Background
The most common infectious diseases are caused by viral agents and occur in the respiratory [
1] and gastrointestinal tract [
2]. In addition to enhanced hand hygiene [
3-
7], social distancing; that is avoidance of public gatherings, crowded public transport vehicles and shopping centers, close contacts with symptomatic persons, and so on, has been suggested as a means of limiting virus transmission during epidemics, including influenza pandemics [
3]. However, it is not clear how feasible social distancing could be in everyday life. One can decide about staying away from public gatherings during free time, whereas using public transport to and from the workplace, and attending scheduled meetings at work may be unavoidable. Likewise, while it might be possible at work to stop shaking hands and to avoid other close contacts with colleagues during epidemics, parents cannot avoid close contact with their sick children. The potential effectiveness of social distancing, and especially avoidance of other persons with symptoms of respiratory tract infection (RTI) or gastrointestinal tract infection (GTI), is partly jeopardized by the fact that virus shedding may already have started before onset of the symptoms [
8] and, on the other hand, continue after cessation of the symptoms. For instance, contrary to previous beliefs, norovirus-infected individuals frequently shed the virus for several days after the gastroenteritis symptoms have ended [
9,
10]. Furthermore, virus-infected persons may remain completely symptomless and still be able to shed the virus.
Very little seems to be known in the literature about self-experienced exposure to other people with RTI or GTI symptoms, and consequent emergence of homologous symptoms in the observer. While this association is expected, a clear documentation of its existence might help to design hand hygiene and behavioral instructions for people in order to limit virus transmission during epidemics.
We have conducted a 16-month cluster-randomized 3-arm occupational health intervention trial, the STOPFLU Study [
11], and reported earlier that in the intervention arm executing enhanced hand hygiene with water and soap, the occurrence of self-reported infection episodes decreased by 16.7%, approximately equating to 1 infection episode per person year [
12]. The data collection took place through a standardized electronic questionnaire sent weekly via personal Emails. In the STOPFLU Study we also collected, on a weekly basis, data on self-recognized exposures to other persons with respiratory or gastrointestinal disease symptoms. Here, we report the association of the self-reported exposures at work or outside the work with self-reported homologous RTI or GTI symptoms occurring in the reporter during the same week or the following week. This analysis is limited to the participants in the control arm only.
Discussion
This study assessed the temporal association of self-reported exposures to persons with symptoms of RTI or GTI with self-reported occurrence of homologous symptoms in the respondent during the same or the following week. An exposure to RTI was reported during more than a quarter of all follow-up weeks while that to GTI was noted in less than one tenth of the weekly reports. Reported exposure to either type of infection clearly increased the relative risk for homologous symptoms in the respondents during both the same week and the following week. The relative risk for homologous disease was higher in persons exposed to GTI than to RTI.
General population-based studies trying to figure out a quantitative relationship between a homologous recognized exposure and subsequent onset of an acute RTI or GTI are difficult to find in the literature. Our current attempt to investigate the matter by the implemented weekly self-reporting principle can be envisaged to have some obvious weaknesses. Firstly, the data collection was by self-reporting of RTI and GTI symptoms, as well as of the exposures, by lay persons rather than through objective assessment of symptoms and signs by health care professionals. In theory, this might decrease the accuracy and consistency of the collected data although the relevant definitions were repeated weekly in the personal Emails requesting the report. Seasonal variation of the monthly rate of the reported exposures was similar to that of the reported onsets of infection episodes in the study population, which was previously found to well reflect the seasonal variation of the epidemic activity of common viral infections in the source population [
12]. This suggests that most exposure reports could be relevant but with the reservations mentioned below.
A second problem is the fact that while the occurrence of RTI and GTI symptoms was reported on a daily basis, the recognized exposures were scored only on a weekly basis [
11]. As a large proportion of the infections were reported to start on Monday or Tuesday [
15] an exposure reported for the same week may have occurred either before or after the onset of the reported infection and, therefore, we cannot use the observed exposure-infection associations for considering potential causative relationship between the two events. Rather, exposures reported for the same week as the onset of infection reflect the overall activity of infection transmission around the reporting participant. On the other hand, because the incubation period of many viral RTIs and GTIs may be as short as 1 to 2 days [
16], and because the duration of a large part of both type of infections in this study was less than 4 days [
15], a putative exposure on Monday may have resulted in a short-duration disease not continuing over the following weekend, and thus might no longer be reported in the following week’s report. Hence, the same week’s associations are likely to give somewhat falsely high figures while the successive week’s associations may result in falsely low figures. Therefore, the described associations of reported exposures with RTI or GTI symptoms reported for either of the 2 weeks must be interpreted with caution as regards the exact numbers.
Although a weekly electronic reporting principle implemented in this study has been considered feasible for epidemiological studies on common infectious diseases [
17], it is clear that daily diaries of both symptoms and recognized exposures would have minimized the recall bias [
17]. We did not use daily symptom diaries because, when designing the data collection system for the STOPFLU trial, we wanted to create an electronic weekly reporting system that was so simple and easy to use, that the trouble of filling it would not be a reason for dropout and not to continue reporting for the desired more than 1-year duration of the trial [
11]. This goal was reached, but as a compromise, we had to limit the details of the data to be collected. An additional reason for choosing this retrospective only-weekly reporting of exposures was our consideration that while it is likely that a person will remember the day of a personal disease onset for 7 days, it is much less probable that he or she can reliably list exposures on a daily basis in the weekly report.
In both types of infections, most of the reported homologous exposures were recorded for the same week as the self-reported symptoms. We cannot exclude the possibility that part of this difference is due to recall bias, as it is a natural behavior for humans to try to remember a potential source for his own infection, whereas it would be easier to forget exposures concerning weeks without symptoms. As for the generalizability of our results, a further note of caution may be necessary. In addition to the inherent differences between a study set-up and normal life, and on top of the above reservations, one has to remember a potential selection bias in our study. Only a proportion of the staff in the study clusters participated in this study, and we have no way to assess how well they represented the entire staff.
A temporal association between an exposure to RTI or GTI and homologous infection cannot be considered big news, especially with the above reservations, but some figures in our results are worth commenting on. The duration of the trial was 16 months, which means that both reported exposures and infections covered a wide range of different causative agents, as we believe, mostly viruses showing different types of seasonal variation of transmission. Seasonality, randomization triplet, cluster effects, and longitudinal associations among observations of each subject were taken into account in our statistical analysis of the exposure-infection relationships. The overall occurrence of reported exposures to RTI was 4 to 5 times higher than that to GTI, a similar ratio as in the reported infection episodes. While a recognized exposure to RTI was associated with reported homologous symptoms in the respondent during the same or the following week more frequently than in the case of GTI, a calculated relative risk of disease associated with an observed exposure was higher for GTI than for RTI. This was based on the much lower rate of GTI infections, as compared to RTI, occurring in association with person weeks without a reported exposure. A possible explanation for the greater relative risk of a homologous disease associated with a reported GTI exposure, as compared to that with an RTI exposure, may be that adults recognized by an outsider to have symptoms of GTI are relatively more infectious than those with an obvious RTI. On the other hand, based on the descriptive assessment of the data the other way around, almost a quarter of the reported designated RTI episodes and as many as about 40% of the GTI episodes occurred without a personal recognition of a homologous exposure during the same or the preceding week. Many viruses are known to be shed by infected persons without clear concomitant symptoms of disease. The difference between RTI and GTI is in line with the fact that RTI symptoms in adults are usually more apparent to outsiders than those of GTI.
Significantly more exposures to both diseases were reported having occurred only-not-at-work than ‘only at work’ and the relative risks of disease associated with the outside work exposures were greater than those occurring ‘only at work’. A possible explanation is that contacts with sick family members are likely to be more intimate, long-lasting and unavoidable than those with symptomatic colleagues at work. Person weeks with recognized RTI exposure ‘both at work and elsewhere’ showed significantly higher relative risk for infection than those with exposures only at one site category. A straightforward explanation would be an additive probability of transmission due to a likely greater number of exposure events. For GTI a similar comparison did not reveal a statistically significant difference, but this may be because of the small number of recorded events.
Conclusions
Self-reported exposure to RTI and, especially, that to GTI remarkably increased the relative risk of reported homologous disease symptoms in the respondents. On the other hand, both types of infection episodes also frequently occurred without an association to a reported exposure. Hence, social distancing as a means of control of both RTI and GTI epidemics may be justified, and should then include both avoiding contacts with persons with RTI or GTI symptoms and people gatherings in general. Likewise, effective hand hygiene can be recommended in support of social distancing through epidemic seasons even in the absence of self-recognized exposures. Reported exposures outside the work appeared to significantly contribute to RTI or GTI in office workers and hence, any intervention aiming at reducing occupational infections should be implemented with the 24 hours a day 7 days a week principle and also cover the free time in order to be effective.
Acknowledgements
The data collected was based on the work of the entire STOPFLU Study Group: Tapani Hovi, Carita Savolainen-Kopra, Thedi Ziegler, Terttu Korpela, Ali Amiryousefi, Pirjo Anttila, Jaason Haapakoski, Pentti Huovinen, Markku Huvinen, Heikki Noronen, Pia Riikkala, Merja Roivainen, Petri Ruutu, Juha Teirilä (deceased), and Erkki Vartiainen.
The study was funded by the Finnish Work Environment Fund (grant 108306) and the National Institute for Health and Welfare, Finland.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
TH designed the study, participated in the coordination of the project and analysis of the results, and drafted the manuscript. JO carried out the final statistical analysis of the results. JH designed the database and participated in the study design and data analysis. AA performed the preliminary statistical analysis. CSK participated in finalization of the study plan, in acquisition of the data, coordination of the project and analysis of the data. All authors read and approved the final manuscript.