In this study, we found that the COVID-19 pandemic modified both the HIAT exposure-response and lag associations in Japan. An overall reduction in the HIAT risk was generally observed during the COVID-19 pandemic, which may partially be due to the attenuation of both the exposure-response and immediate lag associations. Aside from the reduction, notable risk increments were also apparent in the delayed lag associations. Several mechanisms may have contributed to the reduction or increment of HIAT risk during the pandemic. Firstly, we draw comparison on how these observed changes resemble the impact of large-scale events on heat-related health risks in a more familiar concept of heat-health plans. In addition, we subsequently examined how these changes reflect the role of multi-sectoral policies as well as behavioral changes on the reduction and increment of heat-related health risks.
In the absence of true comparison, we presuppose that the COVID-19 pandemic setting serves as a community-level effect modifier, which share similarities to a heat-health plan. Though not entirely the same, the characterization of preventive measures within a heat wave warning system and measures against COVID-19, which, in part, affect the behaviors of people towards exposures, may be assumed to share the same concept of collective consciousness of risk aversion [30]. In Ahmedabad, India, Hess et al. [20] observed a substantial reduction in the heat-related risk after the implementation of a heat action plan (HAP) in response to the 2010 heat wave event. The authors noted an after-to-before HAP unlagged mortality incidence rate ratio (IRR) of 0.95 (95% CI: 0.93–1.22) and 0.73 (95% CI: 0.29–1.81) for maximum temperatures of over 40 °C and 45 °C, respectively. Here, the authors noted that regardless of the cut off, of either over 40 °C or 45 °C, the reductions in the IRR are still discernable. On the other hand, Martínez-Solanas and Basagaña [29] shares a similar observation, whereby the authors observed a weak to non-significant difference between the overlapping effects estimates of pre-HAP (percent change = 28.0; 95% CI: 22.4 – 33.9%) and post-HAP (percent change = 24.9; 95% CI: 21.4 – 28.6%) periods of high temperature effects in the 50 provinces of Spain. The reduction of heat-related risks due to heat-health plans has a resemblance, albeit different, on how COVID-19 attenuated temperature-HIAT association. Here, a population’s response to the risks is influenced mainly by both physical and behavioral factors [21]. The direction of the changes in the risks would also vary depending on the response. However, in this study, we observed an apparent and consistent reduction in the heat-related risks for both exposure-response (Fig. 2A) and same-day lag dimensions (Fig. 3A). Though it is difficult to draw definitive conclusion on the cause of these reductions, it remains plausible that both physical factors, in terms of policy interventions, accompanied with behavioral changes may have resulted to these changes.
The increased remote working environments as well as the availability of alternative services such as telemedicine, home/food deliveries, may have potentially reduced the ambient exposure of the population [41]. Early in the pandemic, on March 2020, the Japanese government encouraged firms to integrate a work-from-home setting to avoid cluster infections within workplaces. In response, 49.6% of the total number of firms adopted the home working system [34]. The remote work set-up was further complemented with innovations in alternative services (telemedicine and food deliveries) which accompanied the new lifestyle. Health facilities offering tele-medicine in the country [22], has increased during the pandemic, with nearly 10,000 additional clinics offering online services [31]. Similarly, food deliveries have gained traction in the country. In an online survey, approximately 39.7% of 1100 respondents used food delivery, 5% of whom are first-time users [8].
Behavioral changes related to heightened health risk aversion may have also contributed to the substantial decrease in the association [38]. In Japan, a recent study noted that more than 75% of 11,342 respondents, aged 20 to 64 years old, had practiced any form of preventive measures (social distancing, handwashing, coughing etiquette, and immunity fortification) [35]. Though mobility restrictions were not that strict, the response of the population towards these governmental requests have been reciprocated with a significant reduction in trips with the number of inter-prefectural travel halved across the country compared to pre-pandemic conditions [15]. Changes in human behavior during the pandemic is a cognitive response to the immediate threat of COVID-19, which has an indirect effect on the reduction of heat exposure [38]. Specifically, behavioral changes related to time spent indoor/outdoor may be related to the variability in heat exposure [27]. In an online survey conducted from 3 to 25 August 2020 in selected major cities and prefectures in Japan (n = 12,872), there was an approximately 13.8% increase in time spent indoors during the pandemic compared to pre-pandemic period, which subsequently corresponded to an 18.8% reduction in outdoor activities across the country [33]. Since HIAT occur mostly in roads (15.6%) and outdoor public areas (12.5%) [16], the increase in time spent indoor may have led to the attenuation of the heat-related health risks during the pandemic.
On the other hand, the pandemic may, in part, have altered the health-seeking behavior of the population. This is quite apparent in the significant difference in Lag 2, whereby pandemic risks were higher compared to the pre-pandemic period. It is plausible that this phenomenon is related to the reduction of same-day risks observed during the pandemic. During the pandemic, several studies have noted the delay of access to medical care [6, 9], with the fear of contracting the disease [5, 45]. In the US, a survey revealed that 41% of the respondents have foregone medical care in the early phase of the pandemic [4]. In another online survey, 12% of 4975 respondents opted to delay or have avoided urgent or emergency medical care [9]. It is reasonable to believe that those populations which have foregone immediate medical services (i.e. hospital/ambulance transport) may have delayed their access to a later period, thus mirroring the reduction in the same-day risk and a subsequent increase in the delayed association.
While we have observed a reduction in the exposure-response association during the pandemic, there was a notable statistically significant delayed lag association at lag 2, with a 16.3% (95% CI: 1.0, 34.0%) increase in the risk of HIAT during the pandemic compared to pre-pandemic period. The increment in the delayed risk during the pandemic period may have potential implications to health service access. In part, this may be loosely related to the delay in accessing health services due to medical care access hesitancy during the pandemic and of other related barriers such as informational/technological resource access [9, 37]. Health service providers and managers would then need to consider these factors and other potential mechanisms in order to overcome the barriers to healthcare access in the pandemic-stricken setting. In terms of heat-related health risk management, access to the appropriate information, such as clinic availability/schedule which provide telemedicine [7] as well as readily-accessible geographical maps of health facilities catering to non-COVID-19 cases [25], may potentially ease up the hesitancy in accessing immediate health care services.
We, however, note several limitations in this study. The ecological nature of the study may not be able to capture the individual-level characteristics, which may possibly explain the changes in the association. Further studies are needed to account for these personal-level data. In the absence of 2019 air pollution data, we were not able to examine the role of air pollution, particularly particulate matter with a diameter less than 2.5 μm in size (PM2.5), on the temperature-HIAT associations; a complete data is needed whenever available. However, it would be prudent to assume that the effect of air pollution on temperature-HIAT associations would be minimal, since 1) Japan has a relatively low ambient air pollution level [46] compared to other developed countries, and 2) that this low ambient air pollution may have been further reduced during the pandemic, which was similarly observed across several countries globally [42]. Also, the pandemic period data coverage is limited to 2020. Data supplementation would be necessary whenever available in order to further establish the robustness of the results. In this study, we utilized a simple binary indicator to represent COVID-19 impact. There is merit for other studies to explore other indicators which could represent the magnitude of the pandemic’s impact, i.e. continuous metrics. Nevertheless, we believe that the results provide substantial evidence on the magnitude of temperature-HIAT association reduction or increment during the pandemic. The results can serve as a guidepost for health managers and policymakers in crafting subsequent heat-health advisories as we enter the second year into the pandemic as well as future disease entities with pandemic potential.