CoVID-19は夏になっても終息しない可能性がある

ヒトコロナウイルスは夏に流行が小さくなる傾向があり、またCoVID-19も実験では温度が上がるほど、不安定になりウイルスが失活することが分かっています。

しかし南半球の国でも流行しているため、楽観はできません。

夏に一旦減るのか、それともダラダラと冬まで続くのかは分かりません。

どちらにしても冬が最も大きな流行になる可能性は高いです。

それまでに持続可能な社会を再構築しないといけません。

　　（※　管理者注　2020/04/21記載）

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新型コロナウイルス感染症（COVID-19）はインフルエンザと違って、暖かくなっても収束に向かう可能性は低いことが、復旦大学（中国）のYe Yao氏が主導した研究で示された。詳細は「European Respiratory Journal」4月9日オンライン版に発表された。

　米国科学アカデミー（NAS）の専門家らも、同団体のプレスリリースで「気候的には夏を迎えているオーストラリアやイランといった国々でも感染が急速に広がっていることを考慮すると、気温や湿度が上昇しても感染者数が減ることはないだろう」との予測を示している。

　NASの専門家らは、「実験室での研究では、気温と湿度の上昇に伴い新型コロナウイルスの生存期間が短縮することが示されている」と述べる一方で、気温や湿度以外にも、ヒトからヒトへの新型コロナウイルス感染に影響を与え得るさまざまな要因があることを強調している。

　Yao氏らは、COVID-19が流行した中国224都市における1月上旬～3月上旬のデータに基づき、毎日の気温と紫外線量、湿度の変化と新型コロナウイルス感染との関連について調べる研究を実施。湿度と紫外線レベルを調整して解析した結果、気温が上昇しても、新型コロナウイルスの感染拡大能力は変化しないことが分かった。また、気温と湿度で調整して解析した場合も、紫外線の量により感染率が変わることはなかった。

　Yao氏は「われわれの研究では、高い気温や紫外線量がCOVID-19の感染拡大を抑え得るとする仮説を支持する結果は得られなかった。現時点では、暖かくなればCOVID-19の感染者数が減ると考えるべきではないだろう」と説明している。こうした特徴は、2012～2013年に流行した中東呼吸器症候群（MERS）に類似していると、同氏らは指摘する。流行当時、アラビア半島では、気温が華氏113度（摂氏45度）まで上昇したにもかかわらず、MERSの感染拡大は続いていた。

　ただし、Yao氏らは、今回の研究結果が確定的なものではないことを強調しており、「今後、より長期の追跡期間で、より幅広い範囲の気温との関係を調べる研究を行う必要がある」としている。

　しかし、希望も残されている。Yao氏らも説明しているが、寒い季節には上気道感染症の患者が増えるが、暖かくなるにつれて感染者数は減る。その明確な理由は不明だが、複数の要因が考えられるという。例えば、夏になって日照時間が増えると人々のビタミンDレベルが上昇し、免疫システムが活性化する可能性があること、また、日光の紫外線がインフルエンザや一般的な風邪の原因となるウイルスを死滅させる可能性があることなどだ。さらに、夏には学校が休みになる国が多く、小児の間で感染が広がりにくくなることも感染の抑制に寄与している可能性があるという。

　この報告を受けて、米ロング・アイランド・ジューイッシュ・フォレスト・ヒルズの感染症専門医Miriam Smith氏は、COVID-19の感染拡大の勢いを最終的に弱めるのは、気候以外の要因だろうとするYao氏らの見解に同意を示している。その上で、「集団免疫の獲得や科学的根拠に基づく有効な治療法の導入、ワクチンの開発が実現するまでは、引き続きソーシャル・ディスタンシング（社会的距離の確保）が感染拡大を阻止するための重要な戦略になるだろう」と話している。

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

https://erj.ersjournals.com/content/early/2020/04/01/13993003.00517-2020

Backgrounds

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 [1]. 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 [4]. In northern Europe, low temperature and low UV Indexes were correlated with peaks of influenza virus activity during 2010–2018 [5]. 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.

Methods

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.

Results

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.

Discussions

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 [8]; 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 [9].

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 [10]. 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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