Seropositivity in blood donors and pregnant women during the first year of SARS-CoV-2 transmission in Stockholm, Sweden
Authors: Dopico Xaquin Castro , Muschiol Sandra , Christian Murray , Hanke Leo , Sheward Daniel J. , Grinberg Nastasiya F. , Rorbach Joanna , Bogdanovic Gordana , Mcinerney Gerald M. , Allander Tobias , Wallace Chris , Murrell Ben , Albert Jan , Hedestam Gunilla B. Karlsson 
Published: December 2020

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 14thMarch 2020-28thFebruary 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.

Structured abstract

Objectives

Sweden did not enforce social lockdown in response to the SARS-CoV-2 pandemic. Therefore, we sought to determine the proportion of seropositive healthy, active adults in Stockholm, the country’s most populous region. Random sampling (of blood donors and pregnant women) was carried out during the first year following virus emergence in the country and prior to vaccination of the general adult population – allowing for an estimate of seroprevalence in response to natural infection.

Design

In this cross-sectional prospective study, otherwise-healthy blood donors (n=2,600) and pregnant women(n=2,500) were sampled at random for consecutive weeks (at four intervals) between 14thMarch and 28thFebruary 2021. Sera from all participants and a cohort of historical controls (n=595) were screened for IgG responses against trimers of the SARS-CoV-2 spike (S) glycoprotein and the smaller receptor-binding domain (RBD). As a complement to standard analytical approaches, a probabilistic (cut-off-independent) Bayesian framework that assigns likelihood of past infection was used to analyze data over time. The study was carried out in accordance with Swedish Ethical Review Authority: registration number 2020-01807.

Setting

Healthy participant samples were selected from their respective pools at random through Karolinska University Hospital.

Participants

None of the participants were symptomatic at sampling. No additional metadata was available from the samples.

Results

Blood donors and pregnant women showed a similar seroprevalence. After a steep rise at the start of the pandemic, the seroprevalence trajectory increased steadily in approach to the winter second-wave of infections, approaching 15% of all individuals surveyed by 13thDecember 2020. By the end of February 2021, when deaths were in decline and at low levels following their winter peak, 19% of the population tested seropositive. Notably, 96% of seropositive healthy donors screened (n=56) developed neutralizing antibody responses at titers comparable to, or higher than those observed in clinical trials of SARS-CoV-2 spike mRNA vaccination, supporting that mild infection engenders a competent B cell response.

Conclusions

These data indicate that in the year since the start of community transmission, seropositivity levels in metropolitan Stockholm had reached approximately one-in-five persons, providing important baseline seroprevalence information prior to the start of vaccination.