Chickenpox: Practice Essentials, Background, Pathophysiology

Varicella, commonly known as chickenpox, is endemic globally. In temperate climates, it predominantly affects children, with the highest incidence observed among preschool and elementary school-aged populations; less than 5% of adults are susceptible. [4] Seasonally, the disease typically manifests in late winter and early spring. In contrast, in tropical climates, varicella infection generally occurs later in childhood and is more prevalent among adults, particularly in sparsely populated areas. The disease incidence in these regions peaks during the driest and coolest months. Its incidence was reduced during the COVID pandemic. In Poland, the incidence of chickenpox by 2022 was found to be back at pre-COVID pandemic levels. [11]

The implementation of the childhood varicella vaccination program in the United States in 1996 has led to substantial reductions in the incidence of this disease. [4] Although varicella remains endemic, the risk for VZV exposure in the United States now is comparatively lower than in most other regions globally. As of 2019, 18% of countries had integrated routine varicella vaccination into their national immunization schedules, and an additional 6% target vaccinations to specific high-risk groups.

Due to the worldwide prevalence of varicella, all travelers susceptible to the virus are at risk for infection during international travel. [4] Additionally, exposure to herpes zoster can result in varicella among susceptible individuals, although it is significantly less transmissible than primary varicella infection. Infants, adults, and immunocompromised individuals lacking immunity are at the highest risk for severe varicella.

United States

In the early 1990s, before the introduction of the chickenpox vaccination program, chickenpox was extremely prevalent in the United States. [12] Annually during this period, over 4 million cases of chickenpox were reported, leading to between 10,500 and 13,500 hospitalizations, and resulting in 100 to 150 deaths. Notably, a significant portion of these cases and hospitalizations, as well as half of the deaths, occurred among children. This underscores the substantial impact of chickenpox on pediatric populations prior to widespread immunization efforts.

International

Countries with tropical and semitropical climates have a higher incidence of adult chickenpox than do countries with a temperate climate (eg, United States, Europe).

In the United States, high vaccination rates are achieved across many states and jurisdictions, with mandates requiring children to be vaccinated against vaccine-preventable diseases before starting school. [13] Conversely, in Japan, whereas the varicella vaccine was incorporated into routine immunization programs in 2014, it is not mandatory for school entry, resulting in lower vaccination rates compared to the United States. The basic reproduction number for varicella, estimated to be between 8 and 10, suggests that the current vaccination coverage in Japan is inadequate to control the spread of the disease, highlighting the need for increased vaccination efforts.

Additionally, although herpes zoster predominantly affects individuals older than 50 years, a decrease in varicella incidence could lead to an increase in herpes zoster cases among younger populations. [13] This shift underscores the importance of herpes zoster vaccination, particularly for those with diminished cell-mediated immunity (CMI) against VZV. Developing methods to assess CMI activity is crucial for identifying individuals at risk and effectively targeting vaccination efforts.