The epidemiology of pertussis in New Zealand and risk factors for pertussis in New Zealand infants

Abstract

Literature review Pertussis mortality and morbidity Mass immunisation was associated with a decrease in pertussis mortality and a profound reduction in pertussis incidence. Despite this pertussis remains prevalent. Infants account for the majority of pertussis deaths and hospitalisations. Immunisation Pertussis vaccines protect against disease rather than infection. Despite immunisation pertussis remains endemic. The efficacy of different whole cell and acellular pertussis vaccines varies considerably. There has only been a small increase in immunisation coverage in New Zealand over the past 25 years. Currently between 80% and 90% of New Zealand children receive the primary immunisation series. Other epidemiological features Bordetella pertussis is a highly infectious organism. Neither infection nor immunisation results in lifelong immunity. Pertussis affects all age groups. It is more severe in females than in males. The incidence has always been highest in infants and children but the reported incidence in adults is increasing. Pertussis epidemics occur at four yearly intervals. The epidemic periodicity has not been changed by immunisation. Risk factors for pertussis Contemporary case control studies from the United States have shown that exposure to someone outside of the home with pertussis increases the risk of introduction of pertussis into the home and that infants of adolescent mothers and of mothers with a preceding coughing illness are at increased risk of pertussis. Small sample size and imprecise measurement of immunisation status have compromised these studies. Other factors associated with an increased risk of pertussis in infants include younger age, low birth weight, the infant's immunisation status and household crowding. Prior to this current case control study there was no knowledge on the effect of infant characteristics, infant immunisation status, parental and household characteristics, or socioeconomic factors on the risk of pertussis in infants. Methods The pertussis mortality and hospital discharge statistics and notification data from 1872 to 2000 were reviewed. The characteristics of children hospitalised with pertussis during the 1995 to 1997 epidemic were described. Risk factors for pertussis in infants were determined using a case control study with two different control groups. A matched case-control design was used to compare infants with pertussis with well control infants from the community. An unmatched design was used to compare infants hospitalised with pertussis to infants hospitalised with other acute respiratory illnesses. Results Historical review of pertussis epidemiology Immunisation was associated with a significant decline in pertussis mortality rates in New Zealand. Pertussis incidence rates in New Zealand are five and 10 times higher than in the United Kingdom or the United States. New Zealand pertussis hospital discharge rates increased from 1920 to 1950, decreased from 1950 to 1970 and have been increasing since then. The severity of disease among those hospitalised in New Zealand is comparable to other developed countries. Case control study of risk factors for pertussis in infants In the community control sample factors associated with incomplete immunisation included poverty and household crowding, advice from a doctor that immunisations be delayed and the caregiver not having a record of the infant's immunisations. Primary and secondary pertussis in case households occurred in all age groups. Over half of the primary cases were infants. Factors associated with an increased risk of pertussis included incomplete immunisation of the infant, children five to nine years of age living in the household, household members with pertussis during the preceding two months and the family doctor advising that an immunisation be delayed. Preschool attendance by a household member was associated with a decreased risk of pertussis. Infants of low birth weight and infants with younger mothers were not at increased risk of pertussis. In a multivariate analysis, non-immunisation of other children in the household and the presence of someone in the household with clinical pertussis were associated with an increased risk of pertussis in infants. The associations between household members with cough and the risk of pertussis varied with the age of the household members and imply an age dependent disease modifying effect of immunisation. For many of the children in the study households it seems unlikely that any health professional knew whether or not they were fully immunised. Conclusions Immunisation reduced pertussis mortality in New Zealand. Pertussis hospitalisation rates are increasing despite improvements in the immunisation schedule. Sustained sub-optimal immunisation coverage appears to be the dominant reason for New Zealand’s excessive pertussis disease burden. Primary school aged children are important in household pertussis transmission.