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 children.

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 carried out.

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).

1. Romanus V, Jonsell R, Bergquist S-O. Pertussis in Sweden after the cessation of general immunization in 1979. Pediatr Infect Dis J 1987; 6: 364-71.

2. Trollfors B, Taranger J, Lagergård T, Lind I, Sundh V, Zackrisson G, et al. A placebo-controlled trial of a pertussis-toxoid vaccine. N Engl J Med 1995; 333: 1045-50.

3. Hewlett EL, Cherry JD. New and improved vaccines against pertussis. In: Levine MM, Woodrow GC, Kaper JB, Cobon GS, editors. New generation vaccines (2nd edition). New York: Marcel Decker, 1997: 387-416.

4. Olin P, Rasmussen F, Gustafsson L, Hallander H, Heijbel H, Gottfarb P. Randomized controlled trial of two-component, three-component, and five-component acellular pertussis vaccines compared with whole-cell pertussis vaccine. Lancet 1997; 350: 1569-77.

5. Gustafsson L, Hallander HO, Olin P, Reizenstein E, Storsaeter J. A controlled trial of a two-component acellular, a five-component acellular and a whole-cell pertussis vaccine. N Engl J Med 1996; 334: 349-55.

6. WHO meeting on case definition of pertussis. Geneva, 10-11 January 1991. MIN/EPI/PERT/91.1:4-5

7. Miller E, Waight P, Farrington P. Safety assessment post-licensure. In: Plotkin S, Brown F, Horaud F, editors. Preclinical and clinical development of new vaccines. Dev Biol Stand 1998; 95: 235-43.