More than a week has passed since a new measles case was reported in connection with the Philadelphia outbreak. City health officials said Jan. 23 that they will continue to monitor the situation, but wind down updates further if no additional cases are confirmed for two weeks. While cases may be winding down there, measles continues to spread in pockets across the U.S.
Aside from the outbreak of nine cases in the Pennsylvania outbreak, exposure warnings and cases have also appeared in Missouri, Colorado, and Georgia in recent months.
This is in sharp contrast to the public health landscape of 2000, when measles was officially declared "eliminated" after more than 12 consecutive months without a single case in the U.S.
Now, measles cases and deaths globally are on the rise, the CDC and World Health Organization warned clinicians in November about the trend, noting that falling vaccination rates are part of the rise.
Global travel is one part of the increase. While the U.S. has had a better handle on controlling the infection due in part to vaccination efforts, that has not been the case in other parts of the world and exposure to the infection can lead to individuals returning to the U.S. with symptoms and infection.
"If there are pockets of unvaccinated individuals that are congregating closely together and that disease gets introduced into that population, you can have large clusters of cases," Thomas Murray, MD, PhD, a professor of pediatrics at the Yale School of Medicine told CNN.
But falling vaccination rates in U.S. children are contributing to the spread of cluster cases like Philadelphia experienced. It's also adding to a decline in herd immunity, which for measles is important to be around at least 95%. Anytime it falls below that is "a wake-up call, because the real number in many communities is probably far below 93%," Peter Hotez, MD, PhD, the co-director of the Texas Children's Hospital Center for Vaccine Development in Houston and a professor of pediatrics at Baylor College of Medicine, told CBS News.
He added that about 20% of measles cases require hospitalization, and falling herd immunity due to low vaccination rates is an indirect cause for this to increase.
"My concern is that we're still going to see additional measles cases …" Dr. Hotez told CBS. "So if this continues, we're going to start seeing hospitalized kids with measles."
School-age children in the U.S. had the highest rate of vaccine exemptions this year than in years prior, according to the CDC, so cases of measles reported among children in various areas of the country are likely to continue, he said.