No Association of COVID-19 transmission with temperature or UV radiation in Chinese cities
Ye Yao, Jinhua Pan, Zhixi Liu, Xia Meng, Weidong Wang, Haidong Kan, Weibing Wang
European Respiratory Journal 2020; DOI: 10.1183/13993003.00517-2020
The Coronavirus (COVID-19) epidemic, which was first reported in December 2019 in Wuhan, China, has caused 80 904 confirmed cases as of 9 March 2020, with 28 673 cases being reported outside of China. It has been declared a pandemic by the World Health Organization which exhibited human-to-human transmissibility and spread rapidly across countries . Although Chinese government has taken various measures to control city-to-city transmission (e.g. shutting down cities, extending holidays) and many countries have implemented measures such as airport screening and testing of patients who have reported symptoms, the number of cases still increases quickly throughout the world.
Previous studies have shown the importance of weather variables in the transmission of infectious diseases, including, but not limited to, influenza and severe acute respiratory syndrome (SARS). For example, a sharp change of ambient temperature was associated with increased risk of SARS [2, 3]. Also, influenza transmission is often enhanced in the presence of cold and/or dry air . In northern Europe, low temperature and low UV Indexes were correlated with peaks of influenza virus activity during 2010–2018 . Therefore, it is hypothesised that COVID-19 transmission may decrease or even disappear when the temperature and UV radiation increase in the summer. In this study, we aim to determine the association of meteorological factors with transmission of COVID-19 in various Chinese cities.
We collected COVID-19 confirmed case information in China reported by the National Health Commission (www.nhc.gov.cn/xcs/xxgzbd/gzbd_index.shtml) and the Provincial Health Commissions of China (http://wjw.hubei.gov.cn/bmdt/ztzl/fkxxgzbdgrfyyq/). We used the cumulative number of confirmed cases from 224 cities (207 outside Hubei, 17 inside Hubei) with no less than 10 cases as of Mar 9, and calculated basic reproduction number (R0) for 62 cities (50 outside Hubei, 12 inside Hubei) with more than 50 cases as of February 10 (COVID-19 peak time in China). R0 means the expected number of secondary cases generated by an initial infectious individual, in a completely susceptible population. If R0<1, then the disease free equilibrium is locally asymptotically stable; whereas if R0>1, then it is unstable. Thus, R0 is a threshold parameter.
Meteorological data, including daily mean temperature and relative humidity, were collected from the China Meteorological Data Sharing Service System. Regarding the UV radiation, daily erythemally weighted daily dose (EDD) data were extracted from the Dutch-Finnish Ozone Monitoring Instrument (OMI) Level 2 UV irradiance products with version 003 (OMUVB V003) at 13 km×24 km resolution. OMI is a nadir-viewing spectrometer aboard the NASA Aura satellite covering UV wavelength from 270 nm to 380 nm. Average of EDD values from OMI pixels matched within the city area was assigned as the daily mean EDD level for the corresponding city.
We used R to assess the associations of meteorological factors (including temperature, relative humidity and UV radiation) with the spread ability of COVID-19. In particular, we averaged daily temperature, maximum temperature, minimum temperature, relative humanity and UV radiation (EDD data) from early January to early March for 224 cities. Multiple regression methods were used to explore the association of meteorological factors with cumulative incidence rate and R0 in the same period.
Among the 224 cities, the mean±standard deviation and range were (5.9±7.5, −17.8–22.0°C) for temperature and (1332.5±594.0, 385.3222.0 J·m−2) for EDD. Temperature and EDD tended to decrease toward high latitude and altitude. The mean±standard deviation and range were (60.3±324.0, 1.9–4509.1/106) for cumulative incidence rate in 224 cities and (1.4±0.3, 0.6–2.5) for R0 in 62 cities. The top 3 cities with the highest R0 and top 15 cities with highest cumulative incidence rate were all in Hubei Province.
After adjustment for relative humidity and UV, as shown in figure 1 left panel, temperature held no significant associations with cumulative incidence rate (χ2=5.03, p=0.28) or R0 (χ2=0.93, p=0.92), in cities both outside (green points) and inside Hubei (blue points), which indicated that the spread ability of COVID-19 would not change with increasing temperature. Similarly, as shown in figure 1 right panel, UV was not significantly associated with cumulative incidence rate (χ2=5.50, p=0.24) and R0 (χ2=0.91, p=0.92) after adjustment for temperature and relative humidity, suggesting that the spread ability of COVID-19 would not change with increasing UV exposure. In addition, we did not find significant associations of relative humanity, maximum temperature and minimum temperature with cumulative incidence rate or R0 of COVID-19.
Previous results on the relationship between respiratory-borne infectious diseases and temperature indicated that both SARS and influenza need to survive under certain temperature conditions, and increasing temperature can reduce the ability of SARS virus and influenza virus to spread [6, 7]. The underlying hypothesis for why warmer seasons tends to decrease the spread of viruses includes higher vitamin D levels, resulting in better immune responses ; increased UV radiation; and no school in the summer (when children are clustered together, transmission rates of flu and measles increase). Reports of UV and respiratory diseases have also been studied, and previous studies have shown that high levels of UV exposure can reduce the spread of SARS-COV virus .
The results from this study, however, do not follow this expected pattern. According to the current results, cumulative incidence rate and R0 of COVID-19 held no significant associations with ambient temperature, suggesting that ambient temperature has no significant impact on the transmission ability of SARS-CoV-2. This is quite similar with MERS epidemic in the Arabian Peninsula where MERS cases continue when temperatures are 45°C . Other newly emergent zoonotic disease, such as Ebola or pandemic strains of influenza, have also occurred in unpredictable patterns. Even though the transmission of SARS, which began in November, 2002, and ended in July, 2003, suggests it might be seasonal, but it also might have been controlled by effective case finding, contact tracing and quarantine.
Our study has limitations. First, our study period may not represent a whole meteorological pattern associated with transmissibility of COVID-19. However, we did not observe reduced transmissibility of COVID-19 in some southern Chinese cities (e.g. Sanya, Haikou, and Danzhou) with average daily temperature already over 20℃ (maximal temperature >30℃), suggesting the robustness of our findings. Certainly, further studies with longer follow-up period and wider temperature range are warranted. Second, given the ecological nature of study, other city-level factors, such as implementation ability of COVID-19 control policy, urbanisation rate, and availability of medical resources, may affect the transmissibility of COVID-19 and confound our findings. Future studies should develop complicated models with high spatial-temporal resolution to assess the relationship between meteorological conditions and epidemiologic characteristics of COVID-19.
In summary, our study does not support the hypothesis that high temperature and UV radiation can reduce the transmission of COVID-19. It might be premature to count on warmer weather to control COVID-19.
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