Exposure to persons with symptoms of respiratory or gastrointestinal infection and relative risk of disease: self-reported observations by controls in a randomized intervention trial

BACKGROUND Little is known about the quantitative relationships between a self-recognized exposure to people with symptoms of respiratory (RTI) or gastrointestinal tract infection (GTI) and subsequent occurrence of homologous symptoms in the exposed person. METHODS Adult office employees, controls in an intervention trial, reported weekly own symptoms of RTI or GTI and exposures to other persons with similar symptoms. To ascertain the reliability of the self-reported data, the participants received both in-advance training and repeated instructions in the weekly Email requests for reports. The relationship of self-reported exposures to self-reported homologous symptoms during the same or the following week was analyzed including, in the statistical models, cluster effects and longitudinal aspects in the data, seasonality, and cluster-specific baseline values. RESULTS Altogether 11,644 weekly reports were received from 230 participants during the 16-month duration of the study. The mean age of the reporters was 42.9 years (standard deviation 11.1 years), and the female/male ratio 157/68 (for 5 participants this information was not available). A reported exposure to RTI was associated with an almost 5-fold higher relative risk for a reported homologous infection during the same week (4.9; 95% confidence interval (CI) 4.0 to 5.9), and with a 3-fold risk during the following week (3.3; CI 2.8 to 3.8). For GTI the corresponding figures were 15.1 (CI 10.4 to 21.8) and 4.3 (CI 3.1 to 5.8), respectively. On the other hand, for 24% of the designated RTI episodes, a homologous exposure had been reported during neither the same nor the preceding week. For GTI this figure was even greater (40%). For both RTI and GTI, weeks with a reported exposure were more frequent outside the workplace than only at the workplace (434 versus 262, and 109 versus 41, respectively). CONCLUSION A reported exposure to persons with obvious symptoms of RTI or GTI significantly increased the relative risk of reported homologous infection in the exposed adult persons. Yet, a substantial part of reported designated RTI and, especially, GTI episodes occurred without a reported exposure during the same or the previous week. TRIAL REGISTRATION ClinicalTrials.gov with an identifier of NCT00821509 (12 March 2009).