Exploration of the effectiveness of social distancing on respiratory pathogen transmission implicates environmental contributions

Abstract

Background: In both military and civilian settings, transmission of respiratory pathogens may be due to person-to-person and environmental contributions. This possibility was explored in a military training setting, where rates of febrile respiratory illness (FRI) often reach epidemic levels.

Methods: Population size and FRI rates were monitored over 10 months in the units of 50-90 individuals. Some units were open to the influx of potentially infectious convalescents (hereafter referred to as "open units," and some were closed to such an influx (hereafter referred to as "closed units"). Virologic testing and polymerase chain reaction analysis were used to detect adenovirus on surface structures.

Results: The odds ratio (OR) associated with FRI in closed units, compared with open units, was 1.13 (95% confidence interval [CI], 0.99-1.28). The OR in units with a population greater than the median size, compared with units with a population lower than the median size was 1.38 (95% CI, 1.23-1.55). Between 5% and 9% of surface samples obtained from selected units harbored viable adenovirus.

Conclusions: FRI rates were not reduced in units that were closed to potentially contagious individuals. These findings imply that the primary source of the pathogen is likely environmental rather than human, and they underscore what is known about other virus types. Diligence in identifying the relative roles of different transmission routes is suggested for civilian settings similar to those described in the current study.

Figure 1

Contrast of populations with a size above the median (High pop) and those with a size below the median (Low pop). The rates of febrile respiratory illness (FRI) for these populations were significantly different. Although the units closed to an influx of potentially infectious convalescing persons (hereafter referred to as “closed units”) generally had higher FRI rates than the units open to such an influx (hereafter referred to as “open units”), the difference was not significant. Logistic regression of the odds of an FRI diagnosis, in terms of the population size and unit type, showed that the difference in population size remained even after adjusting for unit type.

Figure 2

Febrile respiratory illness (FRI) rates and 95% confidence intervals of units grouped according to 4 population size ranges formed around the quartiles.

Table 1

Logistic regression analysis of effect of quartile group on odds of an FRI diagnosis, adjusted for type of unit.

Figure 3

Percentage of surface samples in each barrack that were found to be positive for adenovirus type 4 by means of culture and polymerase chain reaction (PCR) analysis. The figures above the bars denote the number of samples. The asterisk (*) denotes that the DNA test for barrack 1 is unavailable. Barrack 1 was an open-bay unit that was vacant for 3 days, Barracks 2 and 3 were open-bay units that were vacant for 1 week, and the medical clinic (Med clinic) was operating routinely. Multiple sites within each building were sampled.