| ND = Pas de données / No data
Ge = Vaccin à germes entiers / Wc = Whole cell vaccine
ac = acellulaire / acellular
* Basé sur le vaccin à germes entiers ( vaccin acellulaire depuis janvier 1997) /
Based on whole cell vaccine (shift to acellular in January 1997)
** Dans sept régions sélectionnées d'Italie / In seven selected regions of
Italy
*** Une cinquième dose de vaccin acellulaire est recommandée depuis 1998 pour
les 11 - 13 ans / A fifth dose with an acellular vaccine was introduced in 1998 at 11-13
years
† Excepté 4 années
épidémiques / Except 4 epidemic years
In group one, Finland, France, and the Netherlands
have very high coverage (>95%). There are either no official contraindications (as in
the Netherlands) or those that exist do not affect coverage. Consequently, the need for a
change to acellular pertussis vaccine for primary immunisation has not been felt
necessary.
Group one also includes Denmark and England and Wales. In Denmark, coverage with whole
cell vaccine was high for the first dose (96% - 98%), but dropped to 87% - 90% by the
third dose, perhaps because reactions to the first and second doses were thought to
constitute a contraindication. Acellular pertussis vaccine completely replaced the use of
whole cell vaccine in 1997. In England and Wales, where only the whole cell vaccine is
used, coverage has increased steadily from 41% in 1980 to 94% in 1995.
In the second group, (Italy, Sweden, and Germany) coverage has remained, until
recently, much lower. In Sweden the coverage fell dramatically from 90% in 1974 to below
5% in the 1980s. It increased in the early 1990s but remained under 50%. In 1996, the
introduction of the acellular vaccine into the national immunisation schedule, was
followed by a swift rise in coverage to over 95% (P. Olin, personal communication).
In Italy, cluster surveys of voluntary pertussis vaccination in the 1970s and 1980s
suggested that coverage for three doses was between 7.6% and 71%. Uptake has increased
since the introduction of acellular vaccine in 1995, and is now estimated to be 88%. In
Germany, routine immunisation with whole cell pertussis vaccine ceased in 1976. In 1991 it
was again recommended for routine immunisation and, in 1995, an acellular pertussis
preparation was also approved. No data are available on vaccine coverage.
The types of vaccine used differ, but there is little variation in primary vaccination
schedules. Between two and four doses are given in the first year of life: most countries
give three. The total number of doses varies between three and four. In all countries the
maximum age at which children were offered the vaccine (until 1997) was 2 years.
Surveillance and epidemiology
Pertussis is a notifiable disease in all countries except Germany and France. Among
those countries with routine surveillance, only Italy and England and Wales have case
definitions that do not require laboratory confirmation. The data provided by the
participating countries are likely to vary in reliability. Most countries could not
assess the quality of their surveillance systems. One potential source of variation is a
failure to diagnose the disease in older age groups. Therefore comparisons between
countries should be made cautiously. No analysis could be made for Germany, because
no surveillance data exist, or for France, where notification ceased in 1985 and a
hospital based paediatric sentinel surveillance system was implemented in 1996.
Pertussis is still endemic in the two countries that have had very high vaccine
coverage (above 95%) for many years and where annual morbidity data are available (Finland
and the Netherlands). Their annual incidences are between one and five cases per 100 000
and epidemic cycles are still seen. Finland experienced two outbreaks in 1983 and
1990-1991, with rates higher than 12 cases per 100 000. In 1995 and 1996, 63% of cases
were at least 10 years of age. In the Netherlands, peak incidences were noted in 1989 and
1994 (3.5 and 3.4/100 000, respectively). In 1996, however, a sudden larger than expected
increase in the incidence of pertussis (27.3/100 000) occurred before it was due,
according to the usual three and five year cycle. Infant cases (1 to 11 months of age)
accounted for only 6% in 1996 compared with 20% in 1989-1995. A larger proportion of cases
were children aged 5 years and over in 1996 (67%) than in 1989-1995 (55%). Greater
awareness, changes in diagnostic practices, or a lower vaccine coverage could not explain
the epidemic. It has been suggested that changes in the circulating strain of Bortella
pertussis could have resulted in a mismatch with vaccine induced immunity (4). In
France, where the disease was thought to have almost disappeared, a resurgence of
pertussis has been suspected, based on a study carried out in 1993-1994 (5).
The high coverage category also includes Denmark and England and Wales. In Denmark,
very large outbreaks occurred in 1976 and 1977 with rates exceeding 300/100 000, but the
incidence has since declined to <20/100 000 in 1986 and <10/100 000 in 1992-1993,
the last years for which data for the whole population are available. Since 1987, the
three to four year epidemic cycle seems to have virtually disappeared. Between 15% and 20%
of cases from 1980 to 1993 were over 10 years of age. Vaccine coverage in England and
Wales declined dramatically in the 1970s because of concerns about the safety of the whole
cell vaccine preparation. Two major epidemics in England and Wales peaked in 1978 and 1982
(at rates of 130/100 000). Since 1982, coverage has improved gradually, resulting in a
proportional decrease in incidence. The size of peaks has gradually decreased and the
interepidemic period has lengthened since 1990, when high vaccine coverage was achieved
for the first time. The proportion of cases 10 years old or older, based on clinical
surveillance, was between 5% and 10% from 1974 to 1989, but since 1990 has steadily
increased to reach 16% in 1994-1995.
The uptake of pertussis vaccine in Italy is poor, and the incidence of the disease has
not decreased. The four year cycles are well documented and the average incidence is
20/100 000. This is probably an underestimate, given the data from countries with much
higher vaccine coverage. About 10% of cases are over 10 years of age. In Sweden, the
annual reported incidence of culture confirmed cases of pertussis was less than 20/100 000
during the 1970s. After vaccination was discontinued in 1979, the incidence increased
sharply to an annual rate of over 100/100 000 from the mid-1980s onwards.
Discussion
The comparability of pertussis surveillance data is more questionable than for
diphtheria as both the case definition and the accuracy of the surveillance data vary
between countries. Nevertheless, we identified two different situations with regard to the
level of control of the disease. Differences in vaccination schedules between countries
for parameters such as age of completion of the first series and total number of
doses/boosters are few and seem not to play an important role in differences in the
current epidemiological profile of the disease. As for diphtheria, vaccine coverage in
children seems to be the main factor that determines pertussis incidence. The history of
pertussis vaccine coverage shows how varying perceptions by the public and health
professionals of the value of vaccines and their safety can lead to very different
decisions and levels of effectiveness of vaccination programmes (6). Even in countries
where coverage has been very high for a long period, however, B. pertussis is still
circulating and epidemics still occur. The proportion of cases over 10 years of age
increases with coverage as shown by comparisons within and between countries. This is
mainly because vaccine induced immunity is lost in older children in the absence of
natural boosting with wild pertussis bacteria, whose circulation has fallen dramatically
(7). In England and Wales the susceptibility in older cohorts could be also explained by
lower coverage when they were scheduled to receive the vaccine. Unlike some other
communicable diseases, the severity of pertussis decreases with increasing age of
infection, although adult cases may be a reservoir for infants too young to have been
protected by a full series of primary vaccinations (8). These factors have been
used to call for a booster dose in older members of the population.
Concerns about the perceived risk of vaccine related adverse events associated with a
whole cell booster (9), however, have led two of the group one countries to introduce or
plan to introduce the acellular vaccine as a booster. In France, the 1998 immunisation
schedule includes an acellular booster at 11 to 13 years and in the Netherlands, a trial
starts in 1998 on the effect of a booster dose of acellular vaccine at 4 years of
age. In addition, in Sweden, a study of a booster of acellular pertussis vaccine given at
4, 5, or 6 years of age is in progress.
In group two countries the age distribution of cases does not show a significant shift
in infection towards older age groups. Thus increasing and maintaining the coverage of the
primary series to over 95% seems to be the priority rather than introducing booster
doses.
It is too early to assess the impact on the current incidence of the disease of the
very recent introduction of acellular pertussis vaccine either in the primary series or as
a booster dose. Preliminary data show, however, that in countries where the acellular
vaccine for primary immunisation has been adopted, coverage of pertussis vaccine is
increasing tremendously. |