Pediatric Annals


Vaccine-Preventable Outbreaks: Still with Us After All These Years

Daniel Ruderfer, MD; Leonard R. Krilov, MD, FAAP


Outbreaks of vaccine-preventable diseases continue to occur in the United States, and they have been occurring at increasing rates over the past decade. Factors contributing to these outbreaks include importation from abroad, under-vaccination of segments of the population, and incomplete protection or waning immunity with certain vaccines. This article reviews recent outbreaks of measles, mumps, and pertussis in the United States to highlight the extent to which outbreaks of these vaccine-preventable diseases are still occurring and even increasing. Appreciating the magnitude of these illnesses may help the physician in educating families who are hesitant about vaccines. [Pediatr Ann. 2015;44(4):e76–e81.]


Outbreaks of vaccine-preventable diseases continue to occur in the United States, and they have been occurring at increasing rates over the past decade. Factors contributing to these outbreaks include importation from abroad, under-vaccination of segments of the population, and incomplete protection or waning immunity with certain vaccines. This article reviews recent outbreaks of measles, mumps, and pertussis in the United States to highlight the extent to which outbreaks of these vaccine-preventable diseases are still occurring and even increasing. Appreciating the magnitude of these illnesses may help the physician in educating families who are hesitant about vaccines. [Pediatr Ann. 2015;44(4):e76–e81.]

Vaccination is estimated to prevent 2.5 million deaths globally each year.1 Despite great advances in modern medicine and public health, vaccine-preventable diseases (VPDs) continue to remain a major cause of morbidity and mortality in both developing and industrialized nations. Immunization is an essential component of the human right to health, and a responsibility that contains a hierarchy of components: the individual child, the parents, the community, and government. It is only when this entire chain of members are uniformly dedicated to vaccination efforts that true progress can be made toward eliminating VPDs.

Although global immunization efforts have many challenging roadblocks, such as lack of financial resources, weak health care systems, and low awareness of the importance of vaccination, vaccination in the United States has its own set of inherent difficulties. As outlined in the article by Drs. Lori A. Kestenbaum and Kristen A. Feemster in this issue of Pediatric Annals, the United States is faced with a growing “antivaccine” segment of its population that intentionally refuses to vaccinate outright, selectively vaccinates, or delays vaccinating their children. Whether the justification of this antivaccine mentality is religious, philosophic, or politically motivated, the net outcome ultimately results in the propagation of a large community of unprotected children.

When unvaccinated groups cluster together, an unfortunate consequence is the decline in herd immunity. This frequently manifests as outbreaks in certain communities, and not only in the children whose parents refused vaccinations, but also in those children who cannot be vaccinated due to age or immunodeficiency.

The World Health Organization (WHO) defines a disease outbreak as the occurrence of cases of disease in excess of what would normally be expected in a defined community, geographical area, or season.2 In this article, we review recent outbreaks of measles, mumps, and pertussis to raise awareness of the ongoing occurrence of these VPDs.


Measles virus, a member of the paramyxovirus family, causes a VPD that is one of the leading global causes of death among children younger than age 5 years.3 An important contributor to measles outbreaks is its highly contagious nature, with the ability of an infected person to spread measles via airborne transmission or direct contact with nasal or throat secretions.4 An estimated 0.2% to 0.3% of infected persons in the United States die of measles; the death rate in developing countries ranges from 2% to 15%.4

Before widespread vaccination began in 1980, measles caused an estimated 2.6 million deaths globally each year.3 Accelerated global immunization programs have had a major impact on reducing measles deaths, with the WHO estimating 15.6 million deaths having been prevented by vaccination during the period from 2000 to 2013.3 This major decline can be attributed to the significant increase of the world’s children receiving one dose of measles vaccine by their first birthday, from 73% in the year 2000 to 84% in 2013.3 Despite improved vaccination coverage resulting in a 75% reduction in measles deaths worldwide between 2000 and 2013, in 2013 there were still 145,700 measles deaths globally (23,700 more than in 2012).3,5 On average, that is approximately 400 deaths per day or 16 deaths per hour.3

After the development of an effective measles vaccine in 1963,6 the incidence of measles declined significantly in the United States, with the Centers for Disease Control and Prevention (CDC) ultimately declaring the elimination of endemic measles in 2000.7 Unfortunately, cases continue to surge in the United States, caused by importation of the virus from travelers visiting endemic countries, as well as spread occurring largely among unvaccinated individuals. In 88% of the cases reported between 2000 and 2011, the virus originated from a country outside the United States, and 2 of every 3 individuals who developed measles were unvaccinated or of unknown vaccination status.8 Perhaps because of the false allegations by Wakefield et al.9 linking vaccines to autism, Western Europe saw a rise in measles cases from 2010 to 2011, with 33 countries reporting a total of 68,743 measles cases, resulting in more frequent importations of the virus into the Americas.10

This trend of increased measles incidence continued into 2014, with 634 confirmed cases in the United States, an approximate 5-fold increase from the 187 cases in 2013.11 The 2014 figures eclipse the highest previous reported total of the past decade (220 in 2011),11 and are approaching the highest levels seen in almost 20 years (963 cases in 1994).12 In 2014, measles affected children in 27 states, with 23 reported outbreaks (representing 89% of the cases).13

Young children who are not appropriately vaccinated may experience more than a 60-fold increase in risk of disease due to exposure to imported cases from countries where measles is endemic.8 The vaccination rate needed to achieve herd immunity and interrupt measles transmission is estimated at 92% to 94%, which is higher than for most other VPDs.4 The WHO reports that the United States’ vaccination rate for one dose of measles vaccine prior to a child’s second birthday in 2013 was 91%, down from 92% from 2010 to 2012; this is certainly a contributing factor to the higher numbers of cases seen in 2014.14

Among clusters of intentionally unvaccinated communities, the largest outbreak in 2014 was primarily among unvaccinated Amish persons in Ohio, with 138 cases reported as of May 23, 2014.15 It is thought that this outbreak began after Amish missionaries returned from the Philippines, which was experiencing a large measles outbreak, with more than 21,400 confirmed cases having been reported.16 Importation of the virus from endemic countries continues to occur, with at least 48 cases of measles having been brought into the United States by travelers from more than 30 countries during the 2014 United States measles outbreak.4 Health care providers should remind persons traveling to endemic countries of the increased risk for measles and encourage timely vaccination of all persons. One dose of measles, mumps, and rubella (MMR) vaccine is recommended for infants ages 6 to 11 months before travel, and two doses for persons age 12 months or older, with a minimum interval between doses of 28 days.16

Responding to disease outbreaks like measles is not only time consuming, but also has surprisingly high financial costs for state health departments. An outbreak in San Diego in 2008 cost the public health department $124,517 to contain, with additional medical costs and costs incurred by families for quarantining unvaccinated contacts at home.9 Total outbreak costs were $176,980.9 Another outbreak in 2008 affecting several hospitals cost approximately $800,000 to contain (a cost of more than $100,000 per case).10 Using these estimates, the 2014 measles outbreaks will undoubtedly present an enormous financial burden, and may cost as much as $60 million dollars.

The startlingly large number of cases this past year emphasizes the need for health care providers to have a heightened awareness of the potential for measles in their communities and the importance of vaccination to prevent measles. Preventing outbreaks will help ensure that high-risk populations (those children younger than age 1 year and/or those who are immunocompromised) will remain healthy and also save the allocation of considerable health care dollars toward a potentially avoidable illness.


Infection with the mumps virus (also a member of the Paramyxovirus family) usually presents with nonspecific symptoms such as myalgia, malaise, and low-grade fever. Despite 20% of cases being asymptomatic and the disease being self-limited overall, mumps may have complications of aseptic meningitis, orchitis/oophoritis, and most commonly parotitis (30%–40%).17 Considered as contagious as influenza but less than that of measles or varicella, mumps is transmitted via respiratory droplets.17 Although mumps is endemic in a large proportion of the world, reported cases of mumps in the United States have decreased dramatically since the initiation of vaccination programs in 1967. Prior to the vaccine age, approximately 186,000 cases were reported yearly.18 During the course of the past decade, most yearly cases were reported in the several hundreds, with sporadic outbreaks increasing reports to the few thousands. Outbreaks often occur in winter and spring but may occur at any time of year.18 Close-contact environments are strong contributors to outbreaks and these can occur in a variety of settings, such as same classrooms and shared living spaces.

A dramatic rise in reported mumps cases occurred in 2014, with 1,151 confirmed cases, which was nearly a doubling of the 2013 total of 584 cases.11 Bringing media attention to the mumps resurgence has been the recent outbreak of the mumps among professional hockey players in the National Hockey League, with 23 players affected thus far.19 The sports locker room is a classic example of a crowded environment that may become a source for outbreaks. Additionally, close contact among players during the game with spraying of saliva likely contributed to the spread of the virus in this setting.

A large reason for why the mumps continues to persist in the United States is that the mumps vaccine component of the MMR vaccine has a lower effectiveness compared with the measles and rubella components, with estimated effectiveness at a median of 78% (range, 49%–91%) for one dose, and a median of 88% (range, 66%–95%) for two doses.20 This ultimately provides a lower-than-anticipated “herd effect” and explains why outbreaks may still occur in highly vaccinated communities in the United States. The herd immunity threshold to prevent community transmission and outbreaks of mumps may be higher than the previously suggested 88% to 92%.21

Another potential cause for outbreaks is that mumps immunity likely wanes over time. One study showed that the two-dose vaccine efficacy decreased from 99% among 5- to 6-year-old children to 86% among 11- to 12-year-old children.22 This may help explain the shift of age in these outbreaks from 4 to 5 years in the prevaccine era to adolescents and young adults in the current era. Lastly, there is evidence that suggests the vaccine strain (derived from genotype A) may be less effective against serogroup G wild strains (a genotype now seen in the United States).21

The largest mumps outbreak this past decade occurred in the United States in 2006, when 6,584 cases were reported.23 Despite a high coverage rate with two doses of mumps-containing vaccine, college-aged students who were living in multiple college campuses in the Midwest were predominately affected.23 Communal living situations, such as dormitories or boarding schools, are prototypical examples of environments that provide increased opportunities for close contact or saliva exposures and higher dose exposures to mumps virus when introduced. Another large mumps outbreak occurred during the years 2009 and 2010, with 3,502 cases reported in New York City, predominately among Orthodox Jewish boys attending all-male yeshivas (private, traditional Jewish schools).24 A total of 3,419 cases (98%) occurred among persons for whom vaccination is recommended (ie, persons age 12 months or older and born after 1956). Among the 2,317 patients (68%) in this population who had verified vaccination status, 10% were unvaccinated, 14% had received one dose of MMR vaccine, and 76% had received two doses of MMR vaccine.24 This outbreak was characterized by environmental factors such as a densely populated environment, prolonged face-to-face contact, and mixing among infected students. The index case-patient was a twice-vaccinated 11-year-old member of the religious community who had returned from the United Kingdom, where a large mumps outbreak had been occurring.24

The major resurgences in mumps this past year and decade highlight the notion that mumps is still very much a force to be reckoned with, with its ability to spread quickly despite proper vaccination coverage. The suggestion that a third MMR dose may be helpful, or spacing of the second MMR dose into adolescence, should be re-evaluated by public health officials as outbreaks continue to occur. Physicians should continue to promote aggressive vaccination despite its incomplete protection because high vaccination coverage helps limit the size, duration, and spread of mumps outbreaks.


Bordetella pertussis is a highly contagious respiratory pathogen that causes illness in thousands of children yearly. Commonly known as “whooping cough” in America and the “100-day cough” in Chinese medicine, the coughing spells can last for up to 10 weeks or more.25 This respiratory infection can range from chronic cough in adolescents and adults (with significant loss of time at school or work) to mortality in young infants. Hospitalization is most common in infants younger than age 6 months,26 with those hospitalized at risk for apnea (67%), pneumonia (23%), seizures (1.6%), death (1.6%), and encephalopathy (0.4%).27 In the United States from 1997 to 2000, a total of 7,203 cases of pertussis were reported in infants younger than age 6 months, with 4,543 patients (63.1%) hospitalized, 847 patients (11.8%) with pneumonia, and 56 deaths (0.8%). In children between ages 10 and 19 years, 8,273 cases were reported with 174 patients (2.1%) hospitalized, 155 patients (1.9%) with pneumonia, and no deaths.26

Pertussis is currently endemic in the United States. Despite routine childhood vaccination significantly reducing disease, pertussis continues to remain the most poorly controlled bacterial VPD in the United States. With the number of reported pertussis cases steadily increasing since the 1980s, there have been an alarming number of outbreaks in recent years. Cherry28 notes that reported cases likely represent a vast underestimation of the true number of disease and suggests that there may be between approximately 800,000 and 3.3 million cases per year in the United States. The causes for the resurgence of pertussis are multifactorial and attributed to waning immunity, under-vaccination associated with the growth of the antivaccine movement, geographic clustering of communities resistant to vaccination, increased surveillance, more sensitive diagnostics, and the emergence of vaccine-resistant strains (eg, the strain with pertactin deletion).29

The resurgence of pertussis has contributed to its evolving epidemiology, with peaks in both infants younger than age 1 year and children older than age 10 years.26 Early waning of immunity, both postvaccination or even after natural disease,30 has markedly contributed to increasing population-level susceptibility. This is in part due to increased usage of acellular pertussis vaccines, which cause fewer reactions than the whole-cell vaccines that preceded them but seem to offer less prolonged protection to the host. These observations led to the recommendation for routine administration of tetanus, diphtheria, and pertussis (Tdap) vaccine to children ages 11 and 12 years, and booster doses for adults.31

Notable peaks of pertussis outbreaks occurred in the United States in 2004 (25,827 cases, 27 deaths) and 2010 (27,550 cases, 27 deaths).32 An outbreak in 2012 (48,277 cases, 20 deaths)33 represented the most reported cases since 1955 (62,786 cases).34 Although overall reporting of pertussis declined during 2013 (28,639 cases), 13 states and Washington, DC experienced increased cases compared with 2012.35 With 28,660 reported cases in 2014, pertussis continues to remain a serious public health concern.11

The incidence of pertussis in California has been steadily growing since the 1990s, and the state is currently experiencing its worst pertussis epidemic in 70 years.36 In 2014, there were 10,831 reported cases (with two deaths) of whooping cough (an incidence rate of 28 cases per 100,000 people).37 Three hundred and seventy-six patients were hospitalized and 85 (23%) required intensive care. Two hundred and twenty-seven (60%) of the hospitalized patients were infants younger than age 4 months. Among the 2,006 adolescent cases (patients ages 14–16 years), only five patients (0.2%) were hospitalized.36 Among 7,081 (85%) pediatric cases with vaccination history information, 720 (10%) had never received any doses of pertussis-containing vaccine.37 Overall, rates by race/ethnicity are highest for Hispanic infants younger than age 1 year, and white, non-Hispanic children and teens ages 10 to 17 years.37 The 2014 totals eclipsed the 9,159 cases (including 10 deaths) during the state’s last epidemic in 2010,38 and were approximately 5 times higher than California’s 2013 statistics (a total of 2,011 cases, with an incidence of 5.3/100,000 persons).39

A low rate of Tdap vaccination during the third trimester of pregnancy is another reason attributed to this recent outbreak. Despite recommendation by the CDC’s Advisory Committee for Immunization Practices (ACIP), Tdap vaccination coverage among pregnant women in California was 19.5% in 2012, and only 17% during the current outbreak.36 Vaccination after pregnancy does not confer any direct protection to the infant, but third trimester vaccination confers some protection to the fetus via the transplacental passage of antibodies to the fetus. All primary caretakers for young infants should also receive a Tdap booster to prevent transmission of the disease, and thus create a “cocooning” effect around the newborn. This should theoretically protect them until they are old enough to be vaccinated, which can begin as early as 6 weeks. “Cocooning” is an especially important preventive effort in overcrowded living situations.

Another large outbreak occurred in Washington state from January 1, 2012 to June 16, 2012, during which time a total of 2,520 pertussis cases (rate of 37.5/100,000 compared with 4.2 cases/100,000 nationally), with no fatalities reported.31 These numbers represent a 1,300% increase compared with the same period in 2011 and the highest number of cases reported in any year since 1942.31 Of the 155 reported pertussis cases in infants younger than age 1 year, 34 (21.9%) were hospitalized. Of the 2,360 cases involving children age 1 year or older with known outcome, only 14 (0.6%) were hospitalized. In those with vaccination history available, 758 of 1,000 (75.8%) patients ages 3 months to 10 years, 97 of 225 (43.1%) patients ages 11 to 12 years, and 466 of 604 (77.2%) patients ages 13 to 19 years were up-to-date with pertussis vaccination. Estimated diphtheria, tetanus, and pertussis coverage in Washington among children ages 19 to 35 months was 93.2% for three or more doses and 81.9% for four or more doses in 2010; the rate was 70.6% for adolescents ages 13 to 17 years.31 Unvaccinated children have at least an 8-fold greater risk for pertussis than fully vaccinated children.31

There are a variety of obstacles that must be addressed in fighting the current resurgence of pertussis in the United States. Despite its imperfect nature, vaccination continues to be the single most effective strategy to reduce morbidity and mortality caused by pertussis. The current vaccination strategy should be re-evaluated in light of the shift in epidemiology to older adolescents, and perhaps additional booster doses or development of a new vaccine that provides prolonged immunity is needed. Vaccination of pregnant women and primary contacts of infants is critical to emphasize, as young infants represent the group most at risk for developing complications. Joint efforts from the pediatric and obstetrical communities must continue to stress the importance of vaccination to protect infants.


As demonstrated in the outbreaks described in this article, VPDs are still very prevalent, and suboptimal immunization contributes to the spread and persistence. Awareness of these outbreaks and highlighting them to parents who are vaccine hesitant/resistant may help in improving compliance with recommended vaccine schedules.


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Daniel Ruderfer, MD, is a Pediatric Hospitalist, Children’s Medical Center, Winthrop University Hospital. Leonard R. Krilov, MD, FAAP, is the Chief, Pediatric Infectious Disease, and the Vice Chairman, Department of Pediatrics, Children’s Medical Center, Winthrop University Hospital; and a Professor of Pediatrics, State University of New York at Stony Brook School of Medicine.

Address correspondence to Leonard R. Krilov, MD, FAAP, 120 Mineola Boulevard, Suite 210, Mineola, NY 11501; email:

Disclosure: Leonard R. Krilov has performed contracted research for Pfizer and AstraZeneca (MedImmune). The remaining author has no relevant financial relationships to disclose.


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