In Sweden, social restrictions to contain SARS-CoV-2 have to date primarily relied upon voluntary adherence to a set of recommendations and strict lockdowns/regulations have not been enforced, potentially affecting viral dissemination. To understand the levels of past SARS-CoV-2 infection in the Stockholm population before the start of mass vaccinations, healthy blood donors and pregnant women (n=5,100) were sampled at random between 14th March 2020-28th February 2021. All individuals (n=200/sampling week) were screened for anti-SARS-CoV-2 spike (S) trimer- and RBD-specific IgG responses and the results were compared with those from historical controls (n=595). Data were modelled using a probabilistic Bayesian framework that considered individual responses to both viral antigens. We found that after a steep rise at the start of the pandemic, the seroprevalence trajectory increased more steadily (over summer) in approach to the winter second-wave of infections, approaching 15% of all adults surveyed by mid-December 2020. The population seropositivity rate again increased more rapidly as cases rose over the winter period. By the end of February 2021, ~19% (~one-in-five) in this study group tested seropositive. Notably, 96% of random seropositive samples screened (n=56), displayed virus neutralizing responses, with titers comparable to those engendered by recently approved mRNA vaccines, supporting that milder infections generally provoke a competent B cell response. These data offer baseline information about the level of seropositivity in this group of active adults in the Stockholm metropolitan area following a full year of SARS-CoV-2 transmission and prior to the introduction of vaccines.

中文翻译：

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瑞典斯德哥尔摩SARS-CoV-2传播第一年期间，献血者和孕妇的血清阳性

在瑞典，迄今为止，遏制SARS-CoV-2的社会限制主要依靠自愿遵守一系列建议，并且尚未实施严格的封锁/法规，这可能会影响病毒的传播。为了了解大规模疫苗接种开始之前斯德哥尔摩人口中过去SARS-CoV-2感染的水平，在2020年3月14日至2021年2月28日之间随机抽取了健康献血者和孕妇（n = 5,100）。筛选n = 200 /采样周的抗SARS-CoV-2峰值（S）三聚体和RBD特异性IgG反应，并将结果与​​历史对照的结果进行比较（n = 595）。使用概率贝叶斯框架对数据建模，该框架考虑了对两种病毒抗原的个体反应。我们发现，在大流行开始时急剧上升之后，到冬季第二波感染时，血清阳性率轨迹（在整个夏季）稳定增加，到2020年12月中旬已接近调查的所有成年人的15％。随着冬季病例的增加，人群的血清阳性率再次提高得更快。到2021年2月底，该研究组中约有19％（五分之一）检测到血清反应阳性。值得注意的是，筛选出的血清反应阳性样本中有96％（n = 56）显示出病毒中和反应，其滴度可与最近批准的mRNA疫苗产生的滴度相提并论，这表明较温和的感染通常会引起有效的B细胞反应。