In this study, we observed an increase in self-reported tick encounters during the spring and summer of 2020 compared to the same time period in 2019, and an increase in outdoor activities overall. The timing of the initial shift in outdoor activities coincides with COVID-19 stay-at-home orders in the United States, suggesting changes in tick exposure were influenced by 2020 COVID-19-driven behavioral changes. The comparison between pre- and amid-pandemic conditions (before and after March 2020) was enabled by the availability of a baseline dataset encompassing ~7000 responses from ~1000 users each year from the Tick App, a smartphone application designed to capture human behavioral risk factors associated with tick encounters. Although previous studies have shown an overall increase in outdoor activities in other countries in the Global North34,35 and even locally in the United States36, this is the first study to use individual-level data to show a two-fold increase in outdoor activities and a 30% increase in self-reported tick encounters in 2020 compared to 2019. Moreover, we showed a shift towards peridomestic activities compared to recreational activities in green spaces, the latter decreasing by 70% in the spring and summer of 2020.
Throughout the COVID-19 pandemic, outdoor activities and gatherings have been encouraged over indoor activities in order to curb SARS-CoV-2 transmission37. However, accessibility to public green spaces, differences in competing hazards (e.g. ticks, mosquitoes, rodents) between public green spaces and peridomestic settings, and risk perception (for COVID-19 transmission, and vector- or rodent-borne diseases), among other factors may have influenced not only the frequency of outdoor activities, but also where people decided to conduct their activities (peridomestic vs. green spaces). This study stands apart from previous assessments of outdoor activity during the COVID-19 pandemic by using individual-level information about shifts in outdoor activity patterns and self-protection efforts. Other studies have relied on aggregated mobility data to assess changes in park visitation as a proxy for outdoor recreation35,38–40. Individual-level data is essential to fully assess the risk of tick- and other vector-borne diseases, which depends on the background risk (e.g. vector density), exposure (e.g. the type, location and intensity of outdoor activities), and self-protection efforts (e.g. the use of repellents)16,17,41. Behavioral choices related to exposure and self-protection are made through decision-making processes where individuals make complex risk assessments in the face of tradeoffs between mitigating competing hazards. To identify how an individual adapts to changing risk, data collection must explicitly measure the behaviors at multiple time points and scenarios (i.e., changes in the type of outdoor activities). Another strength of the study is that we distinguished between activities undertaken in peridomestic environments vs. green spaces. Because unidentified mobility data cannot distinguish between yards and indoor locations, such datasets completely miss peridomestic activities.
The overall increase in any type of outdoor activity during the spring and summer of 2020 compared to 2019 (pre-pandemic) found in this study (after adjusting for weather and demographic covariates) may have resulted in increased exposure to vectors (ticks and mosquitoes) during the COVID-19 pandemic. This hypothesis is supported by the observed increase in self-reported tick encounters in 2020, verified by picture submissions through the smartphone app. At the individual level, we found that self-reported tick encounters were associated with engagement in outdoor activities, which provides further evidence for the link between increased outdoor activities and tick exposure. However, when evaluating the association of self-reported tick encounters and the type of activity (peridomestic activities or recreational activities in green spaces), no significant association was found with either type. The fact that the engagement in any type of outdoor activities was associated with a reported tick at the individual level but not for a specific type of activity could be explained by similar tick abundances in both settings, although no available data is available to assess this. Furthermore, ticks’ prolonged attachment and increased likelihood of detection as ticks engorge creates uncertainty on the date when the tick was first attached and first spotted, making it difficult to match the tick to the specific activity that resulted in the exposure. Additionally, in the app, users can report several activities on the same day, further hindering the match with the tick report. Nonetheless, the group-level shift towards peridomestic activities and increased tick exposure indicates either an increase in the overall tick population or increased exposure in those settings where participants are less inclined to use preventative measures against tick bites30. A similar pattern was observed for mosquito-borne disease in two studies on dengue infection: i) a simulation model found that COVID-19 stay-at-home measures (in which 70% of the population restricted their mobility to their house) led to an increase in overall infections and increased clustering of cases at the household level42; and b) an empirical study using dengue case report data in Singapur found a 37.2% increase in the number of cases from expected baseline levels attributed to COVID-19 stay-at-home measures43.
Our study population was overrepresented by outdoor enthusiasts29, which is a group that tends to be at a higher risk of tick exposure than the population at-large. This makes The Tick App users a suitable population to explore how COVID-19 might have shifted overall exposure and the context of the exposure (in green spaces vs. peridomestic). Overall, statewide stay-at-home orders had a strong effect on reducing out-of-home mobility in the United States, reflected by a 15.2% increase in the time spent at home after the policies took effect38. In our study, this was also reflected in an increase in peridomestic activities in 2020 compared to 2019 and pre- and post-stay-at-home orders that were implemented in March 2020. This effect was more pronounced immediately after the orders went into effect, even after adjusting for weather variables to account for seasonality effects. This increase in peridomestic activities was reflected in the self-reported impacts of COVID-19 on the user’s outdoor activities: among individuals who reported an impact, the most frequent response was that they spent more time in their yard, particularly in April and May.
Recreational activities in green spaces on the other hand decreased significantly between years and pre- and post-stay-at-home orders, even after adjusting for weather variables. Increased park visitations revealed by Google Mobility Reports44 were used by two studies39,40 as a proxy for changes in outdoor activities in these environments. However, these trends were found to be primarily driven by seasonality at the county-level, providing little empirical support to an effect of COVID-19 risk mitigation responses on outdoor recreation activities45. By comparing the same period in 2019 and 2020 and adjusting for weather variables to account for seasonality, we found a reduction in recreational activities, even after adjusting for urbanicity levels. Consistent with our findings, another logitudinal study that used questionnaires to survey outdoor enthusiasts in the United States46,47 at three different time points after stay-at-home measures took effect (between April and May), found a reduction in the frequency of outdoor activities and the distance traveled to partake in these activities. Additionally, a nationwide online survey in the United States examining the effects of the COVID‐19 pandemic on outdoor recreation trips, showed an estimated 26% reduction in trips per participant to public outdoor recreation sites post‐COVID‐19, as compared to pre‐COVID‐19 trips48. The shift to backyard activities detected in our study was also found in a survey administered to avid birdwatchers worldwide (mostly located in the Global North), 85% of respondents reported that COVID-19 changed their birding behavior, increasing their time focused on this activity but shifting mostly to yard birding49. It should be noted that crowded conditions forced park closures off and on to mitigate COVID-19 transmission risk or offered limited capacity -i.e., the crowds were too large for proper distancing, which could have affected people's engagement in these activities and the distance traveled, particularly in higher urbanicity locations. However, in our study few study participants reported not engaging in recreational activities due to 'no access' to parks (Figure 3B). As our study focused on two specific regions, these results cannot be generalized to the entire United States where activity patterns in recreational areas may have differed during the pandemic.
When taken together, our findings and those in the aforementioned studies, suggest a reduction in recreational activities in green spaces during the first couple of months of the COVID-19 pandemic after stay-at-home orders went into effect. Although our study suggests that the overall increase in any type of outdoor activities during the COVID-19 pandemic were driven mostly by an increase in peridomestic activities, we cannot disregard an increase in recreational activities in grey spaces (e.g., walking around the neighborhood, Supplementary Figure 2); however, there is insufficient data to analyze the effects of COVID-19 on these types of recreational activities. Studies conducted in Canada and the United States showed that, in the first month of the pandemic, the majority of participants engaged in physical activity either in the home environment or in their neighborhood34,46.
When analyzing changes in activity patterns over time, we observed that changes prompted after the stay-at-home orders in effect persisted until the end of the period analyzed (July 31st), even after adjusting for seasonality effects. However, we also found that the Shelter-in-Place Index (used as a proxy for reduced out-of-home mobility), which decreased over time after the first month following the stay-at-home others went into effect, was inversely associated with the daily proportion of peridomestic activities and was not associated with the daily proportion of recreational activities in green spaces. In other words, as stay-at-home orders and human risk mitigation behaviors relaxed over time and out-of-home mobility increased (i.e., a decrease in the Shelter-in-Place Index, Supplementary Figure 4), both the proportion of users reporting any type of outdoor activities and peridomestic activities increased. The COVID-19 incidence rate both at the national and county level showed the same pattern: an inverse association with the proportion of users reporting peridomestic activities but no association with the proportion reporting recreational activities in green spaces. This inverse association between local COVID-19 incidence and peridomestic activities was particularly evident towards the end of our study period in June and July 2020, when cases drastically increased (i.e. the “second wave”) while the Shelter-in-Place Index remained constant (Figure 2A vs. Supplementary Figure 4). Our results suggest that there was a substantial shift in peridomestic activities prompted by the COVID-19 pandemic but, as the time progressed, activity patterns were less influenced by the COVID-19 risk mitigation responses directly (particularly for peridomestic activities) although did not return to pre-pandemic patterns. This temporal pattern was also observed in a national survey to outdoor enthusiasts; after the initial reduction in recreational activities in green spaces, there was a slight shift from neighborhood and city streets back to public lands in late May compared to early April, though still in lower proportions compared to pre-pandemic levels46. These results were supported by an increased percentage of respondents indicating that their new outdoor activity patterns would persist in the long-term towards the end of the study46.
Self-reported impacts of COVID-19 on recreational activities were not homogenous along a rural-to-urban gradient. While peridomestic activities were disproportionately higher in small and medium metropolitan areas and rural areas as compared to large metropolitan areas, the proportion of users reporting recreational activities in green spaces increased with the proportion of users in rural counties. Similarly, Rice et al47, found that outdoor enthusiasts residing in urban areas reduced their frequency of outdoor recreation participation. Accessibility to green spaces might have been a limiting factor in metropolitan areas across the United States. In contrast, in Norway, recreational outdoor activities increased in Oslo during the early spring after COVID-19 mitigation responses were implemented. This increase was associated with the wide availability of green spaces and corridors through the city, as well as the Norwegian principle of common access (allemannsretten) that grants anyone the right, within certain restrictions, to move freely across private and public uncultivated land (Outdoor Recreation Act, 1957)35. Thus, Oslo provides a range of opportunities for recreation along an urban-wilderness continuum that may not be available in many large metropolitan areas in the United States. Nonetheless, variations within Oslo were also observed: “Activities were intensified in the urban periphery, while marginal increases and, in some areas, decreases were evident toward the city center for both pedestrian and cycling activities.”35 In our study, self-reported tick encounters were mostly associated with users living in rural counties, where tick hazards tend to be higher than in more urbanized areas, indicating that human-tick encounters depend on the tick distribution and density as well as human activity patterns that determine exposure, even without accounting for preventative measures such as tick repellents or protective clothing.
Our study has several limitations, as with any observational and longitudinal study. Our conclusions are limited to a population that tends to engage in outdoor activities frequently and cannot be generalized to the general population. Given that The Tick App is marketed as an app to better understand human exposure to ticks, we expect that people at greater risk of tick encounters and previous tick exposure use it more frequently, introducing self-selection bias. Moreover, our population is biased towards older ages, thus limiting conclusions for younger age groups; children and teenagers were not included. Follow-up time and surveys completed by users were highly variable, thus data had to be pooled to study temporal (daily) variations. In addition, the survey was slightly modified in 2020 to capture outdoor activities more accurately. However, new changes were rolled out by late April 2020 and we validated our results by comparing reporting activities between and after roll out of the new version of the app. Lastly, although we accounted for urbanicity levels, pooling users from different geographical areas in the Northeast and the Midwest might mask other aspects affecting outdoor activity patterns and tick encounters occurring at the local level. Nonetheless, this study capitalized on a unique dataset collected that captures human behavioral risk factors for a consistent geographic area over time, allowing us to study the effects of COVID-19 mitigation efforts on outdoor activities and tick-borne disease risk, compared to a pre-pandemic baseline. Unlike other studies conducted after the pandemic started, this dataset is not subject to recall bias. Moreover, the data encompassed reports by a subset of users who completed surveys in both years, providing robust validation of findings. Finally, a limitation of the study is that the size of the dataset precludes assessments of local changes in vector exposure or geographic-specific risk of vector-borne diseases29,30.
While the COVID-19 pandemic continues to represent a significant challenge to public health, we cannot neglect other health risks. In the United States, tick-borne disease cases have doubled during 2004-2016, and nine vector-borne human diseases have newly emerged during the last decade12. The full impact of the pandemic on the transmission of tick-borne, and other vector-borne diseases is yet to be fully realized. This study could provide valuable insights into potential underreporting of tick-borne disease cases during the COVID-19 pandemic10. Our results show increased self-reported tick exposure (particularly among outdoor enthusiasts) in 2020, but its association to disease risk is unknown. We speculate that this increase is influenced by the shifts in outdoor activity patterns found in our study population. However, we cannot disregard effects of an increase in the population of ticks or in awareness of tick-borne disease risk resulting in a more diligent search for ticks after being outdoors. The links between outdoor recreation and wellbeing are well-established50 and particularly during the COVID-19 pandemic51, outdoor activities have been (and should be) encouraged as a way to mitigate transmission risk while providing safe spaces for social interactions. Nonetheless, other hazards occurring in outdoor settings should be taken into account. Our results highlight the need to balance public health resources to avoid neglecting the disease burden caused by tick-borne and other vector-borne diseases.