Introduction

Measles, or rubeola—derived from “rubeo,” the Latin word for red—is a highly contagious, acute viral illness caused by a single-stranded, enveloped RNA virus. It is classified as a member of the genus Morbillivirus in the paramyxoviridae family. Measles is endemic in most of the world, and an estimated 20 million cases occurs annually [1]. Measles remains a leading cause of death globally among young children, despite the availability of a safe and effective vaccine for over 40 years. Nevertheless, the vaccine has had considerable impact, with an estimated 75 % reduction in children’s deaths comparing numbers from 2000 to 2012 [2]. In fact, a new goal to achieve a 95 % reduction of measles worldwide by this year has been set by World Health Organization (WHO) [3].

In the USA, ongoing transmission of the measles virus was declared eliminated (defined as interruption of continuous transmission lasting ≥12 months) in 2000, an achievement attributed to high rates of vaccination coverage [4]. However, importations from other countries where measles remains endemic continue to occur, and poor vaccine coverage has led to recent outbreaks of measles in the USA. The reemergence of measles in recent years has led to renewed interest in the virus. In this article, we discuss its epidemiology, transmission, clinical features, complications, vaccine indications, and public health implications.

Epidemiology and Transmission

Measles is one of the most contagious of the vaccine-preventable diseases, with reproduction rates (R0) estimated at 12 to 18—meaning that the average person with measles is capable of infecting 12 to 18 other people if all close contacts are susceptible [5]. It has a secondary attack rate among susceptible individuals higher than 90 %. The virus can be transmitted in the air (aerosolized) in respiratory droplets or by direct or indirect contact with the nasal and throat secretions of infected persons when an infected person breathes, coughs, or sneezes [6].

Despite being declared eliminated in the USA, there have been several measles outbreaks in the USA in the past few years. Most recently, during January 4–April 2, 2015, a total of 159 measles cases (in 155 US residents and four foreign visitors) from 18 states were reported to the Centers for Disease Control (CDC) [1]. Of those infected, 55 % were adults [58 (36 %) were aged 20–39 years, and 30 (19 %) were aged ≥40 years]. Twenty-two patients (14 %) were hospitalized, including five with pneumonia. The majority of the 159 patients with reported measles were either unvaccinated (71 [45 %]) or had unknown vaccination status (60 [38 %]). The bulk of cases (96 %) were import-associated cases, belying the importance of vaccine coverage and herd immunity (discussed later).

Unfortunately, this pattern of measles resurgence in the USA is also reflected in Europe. Since a decrease in the number of notified measles cases in Europe between 2003 and 2009 [7], there have been a number of measles outbreaks especially in Central and Western Europe, with a peak of cases reported in 2011 (32,124 cases reported). France was the most affected country, with 47 % of cases in Europe in 2011, while several other countries have also reported a considerable number of cases including Bulgaria, Germany, Italy, Romania, Spain, Ukraine, and the UK [7, 8]. More recently, over a period of 1 year, over a thousand suspected measles cases (n = 1073) were reported across the UK [9]. Most confirmed and probable cases occurred within two age groups—infants (too young to be eligible for measles-mumps-rubella (MMR) vaccination) and children aged 10–19 years. This resurgence of measles in countries without endemic transmission of disease and prior control is thought to result from poor vaccination rates [9].

Clinical Presentation

Measles is considered a systemic illness. Following exposure, the incubation period before onset of the first symptoms is usually 10–12 days. The prodromal symptoms, known as the “3Cs” of measles—cough, coryza, and conjunctivitis—occur prior to onset of the rash. Fever accompanies the prodromal phase and may be as high as 105 °F. Koplik spots—small spots with white or bluish-white centers on an erythematous base on the buccal around the second molar—are transient, noted only in 50–70 % of patients, but are highly specific [10].

The rash usually appears 14 days after exposure (range 7–18 days) [11, 12]. The classic rash (Fig. 1) in patients with measles is red, blotchy, maculopapular, and develops in a cephalocaudal and centrifugal distribution—it begins on the face, becomes generalized, and lasts 4–7 days. The fever typically ends once the rash appears. Individuals with measles are considered infectious from 4 days before to 4 days after the onset of rash [11] and should be in airborne isolation precautions during this period if hospitalized.

Fig. 1
figure 1

An adult male with a maculopapular, truncal rash. From Hirai et al. [13] with permission

Full size image

Measles should be suspected in any patient who presents with fever, rash, and epidemiologic risk factors such as travel to endemic areas or areas known to have recent transmission.

Laboratory Diagnosis

In areas such as the USA and Europe where the incidence of measles is low, a clinical diagnosis of measles in the absence of a confirmed outbreak has a low positive predictive value, and clinical signs enumerated above are unreliable as the sole criteria for diagnosis [3]. A number of other infections such as roseola, rubella, rickettsial disease (Rocky Mountain spotted fever), and dengue fever can present with a rash resembling measles; therefore, laboratory assessment is required for accurate diagnosis.

Measles-specific immunoglobulin M (IgM) and immunoglobulin G (IgG) are both produced during the primary immune response and can be detected in the serum within days of rash onset, using a sensitive enzyme-linked immunosorbent assay (ELISA). Approximately 70 % of measles cases are IgM-positive at 0–2 days after the rash onset, and 90 % are positive 3–5 days after rash onset. IgM antibody levels peak after 7–10 days and then decline, rarely detectable after 6–8 weeks. Reexposure to measles induces a strong anamnestic immune response with a rapid boosting of IgG antibodies, preventing clinical disease [3]. Measles virus can be isolated from conventional clinical specimens (nasopharyngeal swab, urine, or peripheral blood mononuclear cells) up to 5 days following onset of the rash and may be detected using polymerase chain reaction (PCR) assays on specimens obtained up to 7 days or more after onset of the rash [14].

Complications

Measles infects multiple systems and targets epithelial, reticuloendothelial, and white blood cells [15]. Complications, when they occur, arise largely by disruption of epithelial surfaces of different organ systems and immunosuppression [16–18]. Approximately 30 % of reported cases of measles involve one or more complications. In developed countries, these include otitis media (7–9 %), pneumonia (1–6 %), diarrhea (6 %), blindness, and post-infectious encephalitis (1 per 1000 cases). The risk of serious measles complications is higher in infants and adults [19]. A summary of complications from measles adapted from an excellent review by Perry and Halsey [20] is listed in Table 1.

Table 1 Complications associated with measles by organ system (adapted from [20], with permission)
Full size table

Measles and its associated complications can be severe in certain populations, such as those with underlying immune deficiencies. In fact, with the rise in measles incidence, it now has to be considered among the reemerging viruses in the transplant population. The incidence of measles in transplant recipients, as well as the proportion with severe disease, is unclear. Two series identified two cases of interstitial pneumonia (one fatal) among 24 hematopoietic stem cell transplantation (HSCT) recipients diagnosed with measles [35]. Subacute measles encephalitis (SME) has also been reported in renal transplant recipients and a single HSCT recipient. The clinical course is one of deteriorating mental status and treatment-refractory seizures [35]. Four of six transplant cases of SME died [36]. Among patients with HIV, case reports validate the severity of measles and its unusual presentation. In particular, several case reports document a delayed, uncharacteristic, or absent rash and the frequent occurrence of pneumonitis in both HIV-infected children [37–41] and adults [37, 42–44] with measles.

Treatment

Like other viral illnesses, the treatment of measles is supportive. There is no specific antiviral therapy for measles. However, the WHO provides guidance for the use of vitamin A for severe measles cases among children, such as those who are hospitalized [45].

Measles Vaccine: Impact, Indications, Efficacy, and Adverse Effects

The measles vaccine is one of the most cost-effective health interventions developed. Before its discovery, infection with measles virus was nearly universal during childhood, and more than 90 % of persons had immunity from the disease by age 15 years. Measles occurred in epidemic cycles and an estimated three to four million persons acquired measles each year [46]. In the USA, approximately 500,000 persons with measles were reported each year, of whom 500 died, 48,000 were hospitalized, and another 1000 had permanent brain damage from measles encephalitis [47]. With its inception, the number of reported measles cases decreased dramatically, with the greatest decrease occurring among children aged <10 years [48, 49].

The measles vaccine, a live-attenuated vaccine, has been available for use since 1963. In that year, both an inactivated (“killed”) and a live attenuated vaccine (Edmonston B strain) were licensed for use in the USA. The inactivated vaccine was withdrawn in 1967 because it was not protective, and recipients frequently developed a unique syndrome, called atypical measles. Over time, there have been further modifications to the original Edmonston B strain to reduce adverse effects, and the Edmonston-Enders strain is currently in use [50].

The measles vaccine was first recommended as a single-dose vaccine. However, measles outbreaks among school-aged children who received one dose of measles vaccine prompted changes in recommendations from a single-dose vaccine to two doses of measles-containing vaccine, preferably as MMR [51]. To date, it is recommended that all US residents born after 1956 should ensure that they receive MMR vaccine or have serologic evidence of measles immunity (Table 2). Vaccine recommendations for those born after 1956 and travelling out of the USA who do not have serologic evidence of immunity include the following: two doses of MMR given subcutaneously, separated by at least 28 days for adults and children aged ≥12 months. Infants age 6–11 months who receive one dose of MMR vaccine should still receive a second dose at ≥1 year [50]. The vaccine is contraindicated in anyone with a history of anaphylactic reactions to neomycin, history of severe allergic reaction to any component of the MMR vaccine, pregnancy, and immunosuppression. A thorough discussion of the contraindications is available in the Summary Recommendations of the Advisory Committee on Immunization Practices (ACIP) [50].

Table 2 Acceptable presumptive evidence of immunity to measles (adapted from [50])
Full size table

Given as a single dose, the measles-containing vaccine administered at age ≥12 months is approximately 94 % effective in preventing measles (range 39–98 %) [52, 53]. The effectiveness of two doses of measles-containing vaccine was ≥99 % in two studies conducted in the USA and 67, 85–≥94, and 100 %, in three studies in Canada [52, 54–57]. Vaccine effectiveness was higher among children given the vaccine at age ≥15 months compared to 12 months [54], belying the importance of timing. Similar estimates of vaccine effectiveness have been reported from Australia and Europe [53]. The measles vaccine induces long lasting immunity in most persons, based on serologic and immunologic data [58]. Approximately 95 % of vaccinated persons examined 11 years after initial vaccination and 15 years after the second dose of MMR (containing the Enders-Edmonston strain) vaccine had detectable antibodies to measles [59–62].

Apart from the MMR vaccine, an aerosolized vaccine against measles is also available and has been used by more than four million children in Mexico since 1980 [63]. However, its efficacy compared to the subcutaneous vaccine has been questioned. A randomized controlled trial published recently in the New England Journal [64] showed that the aerosolized vaccine is immunogenic, but based on predetermined margins, was inferior to the MMR vaccine in terms of antibody seropositivity.

MMR vaccine is generally well tolerated and is rarely associated with serious adverse events. It may cause fever (<15 %), transient rashes (5 %), lymphadenopathy (5 % of children and 20 % of adults), or parotitis (<1 %) [65–69]. The majority of persons vaccinated are otherwise asymptomatic. A recent Cochrane review validates these findings [70]. In persons with immune deficiencies, that are inadvertently given the vaccine, there is evidence that supports a causal relation between MMR vaccination and anaphylaxis, febrile seizures, thrombocytopenic purpura, transient arthralgia, and measles inclusion body encephalitis [71–75], although these are rare. The concern that measles vaccine is causally associated with autism spectrum disorders is not borne out by science [76–78] although the myth continues to be pervasive.

Reemergence of Measles

In the last few years, measles has resurfaced in areas where it is considered rare, or eliminated. These measles outbreaks are alarming, and the reasons behind them must be thoroughly evaluated. Often, outbreaks of vaccine-preventable diseases occur when vaccination rates fall below a certain threshold, placing the community at risk. This so-called threshold theorem [74, 79] underlies the concept of “herd immunity.” Herd immunity can be thought of as a threshold level of immunity in the population above which a disease no longer spreads. For measles, the level of immunity needed to interrupt transmission is higher than the thresholds for almost all other vaccine-preventable diseases—to prevent sustained spread of the measles virus, 92 to 94 % of the community must be protected [80].

Vaccine coverage among certain populations both in the USA and Europe has waned over the last several years. Parental concerns about vaccine safety issues, such as the association between vaccines and autism, though not supported by a credible body of scientific evidence [81–84], have led increasing numbers of parents to refuse or delay their children’s vaccination [85, 86]. This is alarming, as children with nonmedical exemptions are at increased risk for acquiring and transmitting vaccine-preventable diseases [87]. In a retrospective cohort study, for example, children who were unvaccinated were 35 times as likely to contract measles as nonexempt children (relative risk, 35; 95 % confidence interval [CI], 34 to 37) [88]. Other studies [87, 89] have confirmed that areas with high rates of vaccine refusal are at increased risk of outbreaks (2–22 times higher risk of disease outbreak, depending on the disease).

Outbreaks of vaccine-preventable disease often start among persons who refuse vaccination, then spread rapidly within unvaccinated populations (the herd), and subsequently involve other subpopulations. To illustrate, the most recent multistate outbreak of measles in the USA began with an unvaccinated person at Disneyland in California. Among 110 patients, 49 (45 %) were unvaccinated and another 43 % had unknown or undocumented vaccination status [90]. An article from the New England Journal [80] emphasizes that to prevent measles from reestablishing itself as an endemic disease in the USA, the vaccine must be accessible to all who need it—especially to people traveling to and from countries with circulating disease—and hesitant patients and families must be convinced the vaccine is safe and effective.

Carrillo-Santisteve and Lopalco [91] provide some insight regarding the difficulty in convincing patients about the importance of vaccination. They introduce the concept of vaccine paradox, in which vaccines are the victims of their own success. When vaccination coverage increases, a dramatic decrease in the incidence of disease follows, in turn, leading to a decrease in the perceived risk of the disease and its complications. As the disease (such as measles) is no longer remembered as dangerous, real or alleged vaccine adverse events become disproportionately highlighted. These vaccine risks are emphasized and propagated by a relatively few, yet very loud antivaccination proponents, who use several methods [92] to create fear and doubt about vaccine safety. In fact, the most frequent reason for nonvaccination, in a survey among parents of school-aged children, was concern that the vaccine might cause harm [93].

In order to curb the rise of measles and other vaccine-preventable diseases, some health care providers have considered terminating their provider relationship with families that refuse vaccines [94, 95]. In a national survey of members of the American Academy of Pediatrics (AAP), almost 40 % of respondents said that they would not provide care to a family that refused all vaccines, and 28 % said that they would not provide care to a family that refused some vaccines [94]. However, the academy’s Committee on Bioethics advises against discontinuing care for families that decline vaccines and has recommended that pediatricians “share honestly what is and is not known about the risks and benefits of the vaccine in question [96].”

So what can be done to help prevent the reestablishment of measles as an endemic disease? Health care providers play a pivotal role in parental decision-making with regard to immunization. Health care providers are cited by parents, including parents of unvaccinated children, as the most frequent source of information about vaccination [93]. In fact, in a study of the knowledge, attitudes, and practices of primary care providers, a high proportion of those providing care for children whose parents refused vaccination and those providing care for appropriately vaccinated children were both found to have favorable opinions of vaccines [97]. However, those providing care for unvaccinated children had less confidence in vaccine safety and less likely to perceive vaccines as benefitting individuals and communities, underscoring the importance of the provider’s advocacy for and knowledge regarding vaccination.

From a public health perspective, improving the quality of vaccine supply, advocacy activities among vaccine decision makers, and activities involving the general public, should be put in place in order to increase the demand for measles vaccination and immunization in general [91]. Campaigns to increase awareness about the disease itself are likewise paramount—early recognition of measles by clinicians is key in making sure that appropriate infection control procedures are followed to prevent disease spread.

Conclusion

Measles remains a leading cause of death among young children worldwide despite the availability of a very effective vaccine. It is a highly contagious disease, and early recognition of the classic presentation of a febrile illness preceded by cough, coryza, and conjunctivitis is necessary to help prevent spread. The multiple outbreaks that have occurred in certain regions of the world where measles is no longer endemic are very alarming and are likely from poor vaccine coverage. Interventions to address low vaccination rates and vaccine hesitancy are urgently needed. Efforts to increase awareness about the disease and strengthening vaccine advocacy are imperative in order to prevent it from reestablishing itself as an endemic disease.