Varicella (chickenpox) is a common illness with a relatively distinct clinical
picture. Transmission of the varicella virus is by respiratory droplets,
aerosol or direct contact with a patient's skin lesions. Varicella is
a highly contagious disease with a clinical attack rate of 65% to 85%
following household exposure of susceptible individuals . Serological
studies across Europe have shown that antibodies to the pathogen are
mostly acquired before 15 years of age .
Bacterial skin superinfection is the most common complication of varicella,
affecting nearly half of all patients . Serious and life-threatening
complications, such as varicella pneumonia and encephalitis, rarely
occur. Acute cerebellar ataxia is the most common neurological complication
of varicella, and occurs in approximately one in 4000 varicella cases
among children younger than 15 years of age. Varicella pneumonia is
the most common complication in adults and requires hospitalisation
in approximately one of every 400 varicella cases .
After infection, varicella zoster virus (VZV) establishes a life-long
latency in the cranial and dorsal root ganglia. In approximately 15%
of all infected individuals, latent VZV reactivates to cause herpes
zoster over a lifetime (HZ) . Triggers for reactivation are poorly
understood. Factors that seem to be responsible for increased frequency
of HZ include, among others: impaired cell-mediated immunity (CMI),
bone marrow and solid organ transplantation, ageing, UV light, injury,
stress. Impaired CMI has been generally identified as a factor predisposing
to zoster, while the role of other conditions in its development remains
to be elucidated.
Varicella and HZ have not been placed on the list of reportable communicable
diseases according to the EU Directive. Only six member states have
legal provisions for case-based mandatory notification of varicella,
and only two countries provide primary care surveillance data for HZ
In Slovenia, varicella became a notifiable communicable disease in 1977.
Case-based mandatory notification of HZ was enforced by the Communicable
Disease Law in 1995. The data collected include demographics (age,
sex), date of onset of illness, complications, hospitalisation and
outcome. Notifiable varicella complications that are part of the same
dataset (ICD-10 codes) include: B01.0 varicella meningitis, B01.1 varicella
encephalitis and B01.2 varicella pneumonia. For HZ only two ICU-10
codes are notified, i.e. B02.0 zoster encephalitis and B02.1 zoster
meningitis. Deaths are also reported.
Laboratory confirmation of the disease is not required for the notification
of varicella and HZ. The descriptive epidemiology of cases notified
between 1996 and 2005 is presented.
The annual number of notified varicella cases ranged
from 9120 to 15 538 (incidence: 456/100 000 to 777/100 000). Females
outnumbered males, but the difference was not statistically significant
(50.4% versus 49.6%). The majority of cases reported (75%) were in
pre-school children under seven years of age. The incidence was highest
in three year olds [FIGURE 1].
The rate of serious complications, including pneumonia, meningitis, and
meningoencephalitis, was low. The average ten-year incidence of varicella
meningitis and meningoencephalitis was 2.1 per 1 000 000 population (all
age groups). For varicella pneumonia the incidence was even lower (0.8/1
000 000). The hospitalisation rate, defined as the number of hospitalisations
per 1000 varicella cases, is indicated in TABLE 1. No death due to VZV
has been notified.
The number of notified varicella cases increased each month from October
to January, and declined from February to July [FIGURE 2]. The lowest
number of cases was recorded in August and September.
HZ was placed on the list of notifiable communicable diseases in 1995.
The initial number of reported cases was low but has been steadily
increasing. In 2005, 1627 cases of HZ were notified (81.3/ 100 000).
The highest incidence was recorded in the elderly [FIGURE 3]. The 10-14
year age group had a notably higher incidence than young adults aged
20-29 years. As expected, more females (59.5%) than males were affected
by HZ, and the female incidence was higher in almost all age groups.
The average ten-year incidence of zoster meningitis and zoster encephalitis
was 3.05 cases per 1 000 000 inhabitants. Hospitalisation rate, i.e.
the number of HZ hospitalisations per 1000 HZ cases, is shown in TABLE
HZ infection does not seem to possess a seasonal pattern, yet the
largest average number of cases was reported in August and the smallest
in February. The difference, however, did not reach statistical significance
(Student t-test). There is no clear explanation of why a greater number
of cases were notified in the second half of the year than in the first
half [FIGURE 4].
Varicella and herpes zoster are not obligatory reported diseases in
most of the European Union member states. In some states, varicella
epidemics or complicated cases only are notified. Some countries collect
data on varicella provided by the sentinel surveillance system based
on primary care or paediatricians . In Slovenia, the case-based
mandatory notification of varicella has been in place for 30 years.
Varicella is the most commonly notified communicable disease as almost
everyone is infected before reaching adulthood. Case definitions for
varicella or HZ have not yet been formulated. Both diseases run a rather
typical clinical course. There are only a few illnesses with vesicular
rash that may be misdiagnosed as varicella or HZ (e.g. extensive herpes
During the observation period, the overall incidence of notified varicella
cases was 456-770/100 000 (650/100 000). The incidence was highest
in three-year and four-year- olds (104,400/100 000 and 9,400/100 000,
respectively). Children less than one year of age are usually cared
for at home, as the mother’s maternity leave starts one month
before the expected date of delivery and ends eleven months after the
birth. According to the 2005 data provided by the Statistical Office
of the Republic of Slovenia (http://www.stat.si/eng/index.asp),
only 14.1% of children under two years of age but 36.7% of three-year-olds
attended kindergarten. Increased pre-school attendance coincides with
the highest varicella incidence in this age group. In the United States,
the overall incidence of varicella recorded through BRFSS (Behavioral
Risk Factor Surveillance System) in 1998 was 1650/100 000 . The
age-specific incidence was highest in children aged between one and
four years (8260/100 000) and between five and nine years (7640/100
000). BRFSS is an ongoing, random-digit dial telephone survey and gathers
information on health characteristics, risks and preventive behaviours.
According to the data collected in Canada and the United Kingdom for
the period 1979–1997, the average consultation rates for varicella
in children aged 0-4 years were 2 345/100 000 population (Canada) and
3 414/100 000 (UK) . The Canadian data were obtained using annual
physician billing claims from the province of Manitoba, which has a
population of approximately 1 100 000 and a birth cohort of 16 000.
The UK data were derived from a sentinel surveillance programme using
a representative sample of practitioners throughout England and Wales.
The incidence provided by the French sentinel system was 1000 to 1350
cases per 100 000 inhabitants, 80% of them belonging to the age group
of one to nine years . According to the most recently collected
Dutch data (that is, between 2001 and 2002), the average annual incidence
of consultations for varicella was 253.5/100 000; it was highest in
children aged less than one year (3101/100 000) and in children aged
between one and four years (3014/100 000). The data were obtained through
the sentinel network of general practices covering 1% of the Dutch
The overall incidence of notified varicella cases in Slovenia was higher
than that obtained through the sentinel surveillance systems in the UK
and the Netherlands, but lower than the figures provided by the French
sentinel system and BRFSS [2,3,7,8]. The BRFSS collects data through
telephone interviews. Theoretically, all subjects with varicella can
be identified through BFRSS, that is, both those who consult a doctor
and those who do not. Incidences generated by BFRSS are therefore expected
to be higher than incidences calculated on the basis of data collected
through the sentinel surveillance system on patients who consulted their
general practitioner or paediatrician. Limitations of data provided by
BFRSS include the potential for recall bias and uncertainty of self-reported
diagnosis: the accuracy of the reporting is not validated. The higher
incidence of notified cases in Slovenia, compared with data collected
through the sentinel system in the UK and the Netherlands, may be attributed
to social factors. Consultation rates may vary between countries. Children
with varicella are not allowed to attend kindergarten and a parent who
may normally work must stay at home with them. In Slovenia, a caregiver
must obtain a medical certificate from the paediatrician to claim compensation
from the health insurance agency for days missed from work due to child
care. It should be pointed out that the number of varicella cases varies
from one year to another. Differences in the incidences reported may
be due to comparison of different time periods.
Since the 1970s the average age at infection has decreased notably
(data not shown) as a result of a significant increase in nursery and
kindergarten attendance . In recent years, the disease has been
increasingly notified in adults . A similar trend has been observed
in the UK, where the number of varicella cases has doubled in 0-4 year
old children and halved in children aged 5-14 years. This downward
shift in age at contracting varicella may result from increased social
contacts between pre-school children. .
Only two cases of meningitis and meningoencephalitis and less than
one pneumonia case per million population were notified during a ten-year
period. Varicella runs a severe and complicated course in adults and
immunocompromised patients. Due to an increasing number of patients
treated with immunosupresive agents and higher number of adult patients
with varicella, a higher incidence of notified cases with complications
would be expected. The reason of under-reporting severe cases is not
clear. The retrospective study analysed hospitalised patients admitted
to one tertiary care centre with varicella during a four year period
(from 1995 to 1998). Two deaths caused by varicella were identified,
giving specific mortality 1 per 30 000 notified varicella cases in
Slovenia . Serious complications observed in the case-based mandatory
system occurred less frequently than reported in the German study:
a crude incidence of severe varicella complications in previously healthy
children was 0.8/100 000 . The study showed that the most common
complications were neurological complications, with cerebellar ataxia
being the most frequently diagnosed condition, followed by encephalitis.
The highest age-specific hospitalisation rate for adults with varicella
aged over 30 years was reported in the Dutch study .
The seasonal distribution of varicella cases is uneven, with the lowest
incidence during the 10-week school summer holidays. Summer holidays
for primary and secondary schools start on 24 June and end on 1 September.
During this period the number of children in kindergarten drops, as most
children spend their holidays with parents or grandparents. Therefore,
during summer months, children socialise less, and the possibility of
viral transmission is reduced. Other school holidays (autumn, Christmas
and winter holidays) last only one week, which is too short a period
to help reduce the spread of varicella. Two peaks of varicella cases
recorded in some countries are attributable to different school calendars
HZ has been on a list of notifiable diseases for ten years, yet HZ
cases have undoubtedly been under-reported. It is difficult to explain
why varicella cases are more accurately notified than HZ cases. According
to the population-based data, the HZ incidences range from 1.2 to 4.8
per 1000 population [2,8,12,13]. Older studies reported lower numbers
than the more recent ones, most probably as a result of the ageing
population in the developed part of the world.
There has been an increase in the HZ incidence along with prolonged longevity
in the developed world. In 2005, the notified HZ incidence in Slovenia
was 0.81 per 1000 population, which is six-fold less than the highest
incidence published. Most of the cases reported were elderly individuals
over 70 years of age. The important thing to note is that teenagers were
more frequently affected than young adults, an observation that we are
not able to explain. HZ incidence increased with the increasing age in
most but not all studies. As reported by Mullooly, the HZ incidence in
females aged 10-17 years was higher than the incidence in the adjacent
age group of 18-29 years (217/100 000 vs.177/100 000) . In the UK,
the decrease in varicella consultations in schoolchildren coincided with
an increase in the HZ incidence in the same age group . Our surveillance
data showed higher hospitalisation rates for HZ than for varicella, which
accords with the data published by Brisson .
Varicella vaccination coverage in Slovenia is very low. The majority
of vaccinees are immunocompromised patients and seronegative individuals
recently exposed to a child with varicella. Vaccination against varicella
is recommended for healthcare workers who are not immune, as in the
UK . However, vaccination in line with the current recommendations
is practiced in very few healthcare institutions in Slovenia.
An overview of varicella zoster vaccination policies in Europe was
provided by the European Sero-Epidemiology Network 2 (ESEN2), which
comprises 22 European countries and Australia . Germany is the only
European country with routine childhood immunisation against VZV: VZV
vaccination by a single dose given at the age of 11-14 months was incorporated
into the routine immunisation schedule in July 2004. Three more countries
have recently recommended vaccination of children against VZV, and
a further five are considering introducing routine VZV immunisation
The policy of universal vaccination against varicella in childhood
will undoubtedly help reduce varicella disease in the vaccinees .
The introduction of this mass vaccination programme demands meticulous
surveillance of varicella and HZ for at least two reasons: a) to document
the drop of varicella cases after the introduction of varicella vaccine
and to ensure there is no ‘epidemiological shift’ of varicella
cases to older age groups, potentially causing more complications,
and b) to monitor the HZ epidemiology. An upward trend in HZ cases
may occur without natural boosting of immunity which is currently provided
by intensive circulation of VZV in the community. An increase in the
number of HZ cases was predicted by a mathematical model and by a population-based
study conducted in the United States [16,17]. Frequent contacts with
children seem to protect against VZV reactivation .
A comparison of our surveillance data and the sentinel data on varicella
incidence indicates that a large proportion of actual varicella cases
have been reported in Slovenia. The system of reporting HZ cases is
much less reliable, and we estimate that, compared with other studies
published, only between a quarter and a fifth of all cases are notified.
After the introduction of universal vaccination, the case-based surveillance
of varicella should continue to identify shifts in age groups, as well
as outbreaks and breakthrough infections in vaccinated persons. Before
the introduction of a routine varicella vaccination programme, an effort
is needed to enhance surveillance of varicella complications and HZ.