Chicken pox is an acute, common, and highly contagious disease caused by the varicella zoster virus (VZV) 1. Chicken pox is predominantly a childhood disease that is characterized by pruritic vesicular exanthema with systemic symptoms such as fever, loss of appetite, and malaise 2. In nonvaccinated populations, primary infection tends to occur at a younger age 3.
The disease can be benign and self-limiting in children, but in adults and immunocompromised hosts, it can be severe with high morbidity and mortality 4. VZV may induce pneumonia or encephalitis, sometimes with persistent sequelae or death. Secondary bacterial infections of the vesicles may leave disfiguring scars or result in septicemia 5.
Chicken pox is five times more likely to be fatal in pregnant than in nonpregnant women 6. Primary VZV infection during the first two trimesters of pregnancy may result in intrauterine infection in up to a quarter of the cases 7. However, the reported rate of spontaneous abortion following acute varicella infection did not exceed the rate of abortion in pregnant women without chicken pox. However, congenital anomalies described as congenital varicella syndrome can be expected in about 12% of infected fetuses 8.
In Saudi Arabia, chicken pox is a monthly notifiable disease and it is the most commonly reported viral infection. It should be compulsorily reported, and therefore all individuals diagnosed with chicken pox at the emergency department and at all outpatient clinics are reported to the Preventive Medicine Department for further reporting to the Ministry of Health for control measures. Notification rules follow the National guidelines of reporting of communicable diseases that classify this disease as to be notified within 1 week from incidence by the ‘The weekly Epidemiological Reports’ (class II reporting). The Saudi Ministry of Health in 2007 reported that 16–25% of chicken pox cases occurred in adults. Subsequently, complications and hospitalization secondary to chicken pox are very common in Saudi Arabia 9.
A major epidemic was recorded in 2007, which resulted in the introduction of varicella vaccine as a part of the childhood extended program of immunization (EPI) in Saudi Arabia 10,11.
Efforts to prevent and control infections are a cornerstone in the health protection programs set by the Saudi Ministry of Health.
This study aimed to determine the occurrence, age, and sex distribution of registered clinical cases of chicken pox and estimate trend and seasonal variation of registered clinical cases of chicken pox from 2007 to 2012 in the south province – namely, in Khamis Mushyat area.
Materials and methods
Study setting and design
A retrospective record-based study was carried out. The study was conducted in the Preventive Medicine Department of the Armed Forces Hospital of South Region (AFHSR), Khamis Mushyat; this is a 500-bed hospital, with eight satellite outpatient clinics. It serves soldiers, officers, employees, and their dependants, ∼900 000 people.
The research protocol was approved by the Hospital Research and Ethics Committee. As no personal data were recorded for the purposes of the study, written informed consent from patients for study participation was not required.
Data were collected by a retrospective review of the existing anonymous surveillance records and book registries of chicken pox in the Preventive Medicine Department from 2007 to 2012. The collected data included the number, age, and sex of registered cases.
Each patient had a unique hospital identification number that was used to detect and exclude duplicate cases.
- Descriptive analysis: data were presented in the form of numbers and percentages. The age classification used in this study was based on the age classification used in the surveillance system in KSA.
- Time series analyses
- Identifying seasonality: the data of chicken pox cases from 2007 to 2012 were reported in months to test for data pattern (seasonality and trend). Seasonality and trend were tested by plotting the monthly number of clinically diagnosed cases over time period to identify any repeated pattern. The ‘Seasonal Index’ was also calculated (Seasonality Index of a period indicates how much this period typically deviates from the annual average). It is calculated by dividing monthly average over time average multiplied by 100.
- Building and analyzing Exponential Smoothing Model (Winter Model) 12: building a best-fit exponential smoothing model involves determining the model type (does the model need to include trend and/or seasonality) and then obtaining the best-fit parameters for the chosen model. An examination of the autocorrelations and partial autocorrelations of the residuals provide stronger evidence for or against the model. Winter’s multiplicative exponential smoothing has level, trend, and season parameters and is described by the following equation 12:
The goodness-of-fit of the models was evaluated through deviance residuals. A P-value of 0.05 or less was considered as the criterion for statistical significance. Analyses were conducted with SPSS, version 20 (IBM Corp., Armonk, New York, USA).
The total number of patients reported to have clinical chicken pox during the whole study period (6 years) was 2515 (Table 1 and Fig. 1), distributed as follows: 754 in 2007, 523 in 2008, 394 in 2009, 346 in 2010, 271 in 2011, and 227 in 2012, showing an average monthly incidence of 34.9 cases. The highest seasonal index was during March and April (285.7 and 232.7, respectively), whereas the lowest was during summer months (July and August).
As regards sex distribution, the percentage of infected male patients was higher compared with female patients, with nearly the same percentage over the years (Table 2).
The age distribution of registered cases of chicken pox showed that most of the cases (up to 65%) occurred in the age group of 5–14 years, and only 1% of cases occurred in individuals above 45 years (Table 3).
The series in Fig. 2 shows a downward trend; that is, the series values tended to decrease over time. The series also has a distinct seasonal pattern with annual rise during March and April (Fig. 2). The plot of data over time suggested a model with both a trend component and a seasonality component. This implies a Winter’s model, which assumes that the series has a trend and seasonal variation.
The parameters of Winter’s model are presented in Table 4 in which all parameters (alpha, gamma, and delta) were significant. Moreover, the mean absolute error of the model was small (6.2) and R2 was very high (0.945). Analysis of the autocorrelations and partial autocorrelations of a model’s residuals demonstrated that there is no significant peak in either the autocorrelation function or the partial autocorrelation function; the peak is within the confidence bounds (Fig. 3), and Box–Ljung statistics for residual autocorrelation are not significant (0.534). All this indicates that the Winter’s model provides a good fit to the data.
Moreover, the model performance characteristics were verified or validated by comparison of its forecasts with actual data (Fig. 4) in which the forecast values showed good agreement with the actual values, indicating that the model had a satisfactory predictive ability.
This study evaluated the trends of one of the commonly seen infectious diseases in clinical practice in the southern region of Saudi Arabia over a period of 6 years (2007–2012). The number of chicken pox cases showed a downward trend this may be due to the action taken by the Saudi Ministry of Health in 2008 when it introduced varicella vaccine as a part of the EPI program, possibly together with the fact of saturation of susceptible 13.
However, the decline in the number of cases found in this study should not be regarded as a sign that chicken pox is no longer a threat to adults in the Saudi Arabia population. Surveillance and seroprevalence efforts should be continued and sometimes even extended for vaccinated populations, as waning immunity, decrease in natural exposure, immigration, or other reasons may lead to unexpected breakthrough illness in the future.
The incidence of chicken pox was reported to be similar in both sexes in most of the medical literature 14,15, whereas the number of cases is slightly higher in male patients in our study. This matched with the study conducted in Qassim region of Saudi Arabia (2007) 16.
Most of the studies conducted in developing countries reported that chicken pox is more common in the age group 4–10 years. In our study also the highest percentage of cases (58%) occurred in the age group 5–14 years. In the USA, during the prevaccination era, about 90% of cases occurred in individuals who were younger than 15 years 17,18. In our study, the percentage of chicken pox cases in individuals younger than 15 year was 78.9%, which matched with other reports in Saudi Arabia 16,19. The increased risk of chicken pox in this age group may be attributed to increased possibility of contact with sick children at schools, allowing a greater opportunity of virus transmission. Several studies from developed countries reported high proportion of chicken pox in children aged 0–4 years. However, in our study the proportion of cases in that age group was low relative to the age group of 5–14 years. This might be explained by the fact that most of the children in this age group in Saudi Arabia do not attend day-care centers and are therefore less exposed to infection.
Our results revealed that the percentage of cases among adults was about 25%. This has special importance, as chicken pox is a more severe disease among adults: the course of the disease is protracted, and the risk of severe complications is greater and mortality rates are higher. It has been estimated that the risk for death from chicken pox in adults is 23–29 times higher than that in children 19.
Chicken pox is one of the infectious diseases that shows seasonal patterns of incidence. The cause of seasonality may be either environmental or social. High temperature causes the inactivation of varicella virus, whereas overcrowding and social contact enhance the spread of infectious particles 20,21.
The study findings indicated that the peak number of cases remained greatest during March and April months throughout the time period evaluated. These findings are similar to those reported by a study conducted in tropical region in which it has been found that the transmission of the virus was reduced in humid hot climates 22.
There were some limitations to this study. First, there were no data about hospitalizations, complications, or death from varicella, nor on the occurrence of the disease in pregnant women. Second, no laboratory confirmation of the clinical cases was carried out.
Confirming reports from elsewhere, we found evidence for a decreasing trend in chicken pox infection in the Southern region of Saudi Arabia, with seasonal peak during March and April. This suggests that public health interventions targeted at reducing such infectious diseases showed success in controlling the disease among populations in the southern region of Saudi Arabia. However, it was found that about one-quarter of cases recorded were in the age group of 15–45 years. This group most probably comprised those who missed the vaccination when they were children (i.e. the vaccine had not been introduced yet).
The following are recommended: (a) to conduct a future study on the severity of chicken pox infection in adults (hospitalization, complications, and death) so as to determine the burden of the disease in the country; and (b) to conduct a national survey for the seroprevalence of markers of infection with varicella zoster among adults. This estimate may provide a clue about naturally immune individuals and those who need to be vaccinated.
Conflicts of interest
There are no conflicts of interest.
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Keywords:© 2014 Egyptian Public Health Association
chicken pox; seasonal variation; trend; varicella zoster virus; winter model