Pertussis seroepidemiology and associated factors in older adults aged ≥40 years with and without acute myocardial infarction (AMI) were studied to investigate whether unrecognised pertussis may precipitate AMI. Sera were obtained from a previous case-control study investigating the role of influenza in precipitating AMIs. Baseline sera were considered pertussis toxin (PT) IgG seropositive at levels ≥5 IU/mL. Levels ≥v62.5 IU/mL were considered indicative of infection in the previous year, and recent infection was indicative at levels ≥125 IU/mL. Of the serum samples tested, 55% (122/222) were seropositive for PT IgG, 5% (11/222) had evidence of infection in the past year and 1.4% (3/222) had evidence of recent infection. Evidence of infection in the past year was found in 3.2% of those aged 65-74 years. Overall, 47.8% of 40-64 year olds and 43.2% of those aged ≥65 years were seronegative for pertussis. Serological evidence of pertussis was not associated with AMI (46/92, 50.0% cases vs. 76/130, 58.5% controls, p=0.2). After adjusting for age, AMI and self-reported pertussis and GP verified influenza vaccination, females (OR = 2.2, 95% CI = 1.1-4.1, p=0.02) were more likely to be seronegative. Just under half of participants had no detectable pertussis immunity and are therefore susceptible to infection. Our study supports the need for an adult pertussis booster to supplement current recommendations.