**Abstract**

In the early twenty-first century, measles was completely eradicated in the United States of America (USA) and almost eliminated in Canada. This was greatly due to most of the population being vaccinated against the virus. In 2018 and 2019, the USA and Canada experienced a rapidly developing measles virus outbreak due to growing debates about vaccine efficacy and side effects. Therefore, some people refused to vaccinate their children against measles, as well as many other life-threatening preventable diseases. This led to a major measles outbreak and health concern in the USA, Canada, and globally. Some countries including the Democratic Republic of the Congo (DRC) reported a significant number of cases and casualties resulting from measles, mainly due to the lack of funding for vaccines, as well as inadequate vaccination coverage in certain socio-demographic areas. People traveling from these countries can easily transmit the disease, though there has been a steep decline in cases since the travel ban due to coronavirus disease-2019 (COVID-19). The number of unvaccinated children currently in the USA and Canada has quadrupled since 2001. Over the past couple of years, most of the measles cases have been diagnosed in those who either did not receive the measles vaccine or complete the recommended doses of the vaccine. This paper reviews the measles outbreak, in recent years, among unvaccinated individuals in the USA, Canada, and globally.

**Keywords:** measles, child, adult, vaccination, disease outbreaks, United States of America, Canada, global, the Democratic Republic of the Congo

### **1. Introduction**

The measles virus also referred to as rubeola from the *Paramyxoviridae* family, is considered a rapidly transmissible, vaccine-preventable disease affecting global populations. It is an enveloped non-segmented, negative-strand RNA virus that encodes six known structural proteins: N, P, M, F, H, and L; as well as two nonstructural proteins V and C [1]. Virus-host transmembrane fusion requires hemagglutinin and fusion glycoproteins, while only the hemagglutinin glycoprotein is

required for host-cell attachment [1, 2]. In humans, the virus has a receptor binding preference for the CD46 protein [3]. It is possible to trace viral ancestral lines; the wild type, for example, is organized into eight clades, and twenty-two confirmed genotypes [2].

The clinical presentation may provide a relative timeline of primary exposure, as well as the progression of symptoms. The incubation period averages about eleven days until presenting with high fever, cough, coryza, and conjunctivitis. Within two to three days, koplik spots typically appear, and from three to five days a rash may appear on the hairline. Rash grouping could be identified as it spreads downwards to the trunk and limbs, along with increasing high fever. The measles virus does not only have the designation as a highly contagious human disease, but it is also often linked to the global infant, childhood, and adult morbidity and mortality, with the age group five to twenty years, having fewer complications than the elderly. Common complications include diarrhea, otitis media, pneumonia, encephalitis, seizures, and death. It is spread by air droplets and is believed to infect the respiratory tract, initially infecting alveolar macrophages and dendritic cells, spreading deep into the lungs, lymphatics, and then the rest of the body [4, 5].

With over 100,000 deaths annually and growing, the measles virus remains one of the major causes of vaccine-preventable infant death worldwide [6, 7]. With the global emphasis on the COVID-19 pandemic, the number of administered vaccinations decreased, creating the potential for a surge in measles outbreaks [8]. The objective of this paper is to provide a much-needed review and study of measles outbreaks in unvaccinated and partially vaccinated children and adults globally.
