Setting, venue and safety concept
The congress took place in Geneva (Switzerland) between September 2nd and September 4th 2020, at a time when the number of cases was rising again nationwide and after the first COVID-19 wave had been successfully contained. The Geneva region was particularly affected by the COVID-19 pandemic and showed one of the highest COVID-19 incidences in the country (65 cases/100 000 population in Geneva vs. 29 cases /100000 population for Switzerland) during the week of the congress. The congress took place at Palexpo, an exposition and congress centre offering 106’000 square meters of floor space in a single block hall[10]. Only meetings that complied with the conditions imposed by the council of state were allowed to take place. Safety measures included universal masking (with surgical masks) in the exposition centre (except for the dining area); physical distancing during sessions (with a vacant seat between participants); hand hygiene upon admission to the congress (supervised by safety personnel); multiple dispensers of alcoholic hand rub within the congress area; personal data registration at lunch for contact tracing purposes; only seated catering and cancelling of official social events (Table 3). The pictograms used as part of the safety concept of the exposition centre are shown in Figure S1. All congress participants received instructions on the necessary hygiene measures during the congress via email[11].
Recruitment and study procedures
All congress attendees (including participants, industry representatives and congress staff) >16 years old were informed by email before the congress about the study and were invited to participate in this prospective cohort. For study inclusion, oral informed consent was obtained from participants (approved by the local ethics committee).
Capillary dried blood spots (DBS) from a finger-prick applied on a filter paper card were collected from participants by trained personnel during the congress. Participants received an additional DBS collection kit labelled with their unique study ID and were instructed on how to obtain the follow-up blood sample. The kit contained an instruction sheet (English, German or French), alcohol swabs, auto-retractable safety single-use lancet, a filter paper card, plasters and a return label. Participants were requested to collect finger-prick DBS 4 weeks (+/- 7 days) after the congress and to send the filter paper to the study centre by postal mail. DBS cards were kept at -80°C until analysed.
Four weeks after the congress, an anonymized online questionnaire was sent to all congress attendees. Study participants were asked to enter their study ID in order to match the questionnaires with serology results. Congress attendees not included in the serologic investigation were also invited to fill in the questionnaire. Questions included the type of profession, working canton, canton of residence, the number of contacts with COVID-19 confirmed patients, adherence with protective measures before, during and after the congress (at the congress site but also off-site during private gatherings), adherence to safety measures of other congress attendees (i.e. peer rating), perceived risk of COVID-19 at the congress and the estimated impact of congress safety measures, use of public transportation, leisure activities before and after the congress. Also, participants were asked regarding symptoms compatible with COVID-19, and results of SARS-CoV-2 PCR swabs or serologies done before and after the congress.
SARS-CoV-2 serology from DBS
Before performing serology, filter paper cards (standard 903TM Protein Saver Cards, Eastern Business Forms, Greenville, USA) were punched and eluted. The methodology was previously developed and established in our research laboratory as part of another research project[12]. All five circles were completely punched to 8mm discs, incubated with 0.5ml elution buffer for 180 min on a shaker at room temperature. Finally, serology from the eluent was performed by two chemiluminescence immunoassays (ECLIA, pan immunoglobulin with anti-nucleocapsid (N)- and anti-spike (S)-specificity, Roche Diagnostics, Rotkreuz, Switzerland). The latter assay simultaneously detects IgG, IgM and IgA directed against the receptor binding domain (RBD) of the S1 subunit of the S protein of SARS-CoV-2 and has been evaluated by our group before[13] Samples were deemed positive if either assay returned a result above the manufacturer’s cut-off.
Outcome and data analysis
The primary outcome was a composite of self-reported COVID-19 diagnosis (i.e. SARS-CoV-2 positive nasopharyngeal PCR) within 3 weeks or seroconversion within 4 weeks after the congress. The secondary outcome was self-reported occurrence of COVID-19 compatible symptoms within 2 weeks after the congress (i.e. testing criteria of the Federal Office of Public Health). Baseline characteristics were described using number and percentages for categorical, and mean and standard deviation (or median and interquartile range, as appropriate) for continuous variables. The proportion of participants with the primary or secondary outcome was calculated; positive cases including previous diagnosis of COVID-19 were described in detail. We used SPSS (IBM SPSS Statistics 20, Armonk, New York) for all statistical analyses.