Circumstances in Countries of Origin

Varicella, or chickenpox, is a communicable disease caused by the varicella-zoster virus. Descriptive characteristics and risk factors for infection are under-studied in the Eastern Mediterranean Region, particularly in Afghanistan and Syria. ^{Reference Al-Turab and Chehadeh8} Varicella vaccination is not included in Syrian and Afghan routine immunization schedules, and surveillance systems are not robust. ^{Reference Mészner, Wysocki and Richter9} In Afghanistan, outbreaks among adults are not uncommon. In 2010, there was an outbreak among recruits aged 18 to 44 years old from the Afghan National Civil Order Police in Herat, Afghanistan. ^{Reference Cowan and Davis10} Prior to the start of the Syrian Civil War, 91% of female students at a pharmacy school in Syria had immunoglobulins against chickenpox, attributed to community-level virus circulation and varicella immunization programs. ^{Reference Al-Turab and Chehadeh8,Reference Barah11} However, the Syrian Civil War has led to the degradation of health care infrastructure within the country and compromised progress made toward wide-spread immunization coverage for vaccine-preventable diseases. ^{12,Reference Kherallah, Alahfez, Sahloul, Eddin and Jamil13}

Until April 2017, immunization activities among refugees in Greece were coordinated by non-governmental organizations. ^{Reference Mellou, Silvestros and Saranti-Papasaranti14} After April 2017, immunization activities were formalized through the PHILOS program, which was spearheaded by the Greek Ministry of Health (Athens, Greece) and the Hellenic Centre for Disease Control and Prevention (Athens, Greece). However, varicella was not considered a priority vaccine, likely due to the high level of need to increase immunization coverage. Population-level immunity against chickenpox among refugees in camps across mainland Greece was largely unknown (eg, no administrative coverage estimates) and likely dependent on pre-existing, infection-derived immunity.

A 2009 study found that nearly 22% of adolescents in Greece were susceptible to chickenpox and concluded that Greek adolescents with no prior infection and without household exposures should be eligible for presumptive varicella vaccination. ^{Reference Katsafadou, Kallergi, Ferentinos, Goulioti, Foustoukou and Papaevangelou15} Eastern Europe has a high prevalence of chickenpox, creating an additional layer of risk both for refugees in the region and host communities and health systems. ^{Reference Rafila, Pitigoi, Arama, Stanescu and Buicu16} The Mediterranean and Balkan transit pathways place both refugees and host communities at high risk for chickenpox outbreaks. Therefore, while immunization against high morbidity and mortality infectious diseases takes priority in resource-limited settings, varicella vaccinations should be considered in routine immunization activities for refugees in the region.

Natural History of Varicella

Chickenpox is a vaccine-preventable disease caused by the varicella zoster virus. Prodromal symptoms such as malaise, headache, and fever, before the development of a rash; ¹⁷ prodromal symptoms are more common as patient age increases. ¹⁸ The incubation period of chickenpox is 15 days on average, with a range of 10 to 21 days. ^{Reference Ayoade and Kumar19} Cases are most contagious one-to-two days prior to rash onset and continue to be infectious for five-to-seven days after appearance. ^{Reference Freer and Pistello20} The reproductive number for chickenpox is roughly ten, meaning that an individual chickenpox case will, on average, infect ten additional susceptible individuals. ^{Reference Doucleff21}

Seasonality and Climatic Drivers of Chickenpox

Chickenpox is typically considered an early childhood disease in temperate countries, with the majority of the adult population having immunity from prior infection or vaccination. ^{Reference Misra, Gawali, Jain, Khetan and Jain22} However, age-specific immunity patterns are not concrete, especially when considering large-scale population displacement, shifts in population demographics, and changes in climatic drivers of disease dynamics. ^{Reference Tunbridge, Breuer and Jeffery23–Reference Harigane, Sumi, Mise and Kobayashi25} While it is widely accepted that there are differences in the natural history of chickenpox between tropical and temperate zones, these differences are attributed to a range of factors. ^{Reference O’Grady, Merianos, Patel and Gilbert26,Reference Mandal, Mukherjee, Murphy, Mukherjee and Naik27} In tropical climates, chickenpox has a more distinct seasonal pattern - with cases occurring in spring and winter - compared to temperate climates where cases occur throughout the year with lowest case counts during summer months. ^{Reference Sumi24,Reference Critselis, Nastos and Theodoridou28–Reference Todorova31} Across all climatic zones, temperature and rainfall contribute to the seasonality of disease. ^{Reference Hanssen, Schakel, Fontilus-Rohoman and Eeftinck Schattenkerk32,Reference Chen, Sumi, Wang, Zhou and Kobayashi33} A modelling study of chickenpox dynamics in Mallorca, a Mediterranean island, found that solar radiation and water vapor pressure were climatic drivers of chickenpox seasonality, with peaks in case numbers in June. ^{Reference Bakker, Eisenberg, Woods and Martinez29} In a retrospective study of pediatric and adolescent chickenpox cases over a 22-year period in Athens, Greece, case numbers were inversely related with air temperature, peaking in May, dropping, and rising again in late autumn. ^{Reference Critselis, Nastos and Theodoridou28} Academic calendars and other drivers of higher contact rates, such as weather patterns, population density, mobility, and crowding, also contribute to the seasonality, as well as deviations from seasonality of chickenpox infections. ^{Reference Jackson, Mangtani, Fine and Vynnycky34,Reference Karsai, Csuma-Kovács and Dánielisz35}

Relevance of Age- and Sex-Disaggregated Analyses

Age- and sex-disaggregated data analyses of communicable disease outbreaks, particularly in displaced populations, are important for timely and appropriate outbreak control measures. A study characterizing measles outbreaks in Dollo-Ado and Dadaab refugee camps in Ethiopia and Kenya found that immunization activities in the early stages of outbreak response were not targeting the age groups that were driving the outbreak. ^{Reference Navarro-Colorado, Mahamud and Burton36} Similar to measles, the clinical severity of disease increases as chickenpox cases become older. ^{Reference Tunbridge, Breuer and Jeffery23} A study in the United Arab Emirates found that adults with chickenpox had increased risk of severe clinical symptoms and complications such as pneumonia. ^{Reference Abro, Ustadi, Das, Abdou, Hussaini and Chandra37} In European Medicines Agency member countries, adults experienced higher morbidity and mortality due to chickenpox. ^{Reference Riera-Montes, Bollaerts and Heininger38}

The objective of this study is to not only present the scale of chickenpox outbreaks in refugee camps on mainland Greece, but also to describe who makes up the case population in terms of age and sex. These steps are important for informing the implementation of appropriate outbreak control measures in future early childhood, vaccine-preventable disease outbreaks in complex emergency and refugee settings.