SARS-CoV-2 Omicron BA.2變異株全球大流行時代的邊境管制：數理模式分析

Abstract

背景與研究目標：邊境管制為防堵COVID-19疫情跨國擴散的重要公共衛生措施，但也會干擾維持社會經濟活力所需跨境商務及觀光人流。臺灣自2022年3月1日起調整防疫政策為「經濟與防疫並存」，並自2022年3月7日起將入境檢疫時間由14天縮短至10天。但3月底隨即在基隆爆發疫情並擴散至全臺灣，截至6月10日止累積染疫數達2,738,582人、死亡數達2,944人。此波疫情起因是否由縮短入境檢疫期所導致，目前仍缺乏嚴謹的流行病學研究；而當未來全民COVID-19疫苗接種率達理想水準，是否能進一步放寬邊境管制措施，開放更多旅客入境 (觀光客入境) 或免除入境檢疫期，而不危害本土疫情控制，目前亦未有數理模式研究。本研究擬以數理模式分析邊境檢疫期縮短與此波境內疫情爆發之關聯性，並進一步分析當境內COVID-19疫苗接種率達理想水準時，在放寬邊境管制時仍能將本土疫情控制在低度流行的必要條件。 方法：本研究建立考慮超級傳播者、SARS-CoV-2 Omicron BA.2變異株流行病學傳播參數、COVID-19疫苗保護力效果的SEIRS數理傳播模式。本研究分為兩部分：第一部分透過擬合臺灣真實疫情驗證模型正確性並估計本土流行病學參數 (境內口罩/社交距離比例)，進一步分析導致臺灣2022年疫情大規模爆發的因素。第二部分模擬當疫苗接種率達理想水準 (追加劑接種率達95%、第二劑接種為97.5%、第一劑接種率為99%)，在目前的3天入境檢疫期，仍然可將疫情控制在低度流行 (單日新確診人數小於1,000) 的必要條件，並模擬免除檢疫期的可行性。分析時考慮不同入境人數 (商務客方案：6,000人/天；觀光客方案：15,000人/天) 與不同旅客來源國風險等級 (中度風險國家旅客帶原率：2.5%；高度風險國家旅客帶原率：10%)。敏感度分析考慮更高傳播力的新Omicron亞型變異株對研究結果的影響 (基本再生數為Omicron BA.2變異株的1.34倍)。 結果：擬合真實疫情可計算出：2022年3月7日邊境檢疫期縮短至10天後，臺灣民眾口罩/社交距離比例由原本的75%下降至僅50%。若是接觸者追蹤及口罩/社交距離比例能維持在3月以前的高水準 (75%接觸者追蹤率、75%口罩/社交距離)，本土疫情可望仍被控制在低度流行 (新確診個案數在6月10日前均低於85人)，口罩/社交距離比例下降實為2022年上半年疫情大爆發之主要原因。第二部份模擬結果顯示：即使疫苗接種率達到理想水準，在6月15後入境檢疫期縮短至3天的情況下，口罩/社交距離比例必須持續維持在90%，才能將開放商務/觀光客入境後境外移入導致的本土疫情控制在低度流行 (單日新確診數高峰介於65至722)。敏感度分析顯示，若出現傳播力更高的新Omicron亞型變異株，口罩/社交距離比例需持續維持在95%以上。若完全取消邊境檢疫，則無法防止新變異株造成本土疫情大爆發。 結論：2022年3月7日邊境檢疫期縮短至10天後，理論上仍可將本土疫情控制在低度流行。境內口罩/社交距離比例下降實為2022年上半年疫情大爆發之主要原因。在目前入境檢疫期縮短至3天的情境下，即使疫苗接種率達理想水準，仍需持續維持95%以上口罩/社交距離，才能避免新Omicron亞型變異株境外移入造成新一波大規模本土疫情。

Background and Aims: Border control is a crucial public health measure which prevents the cross-countries spread of the COVID-19 pandemic. However, it may also disrupt the international exchange of commerce and tourism. From March 01, 2022, the Taiwan government adjusted the policy of COVID-19 prevention to “Prevent COVID-19 epidemic and maintain economically”, and then shortened border quarantine duration to 10 days after March 7, 2022. After that, a domestic outbreak happened in Keelung city at the end of March and spread throughout Taiwan. There was a lack of epidemiological study that analyzed the relationship between relaxing border quarantine and the onset of this epidemic. Also, whether the border control can be further relaxed without jeopardizing domestic epidemic control, when the COVID-19 vaccination rate reaches the ideal level, was lacking modeling study analysis. This study aims to analyze the role of relaxing border quarantine in the onset of the current SARS-CoV-2 Omicron BA.2 outbreak in Taiwan; and to examine whether border control can be relaxed when the national vaccination rate reaches the ideal level, using mathematical modeling. Methods: We established an SEIRS model which considered the superspreading event, the epidemiological transmission parameters of SARS-CoV-2 Omicron BA.2 as well as the effectiveness of the COVID-19 vaccine. There were two parts in this study: In part 1, we validated the model and estimated domestic epidemiological parameters by fitting the real-world epidemic in Taiwan, and then analyzed the factors causing the outbreak in 2022. In part 2, we simulated the necessary conditions which still controlled the domestic epidemic at a low level (< 1,000 daily new confirmed cases) under current 3 day border quarantine when vaccination rate reached ideal level (3rd dose= 95%, 2nd dose= 97.5%, 1st dose= 99%), and simulated the feasibility of lifting border quarantine. Considered the different number of imported travelers (business project: 3,000/day; Tourism project: 15,000/day) and the SARS-CoV-2 infection rate of travelers (from moderate-risk countries: 2.5%; from high-risk countries: 10%). Sensitivity analyses simulated the effect of higher transmissibility Omicron variants (the basic reproduction number was 1.34 times than that of Omicron BA.2). Results: According to the results of epidemic trajectory fitting, the average proportion of mask-wearing/social-distancing had gradually decreased from 75% to only 50% after shortening border quarantine on March 07. Further analysis showed that if contact tracing rate, mask-wearing/social-distancing were maintained at the high level before March 2022, the domestic epidemic could still be controlled at a low level when the border quarantine was shortened to 10 days. Therefore, the relaxing of mask-wearing/social-distancing after shortening border quarantine was the main reason for the domestic outbreak in Taiwan. The results of part 2 showed: even when vaccination rate reached ideal level, the proportion of mask-wearing/social distancing needed to maintain at least 90% to control the domestic epidemic at a low level (the peak of the daily new confirmed cases was ranged 65 to 722). Sensitivity analysis showed that the proportion mask-wearing/social distancing needed to be maintained above 95% when new Omicron variants emerged. If border quarantine was lifted, it would not be possible to prevent new Omicron variants from causing local outbreaks. Conclusion: If the mask and social distancing are kept at a high level, the SARS-CoV-2 Omicron BA.2 outbreak in Taiwan 2022 can be kept at a low level after relaxing border control. To avoid a new wave of a new variant epidemic, it is necessary to maintain greater than 95% mask-wearing/social-distancing under the current 3+4 border quarantine policy.