We conducted a prospective observational study, including both attendees to the French Intensive Care Society (FICS/SRLF) annual meeting held in Paris between June 9 and June 11, 2021 and matched controls. Indeed, given the specific exposure of ICU healthcare professionals, inclusion of a control grouped seemed mandatory. Matched controls were colleagues of attendees who stayed in ICU throughout the medical conference. Data were collected during the meeting and up to day 21 after the meeting. This report follows the STROBE guidelines .
Requirements to attend the congress
French regulation in place at the time of congress mandated each attendee to comply with at least one the following requirements:
7 days after 2nd administration for dual dose vaccines (Pfizer©, Moderna©, AstraZeneca©);
28 days after administration for single dose vaccines (Janssen/Johnson & Johnson©);
7 days after administration of the vaccine for people who had previously contracted COVID-19 (only 1 injection) more than 6 months ago;
Recovery from COVID-19 attested by the result of a positive SARS-CoV-2 RT-PCR or antigenic test dating from at least 15 days and less than 6 months (associated with a limited risk of reinfection with COVID-19) .
Proof of a negative SARS-CoV-2 RT-PCR or antigenic detection test performed within less than 48 h.
Other mitigation strategies during the congress were implemented and included: availability of hand sanitizers, mandatory wearing of surgical face masks and adequate ventilation of all congress areas. Ventilation of congress areas was assessed by measuring average carbon dioxide (CO2) levels using a CO2 Monitoring device Air Therm (La Mode, London, UK).
Inclusion criteria for the attendees were:
Absence of COVID-19 symptoms over the 2 weeks prior to inclusion,
Not being contact of a case of COVID-19 over the 2 weeks prior to inclusion,
Attending at least one day of the FICS annual meeting (June 9–11 2021).
Exclusion criteria were:
Refusal to participate,
Guardianship or tutorship,
Absent affiliation to the French social security.
Controls were recruited by the attendees. The same inclusion and exclusion criteria were applied to controls except that they did not participate to the annual meeting. Controls were matched to attendees in terms of gender, age (< or > 40 years), profession (medical doctor, nurse, nurse assistant, other) and vaccine status (complete, partial, non-vaccinated).
Self-administered LFT SARS-CoV-2 tests
LFT for SARS-CoV-2 aims to detect infection by recognizing viral proteins. Most LFT use specific labeled antibodies attached to a nitrocellulose matrix strip to capture viral antigens. The Flowflex SARS-CoV-2 Antigen Rapid Test is a lateral flow chromatographic immunoassay for the qualitative detection of the nucleocapsid protein antigen from SARS-CoV-2 in nasal and nasopharyngeal swab specimens directly from individuals who are suspect of COVID-19. Successful binding of the antibodies to the antigen is visually detected through the appearance of a line on the matrix strip. The Flowflex SARS-CoV-2 Antigen Rapid test provides results within 30 min.
LFTs were freely provided at the time of inclusion. 7 ± 1 days after the attendee’s last day of attendance, the attendee and his control were asked to perform a LFT.
Standardized forms were used to record the following data: participant’s baseline characteristics (age, gender, height, weight) and COVID-19 status (previous COVID-19 infection and date of occurrence), risk factors of developing a severe form of COVID-19 (see Additional file 4: Table S1), anti-SARS-CoV-2 vaccination status (number of doses, date of injection, type of vaccine), results of the day 7 self-administered LFT and occurrence of COVID-19 symptoms and/or need of medical assistance for a COVID-19 infection over a three week period following the congress. Follow-up was obtained at day 21 for each participant and control.
The study was approved by French Infectious Disease Society ethics committee (CERMIT N° COVID 2021-08) and was registered on ClinicalTrials.gov (#NCT03600181). Written information was delivered to all participants. All participants entered the study following oral consent.
The primary endpoint was the prevalence of attendees with a positive COVID-19 self-administration LFT at day 7 of participation to the congress.
Secondary objectives were:
COVID-19 prevalence among controls at Day 7,
Proportion of congress attendees with COVID-19 symptoms by Day 21,
Proportion of congress attendees requiring COVID-19-related appointment with a general practitioner by Day 21,
Proportion of congress attendees with COVID-19-related emergency department visits by Day 21,
Proportion of conference attendees with COVID-19-related hospitalization by Day 21.
Given the exploratory nature of our study, we did not calculate a sample size. We aimed at including a convenience sample of 500 attendees and 500 controls (1000 participants).
Qualitative variables were described as number (%) and quantitative variables as mean ± SD if normally distributed and as median [25th–75th percentile] otherwise. Matching factors between attendees and controls were compared by means of Fisher tests. All tests were two-tailed with a significance level of 0.05.
Confidence interval for the probabilities of being infected for attendees and controls was performed by means of one-tailed exact 95% confidence interval for binomial proportions. For the difference in proportions, we used the simple “add 2 success, add 2 failures” method .
All statistical analyses were performed using R statistical software version 4.0.3.