The epidemic cycle of <i>Chlamydia pneumoniae</i> infection in eastern Finland, 1972–1987

SUMMARY The epidemic cycle of Chlamydia pneumoniae infection was examined in two areas in eastern Finland over a period of 15 years, 1972–87. The C. pneumoniae IgG antibody prevalence was determined with 5-year intervals in a random sample of the population aged 25–59 years. The total number of sera studied using immunofluorescence was 2387. In 1972 the antibody prevalence was 57% and it increased to 66% in 1977. Over the next 5 years the prevalence decreased to 44% in 1982, but by 1987 it had again increased to 59%. The temporal variation in prevalence was statistically significant ( P &lt; 0·001) and similar for both genders. Throughout the observation period the overall prevalence Mas 7–11% higher in men than in women ( P &lt; 0·01). The antibody prevalence increased with age, being the highest among the oldest study subjects of both genders. The periods of high and low prevalence alternated in an epidemic cycle ( P &lt; 0·001) of about 10 years