|Immunisation against pertussis with an acellular pertussis
vaccine for children at 3, 5, and 12 months was included in the Swedish vaccination
programme in January 1996, 17 years after the withdrawal of whole cell vaccine in 1979
(1). Within months coverage reached at least 95%, mainly with a three-component pertussis
vaccine given as part of the combined diphtheria, tetanus, and pertussis (DTP) vaccine. In
some areas the three-component acellular pertussis vaccine was given on their own to older
preschool children, and in one area (Gothenburg and surroundings) a catch-up campaign with
a mono-component pertussis vaccine given on its own was started as part of a clinical
Clinical trials have shown that multi-component pertussis vaccines are more
efficacious than one or two-component vaccines both against typical disease and against
mild disease and infection (3,4). The differences may influence long term protection and
sustained herd immunity. Surveillance of pertussis in preschool children in Sweden in
autumn 1997 was intensified to study the long term effectiveness of the different
acellular vaccines, and to study the need for booster doses.
This progress report notes the marked decline in the incidence of pertussis three years
after vaccination was reintroduced in Sweden.
Since 1988, cases of laboratory confirmed pertussis have been voluntarily reported by
local laboratories in Sweden. In 1997, notification of pertussis became mandatory. Culture
confirmed and clinically notified cases of pertussis are reported to the Swedish Institute
for Infectious Disease Control (SIIDC) through a national computer-linked reporting
system, but a clinical case definition has not been established for routine reports. Late
in 1997 we began intensified surveillance of all reported culture confirmed cases in
children born in 1992 or later, and residing anywhere in Sweden except Gothenburg and its
surrounding area. These age groups were chosen to include children for which the age
specific incidence was known to be highest in Sweden, children who had received pertussis
vaccines in the efficacy trials carried out in 1992 (5), and 1993-94 (4), and children
born in or after 1996, who were given acellular DTP at 3, 5, and 12 months of age.
Parental permission was obtained to request medical records as needed. Study nurses
monitored the clinical course and detailed vaccination histories of children by telephone
as in the 1993-94 pertussis vaccine trial (4). Our primary case definition of Bordetella
pertussis infection was confirmed by culture or polymerase chain reaction (PCR)
regardless of symptoms. Typical pertussis was defined as culture or PCR
confirmed pertussis with at least 21 days of paroxysmal cough, in line with the World
Health Organization case definition (6).
In the most recent epidemic year (1994) 13 142 cases of culture confirmed pertussis
were reported, compared with 1463 in 1998. The overall incidence of pertussis in Sweden
(per 100 000 population) rose from 113 and 132 in 1992 and 1993, respectively, to 150 in
1994 then fell in successive years - 121 in 1995, 86 in 1996, 40 in 1997, and 16 in 1998.
The age specific incidences for 1992-95 and 1998 are given in figure 1.
In 1998 we followed 811 children born in or after 1992 with confirmed pertussis,
115 of whom had received pertussis vaccine (59 were fully and 56 were partially
immunised). The clinical course was more severe in unvaccinated children: 90% (627/694) of
unvaccinated cases suffered paroxysmal cough for more than 21 days, compared with 77%
(43/56) of partially vaccinated cases, and 69% (41/59) of fully vaccinated cases.
Thirty-one unvaccinated children were admitted to hospital, eight of whom stayed in
hospital for at least one week. There were six admissions to hospital (duration 1-3 days)
among children who had received one dose of vaccine, and none among fully vaccinated
In 1998, only three years after the introduction of acellular pertussis vaccines, the
reported incidence of pertussis had dropped by between 80% and 90%, and was similar to the
lowest rates observed in the 1960s, when the Swedish whole cell vaccination programme was
still effective (1). The vaccination programme has therefore been highly effective in the
short term, due in part to the very high coverage achieved as soon as the new vaccines
were licensed and the acellular DTP vaccine replaced the diphtheria and tetanus (DT)
vaccine in the infant vaccination schedule. Forthcoming data will show to what extent
catch-up vaccination of older preschool children has contributed to the reduction in the
incidence of pertussis in children born from 1992 to 1995, who - unless in clinical trials
(2,4,5) - did not receive pertussis vaccines in infancy.
Caveats should not be forgotten. Substantial numbers of cases still occur among
unvaccinated children, particularly in the 5 to 9 years age group (figure 2).
Underreporting of disease in vaccinated children (and adults) with mild illness may
overestimate the effectiveness of the present programme. Several acellular pertussis
vaccines (with one or several components) have been licensed and it will be difficult to
assess the effectiveness of individual vaccines and their respective contribution to long
term herd immunity. Data on vaccine coverage must therefore be collected continuously,
keeping track of which brand and batch of vaccine each child has received, as recommended
for post-marketing surveillance of safety (7) to enable a more detailed analysis to be
The rapid decline of pertussis in Sweden is promising, but
it remains to be shown that currently licensed vaccines in the present schedule will
control disease in the long term. Surveillance based on active case finding, laboratory
confirmation (including characterisation of pertussis strains), and the collection of data
on vaccine coverage need to continue.
Financial support was obtained from the National Institute of Allergy and Infectious
Diseases (contract no. N01-AI-15125) and from the following manufacturers: SmithKline
Beecham (Rixensart, Belgium); Pasteur-Mérieux Connaught (Toronto, Canada), and
Pasteur-Mérieux MSD (Lyon, France).