Sanitary safety of the 2021 French Intensive Care Society medical conference: a case/control study

Non-pharmacological interventions aimed at preventing SARS-CoV-2 transmission include contact tracing and quarantine of cases, up to lockdowns when rates of infection are higher as well as physical distancing; which includes limiting the occurrence of mass gathering events (MGE), [1]. Indeed, several outbreaks of SARS-CoV-2 were associated with MGE [2], linked to the attendance of asymptomatic carriers who may nevertheless transmit the virus. Indeed, SARS-CoV-2 transmissibility begins 2 to 3 days before symptom onset; nearly half of all transmissions arise through contact with asymptomatic individuals [3]. Additionally, reverse transcriptase-polymerase chain reaction (RT-PCR) test results are delayed, making it an impractical tool for mass testing strategies prior to a MGE. Alternative point-of-care diagnostic methods include antigen-detecting rapid diagnostic tests. Lateral flow tests (LFT) detect the nucleocapsid protein antigen of SARS-CoV-2 and provide results within 30 min. LTF may be self-administered at home, enabling the rapid identification of people infected by the virus.

Despite improved capacities of detecting SARS-CoV-2 infections, few prospective assessments of the sanitary safety of mass gathering events (MGE) have been performed. In a pivotal study conducted in Spain, out of 465 spectators of a live music event, 13 (3%) subjects were found to be positive for SARS-CoV-2 antigen testing [4]. The same team found in a subsequent study, found that out of 5000 subjects willing to spectate a live music event, 6 were tested positive for SARS-CoV-2 [5]. However, these observations occurred prior to mass vaccination of the population. In France, Delaugerre et al. in a randomized controlled trial with almost 4000 attendees, found than participation in a large, indoor, live gathering without physical distancing was not associated with increased SARS-CoV-2 transmission risk, provided a comprehensive preventive intervention was implemented [6].

Among MGE, medical conferences are of special interest because they bring together healthcare workers who are on the frontline of the fight against COVID-19. Said healthcare workers are exposed on a day-to-day basis to SARS-CoV-2 both in their COVID-19 units and out of hospital. Additionally, healthcare workers are essential to provide adequate care for COVID-19 infected patients; sickness leave due to SARS-CoV-2 exposure or infection may weaken already overstretched healthcare system. This is especially true in intensive care units (ICU) [7]. The sanitary safety of medical congresses is thus of utmost importance.

To assess potential transmission of SARS-CoV-2 during the French Intensive Care Society annual meeting, we conducted a prospective study with systematic LFT tests performed by attendees and controls 7 days after the event. This event took place in a context where vaccination and/or negative SARS-CoV-2 testing were required for attendance.

We found that, in the current area of highly effective vaccines, SARS-CoV-2 transmission during a medical congress was extremely low and similar in controls who did not attend the conference.

A recent review of published studies focusing on the impact of mitigation strategies during the COVID-19 pandemic [10] concluded that “there is currently limited evidence on the effectiveness of measures to prevent SARS-CoV-2 transmission at mass gatherings”. Our results are in line with previous studies on the safety of mass gathering events while respecting physical distancing and simple measures of hygiene. In an earlier study, Flury et al. included 196 participants out of the 365 attendees of the Swiss Societies of Infectious Diseases and Hospital Hygiene congress [11]. The authors found that 5 participants presented a positive SARS-CoV-2 serology at follow-up, all whom were already positive at baseline. In the study by Revollo et al. [4], out of the 495 participants attending a music event, none presented a positive PCR at day 8, resulting in a Bayesian estimate for the incidence between the exposed and the non-exposed of − 0·15% (95% CI – 0·72 to 0·44). In our study, the frequentist estimate for the difference in incidence between the participants and the controls ranged between − 2 to 2%.

In addition to these results, our study highlights several challenges encountered in epidemiological studies assessing SARS-CoV-2 transmission. First, participation in a scientific evaluation of transmission is challenging, even in a population of healthcare workers confronted to COVID-19 since the beginning of the pandemic. Indeed, only around 30% of healthcare workers attending the conference agreed to be included and less than half of these self-administered their LFT at day 7 and reported their status after the congress. This low feed-back from participants in the context of self-delivered COVID-19 testing must be anticipated especially for self-funded research study. Second, we found that the recruitment of controls by participants allowing a fair comparison of infectious disease contamination rates is possible for participants who are healthcare workers in departments highly involved in clinical research [12], with almost as many controls as attendees recruited. We also showed that COVID-19 LFT can be self-performed by participants without major technical challenge. These tests could thus be used to mitigate the risk of COVID-19 transmission in other settings. Indeed, although LFT are less sensitive than COVID-19 PCR and despite commercial kits displaying variable performance, their cost is reduced compared to PCR, greatly facilitating mass testing. The sensitivity of the LFT used in this study was 94.5%, i.e., one of the best tests available at the time of the study [13]. In a study of 5,869 asymptomatic adults, Garcia-Finana et al. found that LFT were likely to detect a minimum of 3/5 and at most 998 out of every 1,000 subjects with a positive RT-qPCR test result and a high viral load [14]. We chose to perform LFT at day 7, since it was the best time window to detect SARS-CoV-2 transmission [15]. Our follow-up rate was 42% (216/520), similar to previous reports, i.e., between 13 and 66% [16]. Lastly, CO₂ levels measured during the event were broadly lower than recommended (between 600 to 1500 parts per million) [17, 18] indicating that the venue was sufficiently ventilated.

In addition, limitations due to the low rate of attendee recruitment and feed-back from participants, and the generalizability of our study must be discussed. First, our study was underpowered to detect a small but potentially clinically relevant difference due to the relatively low incidence of COVID-19 cases in France (40 cases/100.000) and Great Paris area (65 cases/100.000) at the time of the congress. Second, at the time of the congress, the SARS-CoV-2 alpha variant was still dominant; our results may therefore not be true faced with delta or any other highly transmissible variant. Third, our study must be interpreted in the context of this specific mass event: young age of the participants and high rate of participants within 6 months of a complete vaccination scheme. Last, absence of randomization or of stratification by region or by profession, limit causal inference between congress attendance and risk of SARS-CoV-2 transmission.

In our study of a medical congress, screening for SARS-CoV-2 transmission by LFT at day 7, identified no positive cases among congress attendees or matched controls. In the context of low overall SARS-CoV-2 incidence and a predominant alpha variant, we found than mass event gathering can be safely performed if adequate mitigations procedures are applied before and during the congress.