Varicella zoster virus (VZV) is a ubiquitous virus that infects humans. VZV causes a highly contagious disease called varicella or chickenpox, which has an attack rate of 65 to 85% in susceptible individuals followed by household exposure [1, 2]. Although varicella is a self-limiting disease, the risk of hospitalizations and complications following varicella among pre-school and school children is higher than that was reported a decade ago .
VZV infections occur worldwide but the epidemiology differs greatly between tropical and temperate climates. In temperate countries, children are predominantly affected by VZV infections and the sero-conversion to anti-VZV IgG occurs usually during late childhood and thus adults show up to 90 to 95.0% serological evidence for the past exposure to the virus . Avaccine naive 1–17 year old German study sample showed an increasing sero-prevalence for anti-VZV IgG with 60.0% and >90.0% of the children showing immunity to varicella by theage of4 and 9 years, respectively . Moreover, in temperate countries VZV infection rate is high during winter and early spring . The epidemiology of varicella is partly understood in tropical and subtropical regions. Differences in exposure rates to VZV infection in different age groups have been noted with different hypotheses [7, 8]. The exposure to the virus at late childhood or young adulthood causes high morbidity in the high school and university students and young work force in tropical nations. The climatic factors like humidity, socio-economic conditions and cultural practices appear to play a role for the differences in the exposure to the virus in the tropics [9, 10].
In tropical countries, the VZV infections are common in adolescents and adults . The clinical severity of the disease in adults is higher than that in children, while it could be fatal in immunocompromised and elderly individuals. These findings suggest that adults in the tropical countries maybe at high risk for acquiring VZV infection due to late sero-conversion to anti-VZV IgG and thus experiencing morbidity and a mortality rate of up to15.0% . Based on a recent South Korean study, incorporating the universal varicella vaccine to the National Immunization Programme since 2005 has not decreased the incidence of varicella, 22.5 per 100,000 persons in 2006 to 73.2 per 100,000 persons in 2013 . However, a difference in the anti-VZV IgG sero-prevalence rate is noticeable even among different tropical countries. Some of the tropical and subtropical countries such as United Arab Emirates, Saudi Arabia and Iran show sero-prevalence of more than 80.0% against anti-VZV IgG while other tropical countries like Singapore, Pakistan, India and Sri Lanka show lower sero-prevalence rates of 40–60.0%, making a significant proportion of adults susceptible to VZV in these countries [13,14,15]. On the other hand, these countries, Pakistan, India and Sri Lanka, with higher susceptibility to VZV infection than other tropical countries are located closer to the equator.
Varicella was not a notifiable disease until 2005 and only a few studies have been conducted on the epidemiology of VZV in Sri Lanka. Based on a study carried out in a selected urban and rural population in Colombo, none of the children below 5 years in the rural area had detectable anti-VZV IgG. Only 10.0% of the children in the urban population had sero-positivity for anti-VZV IgG. In the same study, a sero-positivity of 17.0% and 24.0% for anti-VZV IgG was observed among children below 15 years in urban and rural populations, respectively. In those aged 60 years, only 50.0% in the rural population were immune to VZV whereas in the urban population, 78.9% were immune to VZV .
There is limited data on the antiVZV IgG sero-prevalence among antenatal women in the country. The economic drain posed by varicella and its complications are currently not studied and the incidence data on varicella are sparse in Sri Lanka. Varicella vaccine is not included in the National Immunization Programme in Sri Lanka . However, the vaccine is available on requestfor those who are at risk of acquiring varicella including the medical and nursing students and the military trainees with negative history of varicella. Age specific community based studies are mandatory at district levels to find out the susceptibility rate among different age groups, which would be helpful in identifying the target population for vaccination in the country.
The objective of the present study was to determine the exposure rate to VZV infection in a selected group of antenatal clinic attendees at a Teaching Hospital in Sri Lanka. The study also aimed to determine theexposure rate to VZV infectionin urban, rural and estate communities; to determine the exposure rate to VZV infection in antenatal women in different age categories; to assess the suitability of using the past history of varicella as a predictor of immunity against VZV infection in the study sample.