The virus is also being traced back to a greater number of places than in previous years, according to the CDC.
As of Thursday, Jan. 29, 84 measles cases had been reported in 14 states. The median number of measles cases from 2001 to 2010 was only 60 cases, said Anne Schuchat, assistant surgeon general for the United States Public Health Service and the CDC's national Center for Immunization and Respiratory Diseases.
"This worries me," Schuchat said, "and I want to do everything possible to prevent measles from getting a foothold in the U.S. and becoming endemic."
The majority of the reported measles cases for the current outbreak stem from an outbreak at the Disneyland resorts in California. Since Dec. 28, 2014, an estimated 67 measles cases have been linked to the resort. Schuchat said the first person to spread measles at Disneyland likely traveled from another country and brought the virus back to the U.S. The virus has been spread to six additional states since the first reported case at Disneyland.
"Measles is extremely contagious," Schuchat said. "If one person has it, 90 percent of the [unvaccinated] people close to the person who has it will also become infected."
"This outbreak reminds us that measles exposure can happen in many settings," she added.
Measles, an airborne virus similar to influenza, can be spread when an infected person coughs, sneezes or breathes. Symptoms include a fever, skin rash, dry cough, runny nose, sore throat, inflamed eyes and tiny white spots inside the mouth.
Schuchat said there are 14 different countries linked to the current measles outbreak in the United States—more sources than any previous outbreak at this stage. The countries that this outbreak can be traced back to include Indonesia, India and Dubai. The majority of the measles cases from 2014 — which had a reported 610 cases— stems from the Philippines, where there were a reported 50,000 cases in the country, but none have been traced back there for the current outbreak.
In the U.S., Schuchat noted that 1 in 12 children are not receiving their measles vaccines on time, making them vulnerable to contracting the disease and spreading it to other unvaccinated people. Ninety-five percent of children 19 to 35 months old are recommended to receive the vaccine, while the remaining children may not be able to receive the series due to a pre-existing health condition.
The first dose of the MMR vaccine series, which provides immunity against measles, mumps and rubella, is recommended for children at 12 months of age. The second dose is recommended between ages 4 and 6, but this dose can be administered sooner. Children who are traveling internationally may receive their first MMR dose when they are six months old.
"It's frustrating that some people have opted out of vaccination," Schuchat said. "We have a generation that hasn't seen these diseases. Whether it's clinicians who haven't taken care of patients before, or parents who think this disease doesn't exist anymore."
According to the CDC, 79 percent of the people who opted out of the measles vaccine in 2013 did so due to a lack of belief in vaccinations. The CDC has not yet compiled data to determine that value for 2014.
Some people may opt not to receive vaccines because of fear they may lead to learning disabilities or autism, but those suspicions "just have not born out," Schuchat said.
She said that less than 1 percent of toddlers in the U.S. receive no vaccine at all, but the CDC does not have data for the whole nation on specific exemptions and their reasoning for 2014.
However, Schuchat said, "We know the last few years that the measles cases we've seen have generally been in people who have been unvaccinated— and many of them due to misbelief in vaccinations."
She also noted that more adults have reported contracting measles this year than in the typical outbreak, but she did not note how many.
"It's not just young children who need to be up to date on their vaccines," Schuchat said. "If adults aren't sure if they've had the vaccine before, we urge you to contact your nurse or doctor. There's no harm in getting another MMR vaccine if you've already been vaccinated."
Schuchat said that a waning of the efficacy of the MMR vaccine has not yet been examined, but that when large populations are vaccinated, the greater the chance that there will be some two-dose failures. The CDC estimates that the two-dose MMR series is 97 percent effective.
"We like to keep an open mind and think these things through and fully investigate, but what we're seeing so far is a highly effective vaccine," she said.
Schuchat added that measles is still common around the world, with 20 million new cases reported globally each year. In 2013 alone, 145,700 people died of the virus worldwide. For every 1,000 children that get measles, one to three die despite treatment. Between 2001-2013 in the U.S., 28 percent of children with measles had to be treated in the hospital.
Schuchat said that while measles is different from Ebola because no final vaccine exists to treat the latter virus, both outbreaks serve as a reminder of the interconnectedness of countries.
Schuchat also addressed concerns over this weekend's Super Bowl in Arizona, one of the six states with measles cases that can be traced back to the Disneyland measles outbreak.
"I wouldn't expect the Super Bowl to be a place where many unvaccinated people are congregating," she said. "No special precautions are being taken. If people are having fever or rash, they need to let their doctor or nurse know about that, and clinicians caring for people with rash or fever need to ‘think measles.'"
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