A deep dive into chickenpox epidemiology and outbreaks: A retrospective study in a tribal-dominated district of Western India

INTRODUCTION

Chickenpox is a contagious viral infection caused by the Varicella-Zoster virus (VZV). It spreads through the air or direct contact with blisters.^[1] The diseases can affect people of all ages, genders, and ethnicities.^[2] However, it is commonly seen in children under the age of 10 years.^[3] The symptoms of chickenpox are usually mild but can lead to serious complications, especially in people with weakened immune systems or pregnant women.^[4] The most serious complications from chickenpox include pneumonia, encephalitis (brain inflammation), and sepsis (a severe infection of the bloodstream).^[1] The overall mortality rate for chickenpox is low, but the disease is still responsible for a significant number of hospitalizations and deaths in unvaccinated populations.^[1,2,5] Potential transmission of chickenpox is believed to be affected by climate, and some studies have examined the relationship between weather variability and the incidence of infectious diseases.^[6-8] The exact timing of chickenpox transmission can vary from year to year and from region to region, but the disease is generally more common during the colder and spring months.^[3,9] Regardless of climate, the timing of chickenpox transmission is influenced by several factors, including vaccination coverage, population density, and the presence of susceptible individuals.^[4,10] Vaccination is the sole preventative measure for chickenpox and is readily available for children.^[10,11] Developed nations have observed a decline in chickenpox cases due to vaccination while developing nations remain at risk since the vaccine is optional in countries like India. Without vaccination, there is a gradual rise in seroprevalence with age, peaking in adolescents and adults.^[2,12] Several outbreaks of chickenpox have been reported in different parts of the world and India from time to time.^[13,14] These outbreaks highlight the importance of continued monitoring and surveillance of chickenpox to detect outbreaks early and prevent their spread.

The study explores chickenpox outbreaks and epidemiology in Dadra and Nagar Haveli (D&NH), a district in Western India with a large tribal population spanning the last nine years. The goal is to gather essential data on the disease’s spread, enabling the development of effective public health strategies to reduce chickenpox incidence and associated complications.

MATERIALS AND METHODS

RESULTS

Between 2014 and 2022, 2554 clinical chickenpox cases were reported in the D&NH district. Of these, 31.87% (814 cases) were reported during outbreaks, with the highest number of cases reported in the urban area (37.71%), followed by the tribal areas (28.94%), semi-urban/industry areas, and outof-district areas (9.95% each). A geographical distribution of the cases can be seen in Figure 1. The most vulnerable age group for chickenpox was 6–10 years old (27.74%), followed by 1–5 years old (17.08%), and 11–15 years old (15.74%). Males were outnumbered by females. Most cases (97.26%) had mild-to-moderate symptoms, not needing hospitalization. Only 2.74% were severe, requiring hospital care. Cases varied widely annually: highest in 2017 (538) and lowest in 2014 (156). Most cases occurred from February to November (50.23%), with a peak in January 2019 (232 cases). The fewest cases were in June 2022 (7). Majority were reported between March–June (32.93%), and July–October (16.84%) [Table 2].

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Figure 1:

The spatial distribution of reported cases of chickenpox and outbreaks from 2014 to 2022.

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During the past 9 years, 25 outbreaks and 3 early warning signals of chickenpox have been recorded in the district. The majority of events were reported from November to February, followed by March to June and July to October. The highest number of outbreaks occurred in tribal areas, then semi-urban/industrial areas, and lastly, urban areas. The most common locations for outbreaks and early warning signals were anganwadis/play schools/schools (17 events, 60.71%) followed by chawls/slums and communities (11 events, 39.29%). During the outbreaks, 60.71% had low intensity (attack rates <0.5), 17.86% had rates between 0.51% and 1.0%, 14.29% had rates between 1.01% and 1.5%, and 7.14% had rates above 1.5%. The most susceptible age group was 6–10 years, followed by 1–5 years, 11–15 years, 16–20 years, and those over 20 years old. The duration of chickenpox outbreaks varied widely, with 11 events (39.29%) lasting for over 10 weeks and 2 events (7.14%) lasting only 1 or 2 weeks. The remaining outbreaks lasted between 3 and 4 weeks (7 events or 25.00%), 5–6 weeks (4 events or 14.29%), 7–8 weeks (3 events or 10.71%), and 9–10 weeks (1 event or 3.57%) [Table 3]. The samples of 11 outbreaks (39.29%) were sent to the referral laboratory for laboratory confirmation. Furthermore, the results confirmed the presence of the clade-1 VZV as the causative organism in the district.

DISCUSSION

The present study is an observational retrospective study conducted in the district of D&NH of Western India. The study aims to investigate the epidemiology of chickenpox and its outbreaks in the district over the past 9 years. The current study’s findings indicate a variation in the incidence rates of chickenpox according to age, with the highest rates being observed among children aged 6–10 years. This age group has been identified as particularly susceptible to chickenpox, consistent with previous studies conducted in Iran, China, and the USA.^[9,15,16] The higher incidence of chickenpox in certain age groups may be due to a combination of factors such as higher exposure to the virus, weaker immune systems, and variations in vaccination coverage.^[3] Furthermore, our study found that the majority of cases were mild-to-moderate and did not require hospitalization, which is consistent with the findings of previous studies.^[13] The highest number of cases and outbreaks occurred in the winter and spring months (January–April), and the lowest number of cases occurred in the summer and fall months (June–November). This is consistent with previous studies conducted in different regions of the world, including Taiwan,^[7] China^[8], and the United States.^[9] One possible explanation for this seasonal variation is the impact of temperature and humidity on the survival of the varicella virus. The virus is known to survive longer in cooler and less humid conditions, which may explain why chickenpox is higher during the colder months.^[3] In addition, the winter months are associated with decreased sunlight exposure, which may lead to lower levels of Vitamin D and a weakened immune system, making individuals more susceptible to infections like chickenpox.^[17] In addition, close proximity in the house due to cold weather and school schedules may also influence the incidence of chickenpox, as children are more likely to come into close contact with each other during the winter and spring months.^[2,13,16] The higher number of reported chickenpox cases in urban areas, compared to semi-urban and tribal areas, could be attributed to several factors, including population density, better reporting systems, and high migration. Yang et al. conducted a study in China, which found that higher population density was significantly associated with an increased risk of chickenpox incidence.^[8] Moreover, urban areas may have better health-care facilities and reporting systems, leading to higher detection and reporting of cases. Chickenpox incidence has been found to be slightly higher in males than in females, which could be due to a combination of biological and behavioral factors, as male children were at a higher risk of developing chickenpox than female children, possibly due to a greater tendency for boys to engage in outdoor activities and have more close contact with other children. The study highlights the epi-centers of chickenpox outbreaks in different settings. The results suggest that outbreaks were reported in four different types of settings: anganwadi/play school, school, industry chawl, and community/slum. These settings are known to have high population density and close contact between individuals, which may facilitate the spread of the disease.

The present study calculated the attack rate for various events during the outbreak of chickenpox. Results showed that 60.71% of events had an attack rate of <0.5, 17.86% had an attack rate between 0.51% and 1.0%, 14.29% had an attack rate between 1.01% and 1.5%, and 7.14% had an attack rate > 1.5. The low attack rate in majority of events suggests that appropriate preventive measures have effectively controlled the outbreak. However, the higher attack rate in few events highlights the need for targeted interventions, such as vaccination campaigns, isolation and quarantine of infected individuals, and health education programs to prevent further disease transmission. Previous studies have also reported on attack rates during chickenpox outbreaks. For instance, a study conducted in rural India found an attack rate of 7.4%, which was higher than the attack rate observed in the present study.^[14] Conversely, a study conducted in a school in Taiwan reported an attack rate of 1.3%, which was similar to the attack rate observed in the present study.^[7] The present study analyzed the duration of 28 outbreak events and found that 25.00% of outbreaks lasted more than 10 weeks, while 7.14% lasted 1–2 weeks. The varying durations of outbreaks are consistent with previous studies. A study conducted in a school in Taiwan reported outbreaks that lasted for 3–6 weeks, while another study in primary schools in China reported an outbreak that lasted for 10 weeks.^[7,16] The duration of outbreaks can serve as an indicator of the effectiveness of control measures implemented. Shorter durations of outbreaks suggest that prompt control measures were implemented, while longer durations may indicate a failure in control measures or a delay in their implementation. According to a study conducted between 2000 and 2006, the death rate from chickenpox was reported to be 0.26/1 million population, with the majority of deaths occurring in unvaccinated individuals or those with unknown vaccination status.^[3] In contrast, the present study did not encounter any deaths related to chickenpox. In this study, 97.26% of chickenpox cases were treated on an outpatient basis, while 2.74% required indoor admission for treatment. This finding is consistent with previous studies that have reported similar distributions of hospitalization status in chickenpox cases.^[18] The confirmation of chickenpox outbreaks through laboratory diagnosis is essential for the appropriate management of the disease and the implementation of control measures. In this study, 39.29% of outbreak samples were sent to a referral laboratory for confirmation, and all of them were confirmed using both serological and molecular methods. The detection of the clade-1 VZV strain in D&NH is consistent with prior research,^[19,20] emphasizing the importance of ongoing monitoring and surveillance of VZV genotypes, particularly in areas with high incidences of chickenpox outbreaks.

CONCLUSION

The study was conducted in D&NH, India, spanning 9 years, revealed a significant chickenpox burden. Children aged 6–11 were most vulnerable. Cases were mostly mild, peaking in winter and spring. Clade-1 VZV was identified as the cause. While prevention measures helped, targeted actions like vaccinations are vital to curb future outbreaks.