive the first dose when they’re 12 to 15 months old and the second dose when they’re 4 to 6 years old.

However, Liu said, it’s important for parents to know that their child can be vaccinated when they’re as early as 6 months old if they are planning to travel to a country where measles is common, their child has been exposed to the virus, or there is an outbreak nearby.

For parents who are nervous about vaccines, Liu recommended that they check out healthychildren.org — a parenting website run by the American Academy of Pediatrics. The website has easy-to-understand and well-researched articles about a number of topics, including the risks and benefits of vaccines.

Despite the recent measles outbreaks, the virus is much less common than it once was. Between 1989 and 1991, outbreaks — many of them clustered in areas where immunization rates were low — sickened more than 55,000 Americans and killed 123.

For comparison, between 2022 and 2023, 179 cases were reported to the CDC. Nguyen may have seen people with measles when she was a young child living in Vietnam, she said, but she certainly hasn’t seen any patients with the virus since she became a physician.

But the success of vaccines — considered to be one of the greatest public health advancements in history — can have some ironic consequences, Liu said.

“As parents, we want to protect our kids. That’s a very natural response,” she said. “Because we don’t see the devastation of these actual illnesses because we have better control in this country, I think it’s easier to fear the vaccine more than the illnesses.”

“I really tell parents,” she continued, “‘Make sure you understand what it is that we’re trying to protect your child from.’”