Deadly disease resurgence: Outbreaks linked to waning vaccine protection

By Liz Meszaros, MDLinx
Published October 23, 2018

Key Takeaways

The surprising recent outbreaks of numerous vaccine-preventable diseases such as mumps, measles, and pertussis (whooping cough) have garnered a lot of attention and caused considerable concern for physicians, parents, and infectious disease experts. The unknown etiology of such outbreaks has led to closer scrutiny of vaccine efficacy rates, waning immunity, and the potential effects of pockets of unvaccinated/undervaccinated children.

Both old and new diseases alike are causing current outbreaks in the United States. Cases of long-eradicated infections like the centuries-old plague, for instance, are springing up. According to the Centers for Disease Control and Prevention (CDC), an average of seven human plague cases (range: 1-17) have been reported each year in recent decades, with over 80% confirmed as the bubonic form, also known as the Black Death. Septicemic plague and the more aggressive pneumonic plague can, however, develop from the untreated bubonic form. Thankfully, all forms are currently treatable with modern antibiotics and early diagnosis.

Of more concern recently, the CDC has reported 62 cases of acute flaccid myelitis (AFM), a new fast-acting polio-like illness, that has occurred in children across 22 states this fall alone. AFM is thought to be caused by one or more viruses, including Enterovirus D68, which—like polio—is a member of the Picornaviridae family. AFM is, however, rare. CDC estimates hold that less than 1 of every 1 million people in the United States will contract AFM each year. No vaccines exist for AFM, but according to infectious disease experts, it is cyclical and occurs in waves about every 2 years. AFM may seem to be a new and frightening disease, but infectious disease experts have been actively investigating it since 2014.

Here are the latest tallies for outbreaks of more common preventable diseases—mumps, measles, and pertussis:

  • Mumps: In 2016, multiple outbreaks occurred, totaling 6,366 cases in the United States. Compared with the 229 reported cases in 2012, this was a significant increase.
    • In 2016 and 2017, many young adults were affected, which came to be associated with the college environment. According to the CDC, these outbreaks were due to a combination of factors, including vaccine effectiveness, decreasing immunity after vaccination, and the intensity of virus exposure in close-contact settings coupled with behaviors that increase the risk of transmission.
    • These outbreaks occurred despite the approximately 90% of children who received the recommended vaccinations for measles/mumps/rubella (MMR), polio, and chickenpox, according to a 2015 CDC report.
  • Measles: In 2000, the measles virus was declared eradicated from the United States, and in 2004, 37 cases of measles were reported.
    • Yet, just recently, the number of confirmed cases has risen. Of note, a recent outbreak linked to international travelers returning from Israel resulted in 17 cases in New York. Rockland County was the location of 11 of these cases, which forced the Rockland health commissioner to issue a letter to schools attended by affected students declaring that all unvaccinated students must remain at home until at least November 3.
    • Because measles is a highly infectious virus, it can infect 90% of those who are susceptible if exposed and, once a measles outbreak starts in an area with a low vaccination rate, it can be difficult to control.
  • Pertussis: Between 10,000 to 40,000 cases of pertussis occur annually, according to CDC estimates, and cause up to 20 deaths. In 2014, California reported an outbreak that infected 9,934 people; in 2012, Washington state had a reported 2,530 cases.

Why are these outbreaks occurring?

The recommended doses of the MMR vaccine offer protection for an average of 27 years. According to the CDC, the MMR vaccine prevents most cases of mumps, and the full two doses of the vaccine are 88% effective (one dose is 78% effective).

“People who have received two doses of the MMR vaccine are about nine times less likely to get mumps than unvaccinated people who have the same exposure to mumps virus. However, some people who receive two doses of MMR can still get mumps, especially if they have prolonged, close contact with someone who has the disease. If a vaccinated person does get mumps, they will likely have less severe illness than an unvaccinated person,” wrote the CDC.

Yet, a waning of immunity can also occur, and a strong exposure may be enough to overcome this, according to study recently published in Science Translational Medicine.

Authors Joseph A. Lewnard, PhD, assistant professor, and Yonatan H. Grad, assistant professor, Immunology and Infectious Disease, Harvard T.H. Chan School of Public Health, Boston, MA, admit that, although dwindling immunity after vaccination may have played a role in these outbreaks, there are some unexplained inconsistencies. 

“Outbreaks from 2006 to the present among young adults, and outbreaks in the late 1980s and early 1990s among adolescents, aligned with peaks in mumps susceptibility of these age groups predicted to be due to loss of vaccine-derived protection. In contrast, evolution of mumps virus strains escaping immune pressure would be expected to cause a higher proportion of cases among children, not adolescents and young adults as observed,” they wrote.

Perhaps, they added, a third vaccine dosed at 18 years of age or booster doses throughout adulthood could circumvent the re-emergence of mumps.

In response to these outbreaks, the CDC’s Advisory Committee on Immunization Practices (ACIP) will review vaccination recommendations for mumps and consider implementing one for booster shots during such outbreaks. Encouragingly, fewer cases of mumps have been reported for 2018.

The efficacy of vaccination against pertussis is also known to diminish over time. Two vaccines are used: DTaP—which protects infants and children against diphtheria, tetanus, and pertussis—and Tdap, a booster immunization that protects preteens, teens, and adults. According to the CDC, only 3 or 4 out of 10 people are protected against pertussis 4 years after vaccination.

To circumvent diminishing immunity, the CDC now recommends a single dose of Tdap for adults aged 19 years or older who have not been vaccinated with Tdap, and a tetanus and diphtheria booster (Td) every 10 years. Conveniently, the recommended 10-year Td booster can be replaced with Tdap. This is important because the CDC has stressed that being up to date with one dose of Tdap is critical for adults who are around babies because even fully vaccinated adults can get pertussis.

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Unvaccinated children

In addition to the careful study and reassessment of current vaccines, vaccination rates for these diseases have also come under close scrutiny. The statistics point to a disturbing trend.

According to the CDC, an estimated 100,000 young children have not been vaccinated against any of the 14 diseases for which vaccination is recommended.

These findings highlight a trend. In 2017, 1.3% of children born in 2015 were completely unvaccinated, an increase from 0.9% for children born in 2011.

Despite this increase in the number of unvaccinated children, experts at the CDC are encouraged.

“Although the number of children who have received no vaccinations by age 24 months has been gradually increasing, most children are still routinely vaccinated. Continued evaluation of prevalence and reasons for non-vaccination is needed, as are improvements in access to and delivery of age-appropriate vaccinations to all children. CDC continues to examine barriers to early childhood vaccination, including assessing obstacles to and parents’ experiences with accessing vaccination services,” they concluded.

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