Measles made a comeback in 2019.
The Centers for Disease Control and Prevention reported that, as of Dec. 5, 2019, 1,276 individual cases of measles of measles were confirmed in 31 states, the largest number since 1992. This number is a major uptick in cases, compared with previous years since 2000 when the CDC declared measles eliminated from the United States. No deaths have been reported for 2019.
Three-quarters of these cases in 2019 were linked to recent outbreaks in New York and occurred in primarily in underimmunized, close-knit communities and in patients with links to international travel. A total of 124 of the people who got measles this year were hospitalized, and 61 reported having complications, including pneumonia and encephalitis. The overall median patient age was 6 years (31% aged 1-4 years, 27% aged 5-17 years, and 29% aged at least 18 years).
The good news is that most of these cases occurred in unvaccinated patients. The national vaccination rate for the almost 4 million kindergartners reported as enrolled in 2018-2019 was 94.7% for two doses of the MMR vaccine, falling just short of the CDC recommended 95% vaccination rate threshold. The CDC reported an approximate 2.5% rate of vaccination exemptions among school-age children.
The bad news is that, despite the high rate of MMR vaccination rates among U.S. children, there are gaps in measles protection in the U.S. population because of factors leaving patients immunocompromised and antivaccination sentiment that has led some parents to defer or refuse the MMR.
In addition, adults who were vaccinated prior to 1968 with either inactivated measles vaccine or measles vaccine of unknown type may have limited immunity. The inactivated measles vaccine, which was available in 1963-1967, did not achieve effective measles protection.
A global measles surge
While antivaccination sentiment contributed to the 2019 measles cases, a more significant factor may be the global surge of measles. More than 140,000 people worldwide died from measles in 2018, according to the World Health Organization and the CDC.
“[Recent data on measles] indicates that during the first 6 months of the year there have been more measles cases reported worldwide than in any year since 2006. From Jan. 1 to July 31, 2019, 182 countries reported 364,808 measles cases to the WHO. This surpasses the 129,239 reported during the same time period in 2018. WHO regions with the biggest increases in cases include the African region (900%), the Western Pacific region (230%), and the European region (150%),” according to a CDC report.
Studies on hospitalization and complications linked to measles in the United States are scarce, but two outbreaks in Minnesota (2011 and 2017) provided some data on what to expect if the measles surge continues into 2020. The investigators found that poor feeding was a primary reason for admission (97%); additional complications included otitis media (42%), pneumonia (30%), and tracheitis (6%). Three-quarters received antibiotics, 30% required oxygen, and 21% received vitamin A. Median length of stay was 3.7 days (range, 1.1-26.2 days) (Pediatr Infect Dis J. 2019 Jun;38[6]:547-52. doi: 10.1097/INF.0000000000002221).
‘Immunological amnesia’
Infection with the measles virus appears to reduce immunity to other pathogens, according to a paper published in Science (2019 Nov 1;366[6465]599-606).
CDC/Dr. Edwin P. Ewing, Jr.
The hypothesis that the measles virus could cause “immunological amnesia” by impairing immune memory is supported by early research showing children with measles had negative cutaneous tuberculin reactions after having previously tested positive.
“Subsequent studies have shown decreased interferon signaling, skewed cytokine responses, lymphopenia, and suppression of lymphocyte proliferation shortly after infection,” wrote Michael Mina, MD, from Brigham and Women’s Hospital in Boston, and coauthors.
“Given the variation in the degree of immune repertoire modulation we observed, we anticipate that future risk of morbidity and mortality after measles would not be homogeneous but would be skewed toward individuals with the most severe elimination of immunological memory,” they wrote. “These findings underscore the crucial need for continued widespread vaccination.”
In this study, researchers compared the levels of around 400 pathogen-specific antibodies in blood samples from 77 unvaccinated children, taken before and 2 months after natural measles infection, with 5 unvaccinated children who did not contract measles. A total of 34 children experienced mild measles, and 43 had severe measles.
They found that the samples taken after measles infection showed “substantial” reductions in the number of pathogen epitopes, compared with the samples from children who did not get infected with measles.
This amounted to approximately a 20% mean reduction in overall diversity or size of the antibody repertoire. However, in children who experienced severe measles, there was a median loss of 40% (range, 11%-62%) of antibody repertoire, compared with a median of 33% (range, 12%-73%) range in children who experienced mild infection. Meanwhile, the control subjects retained approximately 90% of their antibody repertoire over a similar or longer time period. Some children lost up to 70% of antibodies for specific pathogens.
Maternal-acquired immunity fades
In another study of measles immunity, maternal antibodies were found to be insufficient to provide immunity to infants after 6 months.
The study of 196 infants showed that maternal measles antibodies had dropped below the protective threshold by 3 months of age – well before the recommended age of 12-15 months for the first dose of MMR vaccine.
The odds of inadequate protection doubled for each additional month of age, Michelle Science, MD, of the University of Toronto and associates reported in Pediatrics (2019 Dec 1. doi 10.1542/peds.2019-0630).
“The widening gap between loss of maternal antibodies and measles vaccination described in our study leaves infants vulnerable to measles for much of their infancy and highlights the need for further research to support public health policy,” Dr. Science and colleagues wrote.
The researchers randomly selected 25 samples for each of eight different age groups: up to 30 days old; 1 month (31-60 days), 2 months (61-89 days), 3 months (90-119 days), 4 months, 5 months, 6-9 months, and 9-11 months.
Just over half the babies (56%) were male, and 35% had an underlying condition, but none had conditions that might affect antibody levels. The conditions were primarily a developmental delay or otherwise affecting the central nervous system, liver, or gastrointestinal function. Mean maternal age was 32 years.
To ensure high test sensitivity, the researchers used the plaque-reduction neutralization test to test for measles-neutralizing antibodies instead of using enzyme-linked immunosorbent assay, because “ELISA sensitivity decreases as antibody titers decrease,” Dr. Science and colleagues wrote. They used a neutralization titer of less than 192 mIU/mL as the threshold for protection against measles.
When the researchers calculated the predicted standardized mean antibody titer for infants with a mother aged 32 years, they determined their mean to be 541 mIU/mL at 1 month, 142 mIU/mL at 3 months (below the measles threshold of susceptibility of 192 mIU/mL), and 64 mIU/mL at 6 months. None of the infants had measles antibodies above the protective threshold at 6 months old, the authors noted.
Children’s odds of susceptibility to measles doubled for each additional month of age, after adjustment for infant sex and maternal age (odds ratio, 2.13). Children’s likelihood of susceptibility to measles modestly increased as maternal age increased in 5-year increments from 25 to 40 years.
Children with an underlying conditions had greater susceptibility to measles (83%), compared with those without a comorbidity (68%, P = .03). No difference in susceptibility existed between males and females or based on gestational age at birth (ranging from 37 to 41 weeks).
The Advisory Committee on Immunization Practices permits measles vaccination “as early as 6 months for infants who plan to travel internationally, infants with ongoing risk for exposure during measles outbreaks and as postexposure prophylaxis,” Huong Q. McLean, PhD, of Marshfield (Wisc.) Clinic Research Institute, and Walter A. Orenstein, MD, of Emory University, Atlanta, noted in an editorial.
The research was funded by the Public Health Ontario Project Initiation Fund. The authors had no relevant financial disclosures.
Bianca Nogrady and Tara Haelle contributed to this story.