Age-dependent effects on infectivity and susceptibility to SARS-CoV-2 infection: results from nationwide contact tracing data in Greece

Understanding the factors that affect the transmissibility of SARS-CoV-2 remains important to keep transmission low and maximize the health benefits of vaccination. We assessed the factors associated with the transmissibility of SARS-CoV-2 based on contact tracing data.
From 1 October to 9 December 2020, 29,385 laboratory-confirmed SARS-CoV-2 cases (index cases, i.e. the first identified laboratory-confirmed cases or with the earliest symptom onset in a setting) and 64,608 traced contacts were identified in Greece. We assessed the prevalence of symptoms in cases, calculated secondary attack rates and assessed factors associated with infectivity and susceptibility to infection.
There were 11,232 contacts secondarily infected (secondary attack rate: 17.4%, 95% CI:17.0–17.8). Contacts aged 0–11 and 12–17 years were less susceptible to infection than adults 65 years or older (odds ratio (OR) [95% CI]: 0.28 [0.26–0.32] and 0.44 [0.40–0.49], respectively). Index cases aged 65 years or older were more likely to infect their contacts than other adults or children/adolescents. The odds of infection [95% CI] were higher in contacts exposed within the household (1.71 [1.59–1.85] vs. other) and in cases with cough (1.17 [1.11–1.25] vs. no cough). There was an interaction between the age of the index and the age of the contact with contacts 65 years or older having a higher probability of infection when exposed to cases of similar age than to children.
Our findings highlight the role of age and age mixing in infectivity and susceptibility to SARS-CoV-2 infection. Precautions are necessary for individuals 65 or older as they have higher infectivity and susceptibility in contact with their peers.

明确影响新型冠状病毒（SARS-CoV-2）传播能力的相关因素，对于持续降低病毒传播水平、最大化疫苗接种的健康收益仍具有重要意义。本研究基于接触者追踪（contact tracing）数据，对与SARS-CoV-2传播能力相关的影响因素展开评估。
2020年10月1日至12月9日期间，希腊共甄别出29385例经实验室确认的SARS-CoV-2感染者（首发病例，即某一场景中首个经实验室确诊的病例，或症状出现最早的病例），以及64608名追踪到的密切接触者。本研究对感染者的症状发生情况进行了统计，计算了继发攻击率（secondary attack rate），并对与病毒感染性、接触者易感性相关的影响因素进行了评估。
本次研究中共发现11232例续发感染接触者，继发攻击率为17.4%（95%置信区间[CI]：17.0~17.8）。0~11岁及12~17岁的接触者相较于65岁及以上的成年人，感染易感性更低（优势比[odds ratio, OR] [95% CI]分别为0.28[0.26~0.32]与0.44[0.40~0.49]）。65岁及以上的首发病例，相较于其他成年人群体或儿童/青少年群体，更易将病毒传播给接触者。与非家庭暴露的接触者相比，家庭内暴露的接触者感染风险更高（OR [95% CI]：1.71[1.59~1.85]）；伴有咳嗽症状的首发病例，其接触者的感染风险也高于无咳嗽症状的首发病例（OR [95% CI]：1.17[1.11~1.25]）。此外，首发病例年龄与接触者年龄之间存在交互效应：当65岁及以上的接触者暴露于同年龄段的首发病例时，其感染概率显著高于暴露于儿童/青少年病例的情况。
本研究结果凸显了年龄及年龄混合模式在SARS-CoV-2感染的感染性与易感性中的作用。对于65岁及以上的人群，需采取针对性防护措施，因为该群体在与同龄人接触时，既具有更高的病毒传播能力，也具有更高的感染易感性。