Impact of SARS-CoV-2 Delta variant on incubation, transmission settings and vaccine effectiveness: Results from a nationwide case-control study in France

BACKGROUND

We aimed to assess the settings and activities associated with SARS-CoV-2 infection in the context of B.1.617.2 (Delta) variant circulation in France, as well as the protection against symptomatic Delta infection.

METHODS

In this nationwide case-control study, cases were SARS-CoV-2 infected adults recruited between 23 May and 13 August 2021. Controls were non-infected adults from a national representative panel matched to cases by age, sex, region, population density and calendar week. Participants completed an online questionnaire and multivariable logistic regression analysis was used to determine the association between acute SARS-CoV-2 infection and recent activity-related exposures, past history of SARS-CoV-2 infection, and COVID-19 vaccination.

FINDINGS

We did not find any differences in the settings and activities associated with Delta versus non-Delta infections and grouped them for subsequent analyses. In multivariable analysis involving 12634 cases (8644 Delta and 3990 non-Delta) and 5560 controls, we found individuals under 40 years and attending bars (aOR:1.9; 95%CI:1.6-2.2) or parties (aOR:3.4; 95%CI:2.8-4.2) to be at increased risk of infection. In those aged 40 years and older, having children attend daycare (aOR:1.9; 95%CI:1.1-3.3), kindergarten (aOR:1.6; 95%CI:1.2-2.1), primary school (aOR:1.4; 95%CI:1.2-1.6) or middle school (aOR:1.3; 95%CI:1.2-1.6) were associated with increased risk of infection. We found strong protection against symptomatic Delta infection for those with prior infection whether it was recent (2-6 months) (95%; 95%CI:90-97) or associated with one dose (85%; 95%CI:78-90) or two doses of mRNA vaccine (96%; 95%CI:87-99). For those without past infection, protection was lower with two doses of mRNA vaccine (67%; 95%CI:63-71).

INTERPRETATION

In line with other observational studies, we find reduced vaccine effectiveness against symptomatic Delta infections. The settings and activities at increased risk of infection indicate where efforts to reinforce individual and public health measures need to be concentrated.

Impact of original, B.1.1.7, and B.1.351/P.1 SARS-CoV-2 lineages on vaccine effectiveness of two doses of COVID-19 mRNA vaccines: Results from a nationwide case-control study in France

BACKGROUND

We aimed to assess the effectiveness of two doses of mRNA COVID-19 vaccines against COVID-19 with the original virus and other lineages circulating in France.

METHODS

In this nationwide case-control study, cases were SARS-CoV-2 infected adults with onset of symptoms between 14 February and 3 May 2021. Controls were non-infected adults from a national representative panel matched to cases by age, sex, region, population density and calendar week. Participants completed an online questionnaire on recent activity-related exposures and vaccination history. Information about the infecting virus was based on a screening RT-PCR for either B.1.1.7 or B.1.351/P.1 variants.

FINDINGS

Included in our analysis were 7 288 adults infected with the original SARS-CoV-2 virus, 31 313 with the B.1.1.7 lineage, 2 550 with B.1.351/P1 lineages, and 3 644 controls. In multivariable analysis, the vaccine effectiveness (95% confidence interval) seven days after the second dose of mRNA vaccine was estimated at 88% (81-92), 86% (81-90) and 77% (63-86) against COVID-19 with the original virus, the B.1.1.7 lineage, and the B.1.351/P.1 lineages, respectively. Recent (2 to 6 months) history of virologically confirmed SARS-CoV-2 infection was found to be 83% (76-88), 88% (85-91) and 83% (71-90) protective against COVID-19 with the original virus, the B.1.1.7 lineage, and the B.1.351/P.1 lineages, respectively; and more distant (> 6 months) infections were 76% (54-87), 84% (75-90), and 74% (41-89) protective against COVID-19 with the original virus, the B.1.1.7 lineage, and the B.1.351/P.1 lineages, respectively.

INTERPRETATION

In real-life settings, two doses of mRNA vaccines proved to be effective against COVID-19 with the original virus, B.1.1.7 lineage and B.1.351/P.1 lineages.

FUNDING

Institut Pasteur, Research & Action Emerging Infectious Diseases (REACTing), Fondation de France (Alliance "Tous unis contre le virus").

Exposures associated with SARS-CoV-2 infection in France: A nationwide online case-control study

BACKGROUND

We aimed to assess the role of different setting and activities in acquiring SARS-CoV-2 infection.

METHODS

In this nationwide case-control study, cases were SARS-CoV-2 infected adults recruited between 27 October and 30 November 2020. Controls were individuals from the Ipsos market research database matched to cases by age, sex, region, population density and time period. Participants completed an online questionnaire on recent activity-related exposures.

FINDINGS

Among 3426 cases and 1713 controls, in multivariable analysis, we found an increased risk of infection associated with any additional person living in the household (adjusted-OR: 1•16; 95%CI: 1•11-1•21); having children attending day-care (aOR: 1•31; 95%CI: 1•02-1•62), kindergarten (aOR: 1•27; 95%CI: 1•09-1•45), middle school (aOR: 1•30; 95%CI: 1•15-1•47), or high school (aOR: 1•18; 95%CI: 1•05-1•34); with attending professional (aOR: 1•15; 95%CI: 1•04-1•26) or private gatherings (aOR: 1•57; 95%CI: 1•45-1•71); and with having frequented bars and restaurants (aOR: 1•95; 95%CI: 1•76-2•15), or having practiced indoor sports activities (aOR: 1•36; 95%CI: 1•15-1•62). We found no increase in risk associated with frequenting shops, cultural or religious gatherings, or with transportation, except for carpooling (aOR: 1•47; 95%CI: 1•28-1•69). Teleworking was associated with decreased risk of infection (aOR: 0•65; 95%CI: 0•56-0•75).

INTERPRETATION

Places and activities during which infection prevention and control measures may be difficult to fully enforce were those with increased risk of infection. Children attending day-care, kindergarten, middle and high schools, but not primary schools, were potential sources of infection for the household.

FUNDING

Institut Pasteur, Research & Action Emerging Infectious Diseases (REACTing), Fondation de France (Alliance" Tous unis contre le virus").

SARS-CoV-2 incubation period across variants of concern, individual factors, and circumstances of infection in France: a case series analysis from the ComCor study

BACKGROUND

The incubation period of SARS-CoV-2 has been estimated for the known variants of concern. However, differences in study designs and settings make comparing variants difficult. We aimed to estimate the incubation period for each variant of concern compared with the historical strain within a unique and large study to identify individual factors and circumstances associated with its duration.

METHODS

In this case series analysis, we included participants (aged ≥18 years) of the ComCor case-control study in France who had a SARS-CoV-2 diagnosis between Oct 27, 2020, and Feb 4, 2022. Eligible participants were those who had the historical strain or a variant of concern during a single encounter with a known index case who was symptomatic and for whom the incubation period could be established, those who reported doing a reverse-transcription-PCR (RT-PCR) test, and those who were symptomatic by study completion. Sociodemographic and clinical characteristics, exposure information, circumstances of infection, and COVID-19 vaccination details were obtained via an online questionnaire, and variants were established through variant typing after RT-PCR testing or by matching the time that a positive test was reported with the predominance of a specific variant. We used multivariable linear regression to identify factors associated with the duration of the incubation period (defined as the number of days from contact with the index case to symptom onset).

FINDINGS

20 413 participants were eligible for inclusion in this study. Mean incubation period varied across variants: 4·96 days (95% CI 4·90-5·02) for alpha (B.1.1.7), 5·18 days (4·93-5·43) for beta (B.1.351) and gamma (P.1), 4·43 days (4·36-4·49) for delta (B.1.617.2), and 3·61 days (3·55-3·68) for omicron (B.1.1.529) compared with 4·61 days (4·56-4·66) for the historical strain. Participants with omicron had a shorter incubation period than participants with the historical strain (-0·9 days, 95% CI -1·0 to -0·7). The incubation period increased with age (participants aged ≥70 years had an incubation period 0·4 days [0·2 to 0·6] longer than participants aged 18-29 years), in female participants (by 0·1 days, 0·0 to 0·2), and in those who wore a mask during contact with the index case (by 0·2 days, 0·1 to 0·4), and was reduced in those for whom the index case was symptomatic (-0·1 days, -0·2 to -0·1). These data were robust to sensitivity analyses correcting for an over-reporting of incubation periods of 7 days.

INTERPRETATION

SARS-CoV-2 incubation period is notably reduced in omicron cases compared with all other variants of concern, in young people, after transmission from a symptomatic index case, after transmission to a maskless secondary case, and (to a lesser extent) in men. These findings can inform future COVID-19 contact-tracing strategies and modelling.

FUNDING

Institut Pasteur, the French National Agency for AIDS Research-Emerging Infectious Diseases, Fondation de France, the INCEPTION project, and the Integrative Biology of Emerging Infectious Diseases project.

Risk of SARS-CoV-2 Infection Among Households With Children in France, 2020-2022

Importance

Understanding the contribution of children to SARS-CoV-2 circulation in households is critical for designing public health policies and mitigation strategies.

Objective

To identify temporal changes in the risk of SARS-CoV-2 infection in people living with children.

Design, Setting, and Participants

This case-control study included online questionnaire responses from French adults between October 2020 and October 2022. Eligible cases were adults with ongoing SARS-CoV-2 infection with an email address on record with the national health insurance system, which centralized all new diagnoses in France. Eligible controls were adults who had never tested positive for SARS-CoV-2 until February 2021, when eligibility was extended to all adults without ongoing SARS-CoV-2 infection.

Exposure

Transmission of SARS-CoV-2 from a child (aged under 18 years) within the household in the descriptive analysis, as reported by the participating case. Sharing household with a child (of any age or broken down by school level) in the case-control analysis.

Main Outcome and Measures

Ongoing SARS-CoV-2 infection diagnosed by reverse transcription-polymerase chain reaction or supervised rapid antigen test (ie, not self-tests).

Results

A total of 682 952 cases were included for the descriptive analysis (68.8% female, median [IQR] age, 44 [34-55] years). Among those, 45 108 (6.6%) identified a household child as the source case; this proportion peaked at 10.4% during the Omicron BA.1 wave (December 20, 2021, to April 8, 2022). For the case-control analysis, we matched 175 688 cases (with a 4:1 ratio) for demographic characteristics with 43 922 controls. In multivariable logistic regression analysis, household exposure to children was associated with an increased risk of infection mainly at the end of summer 2021 (receding Delta wave) and during winter 2022 (Omicron BA.1 wave). In subgroup analysis by school level of the child, living with children under the age of 6 was associated with increased odds of infection throughout the study period, peaking at an odds ratio (OR) 1.8 (95% CI, 1.6-2.1) for children looked after by professional in-home caregivers, 1.7 (95% CI, 1.5-1.7) for children in day care facilities, and 1.6 (95% CI, 1.4-1.8) for children in preschool. The ORs associated with household exposure to children aged 6 to 14 years increased during the Delta (August 14, 2021, to December 19, 2021) and Omicron BA.1 waves, reaching 1.6 (95% CI, 1.5-1.7) for primary school children and 1.4 (95% CI, 1.3-1.5) for middle school children. Exposure to older children aged 15 to 17 years was associated with a moderate risk until April 2021, with an OR of 1.2 (95% CI, 1.2-1.3) during curfew in early 2021 (December 4, 2020, to April 8, 2021).

Conclusions and Relevance

The presence of children, notably very young ones, was associated with an increased risk of SARS-CoV-2 infection in other household members, especially during the Delta and Omicron BA.1 waves. These results should help to guide policies targeting children and immunocompromised members of their household.

Exposures associated with SARS-CoV-2 infection in France: A nationwide online case-control study

BACKGROUND

We aimed to assess the role of different setting and activities in acquiring SARS-CoV-2 infection.

METHODS

In this nationwide case-control study, cases were SARS-CoV-2 infected adults recruited between 27 October and 30 November 2020. Controls were individuals from the Ipsos market research database matched to cases by age, sex, region, population density and time period. Participants completed an online questionnaire on recent activity-related exposures.

FINDINGS

Among 3426 cases and 1713 controls, in multivariable analysis, we found an increased risk of infection associated with any additional person living in the household (adjusted-OR: 1•16; 95%CI: 1•11-1•21); having children attending day-care (aOR: 1•31; 95%CI: 1•02-1•62), kindergarten (aOR: 1•27; 95%CI: 1•09-1•45), middle school (aOR: 1•30; 95%CI: 1•15-1•47), or high school (aOR: 1•18; 95%CI: 1•05-1•34); with attending professional (aOR: 1•15; 95%CI: 1•04-1•26) or private gatherings (aOR: 1•57; 95%CI: 1•45-1•71); and with having frequented bars and restaurants (aOR: 1•95; 95%CI: 1•76-2•15), or having practiced indoor sports activities (aOR: 1•36; 95%CI: 1•15-1•62). We found no increase in risk associated with frequenting shops, cultural or religious gatherings, or with transportation, except for carpooling (aOR: 1•47; 95%CI: 1•28-1•69). Teleworking was associated with decreased risk of infection (aOR: 0•65; 95%CI: 0•56-0•75).

INTERPRETATION

Places and activities during which infection prevention and control measures may be difficult to fully enforce were those with increased risk of infection. Children attending day-care, kindergarten, middle and high schools, but not primary schools, were potential sources of infection for the household.

FUNDING

Institut Pasteur, Research & Action Emerging Infectious Diseases (REACTing), Fondation de France (Alliance" Tous unis contre le virus").

Source of SARS-CoV-2 infection: results from a series of 584,846 cases in France from October 2020 to August 2022

BACKGROUND

We aimed to study the source of infection for recently SARS-CoV-2-infected individuals from October 2020 to August 2022 in France.

METHODS

Participants from the nationwide ComCor case-control study who reported recent SARS-CoV-2 infection were asked to document the source and circumstances of their infection through an online questionnaire. Multivariable logistic regression was used to identify the factors associated with not identifying any source of infection.

RESULTS

Among 584,846 adults with a recent SARS-CoV-2 infection in France, 46.9% identified the source of infection and an additional 22.6% suspected an event during which they might have become infected. Known and suspected sources of infection were household members (30.8%), extended family (15.6%), work colleagues (15.0%), friends (11.0%), and possibly multiple/other sources (27.6%). When the source of infection was known, was not a household member, and involved a unique contact (n = 69,788), characteristics associated with transmission events were indoors settings (91.6%), prolonged (> 15 min) encounters (50.5%), symptomatic source case (64.9%), and neither the source of infection nor the participant wearing a mask (82.2%). Male gender, older age, lower education, living alone, using public transportation, attending places of public recreation (bars, restaurants, nightclubs), public gatherings, and cultural events, and practicing indoor sports were all independently associated with not knowing the source of infection.

CONCLUSION

Two-thirds of infections were attributed to interactions with close relatives, friends, or work colleagues. Extra-household indoor encounters without masks were commonly reported and represented avoidable circumstances of infection.

TRIAL REGISTRATION

ClinicalTrials.gov registration number: NCT04607941.

Correlates of Protection Against Symptomatic COVID-19: The CORSER 5 Case-Control Study

Background

Establishing correlates of protection often requires large cohorts. A rapid and adaptable case-control study design can be used to identify antibody correlates of protection against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in serum and saliva.

Methods

We designed a case-control study to compare antibody levels between cases of SARS-CoV-2 infection within 5 days of symptom onset and uninfected controls. Controls were matched on age, number of coronavirus disease 2019 vaccine doses, time since last dose, and past episodes of infection. We quantified anti-SARS-CoV-2 and seasonal coronavirus immunoglobulin (Ig) G in serum and saliva at inclusion, 1 month, and 6 months.

Results

We included 90 cases and 62 controls between February and September 2022. A boost and decay pattern of serum antibodies was observed in cases at 1 and 6 months, respectively, but not in controls. Anti-SARS-CoV-2 antibody levels were significantly higher in controls at inclusion both in serum (particularly antinucleocapsid IgG: 4.14 times higher compared with cases; 95% CI, 2.46-6.96) and saliva (particularly antispike for Delta variant IgG: 4.89 times higher compared with cases; 95% CI, 2.91-9.89). Saliva antibodies generally outperformed serum antibodies for case/control differentiation.

Conclusions

In this case-control study, we provided evidence of correlates of protection of anti-SARS-CoV-2 IgG in saliva and serum, with saliva antibodies often outperforming serum. The finding that antibodies in saliva are a better correlate of protection than antibodies in serum may inform vaccine development by highlighting the importance of robust induction of mucosal immune responses. This study design may be used during future epidemics for the prompt assessment of correlates of protection.

Risk of SARS-CoV-2 infection in professional settings, shops, shared transport, and leisure activities in France, 2020-2022

PURPOSE

The aim of the study was to identify settings associated with SARS-CoV-2 transmission throughout the COVID-19 pandemic in France.

METHODS

Cases with recent SARS-CoV-2 infection were matched with controls (4:1 ratio) on age, sex, region, population size, and calendar week. Odds ratios for SARS-CoV-2 infection were estimated for nine periods in models adjusting for socio-demographic characteristics, health status, COVID-19 vaccine, and past infection.

RESULTS

Between October 27, 2020 and October 2, 2022, 175,688 cases were matched with 43,922 controls. An increased risk of infection was documented throughout the study for open-space offices compared to offices without open space (OR range across the nine periods: 1.12 to 1.57) and long-distance trains (1.25 to 1.88), and during most of the study for convenience stores (OR range in the periods with increased risk: 1.15 to 1.44), take-away delivery (1.07 to 1.28), car-pooling with relatives (1.09 to 1.68), taxis (1.08 to 1.89), airplanes (1.20 to 1.78), concerts (1.31 to 2.09) and night-clubs (1.45 to 2.95). No increase in transmission was associated with short-distance shared transport, car-pooling booked over platforms, markets, supermarkets and malls, hairdressers, museums, movie theatres, outdoor sports, and swimming pools. The increased risk of infection in bars and restaurants was no longer present in restaurants after reopening in June 2021. It persisted in bars only among those aged under 40 years.

CONCLUSION

Closed settings in which people are less likely to wear masks were most affected by SARS-CoV-2 transmission and should be the focus of air quality improvement.

CLINICALTRIALS

GOV (03/09/2022): NCT04607941.

Contributions of the qualitative Qualicor study embedded in a cohort study on the circumstances of SARS-CoV 2 infection in France

OBJECTIVES

This study aims to understand a major result of ComCor, an online epidemiological study conducted to identify the circumstances of COVID-19 infection in France from 2020 to 2022: One third of respondents reported ignoring the circumstances of their infection.

METHODS

We conducted a qualitative study through semi-structured interviews, diagnosed in spring or summer 2021. Interviews were audio recorded, transcribed, and thematically analyzed.

RESULTS

Fifty interviews were conducted. Half of the participants in Qualicor were able to identify several at-risk situations, most often involving their entourage (family, friends, colleagues), but were uncertain as to which specific situation was the source of infection. Less than one quarter strongly suspected a specific situation without certainty, a similar proportion were unable to identify any circumstances, and only two people were certain about the origin of the infection. Several factors contributed to this lack of knowledge: a desire to conceal these circumstances (in a few rare cases), limitations of the questionnaire, lack of knowledge about how the virus is transmitted, selective perception of at-risk situations, co-existence of several possible sources of infection, and the difficulty of taking an objective view of certain circumstances of transmission.

CONCLUSION

Our study shows the benefits of a mixed approach designed to better understand the perception of Covid 19 contamination circumstances in the French population. It also highlights the need to strengthen or improve communication on modes of virus transmission, especially airborne transmission, and the importance of maintaining certain preventive behaviors after vaccination.

Benefit-Risk Assessment of the French Surveillance Protocol of Apparently Healthy Biting Dogs and Cats for Human Rabies Prevention

In France, apparently healthy dogs and cats that bite humans must undergo an observation period of 15 days with three veterinary visits to ascertain that they remain healthy, indicating that no zoonotic transmission of rabies virus occurred via salivary presymptomatic excretion. This surveillance protocol is mandatory for all pets that have bitten humans, despite France’s rabies-free status in non-flying mammals (i.e., a very low rabies risk). In this context, we aimed to perform a benefit–risk assessment of the existing regulatory surveillance protocol of apparently healthy biting animals, as well as alternative surveillance protocols. A scenario-tree modelling approach was used to consider the possible successions of events between a dog or cat bite and a human death attributed to either rabies or to lethal harm associated with the surveillance protocol (e.g., lethal traffic accidents when traveling to veterinary clinics or anti-rabies centers). The results demonstrated that the current French surveillance protocol was not beneficial, as more deaths were generated (traffic accidents) than avoided (by prompt post-exposure prophylaxis administration). We showed here that less stringent risk-based surveillance could prove more appropriate in a French context. The results in this study could allow policy-makers to update and optimize rabies management legislation.

Are psychological attitudes towards vaccination an expression of personality? A cross-sectional study on COVID-19 vaccination in France

BACKGROUND

The capacity of the 7C model's psychological antecedents, which include confidence in vaccines, complacency, convenience, calculation, collective responsibility, confidence in the wider system, and social conformism, to explain variance in COVID-19 vaccine intentions and behaviours has been documented. However, it remains unclear whether the attitudes represented by the 7C psychological antecedents are specific to vaccination or if they are, in fact, an expression of underlying personality traits.

METHODS

From February to June 2022, French adults completed self-administered questionnaires assessing COVID-19 vaccination history, the 7C antecedents, and personality traits ("ComCor" and "Cognitiv" studies). Vaccination behaviours were studied through three outcomes: at-least-one-dose vaccination status by 2022 (N = 49,019), up-to-date vaccination status (N = 46,566), and uptake speed of first dose (N = 25,998). Personality traits were evaluated using the French version of the Big Five Inventory (BFI-Fr). Multivariable logistic regressions and Cox models predicting vaccine behaviours were run with the 7C antecedents, both with and without personality traits.

RESULTS

Among the 49,019 participants, 95.0% reported receipt of at least one dose and 89.8% were up to date with recommendations. All 7C antecedents were significantly associated with the outcomes. The inclusion of personality traits did not substantially alter the effect estimates of the association between the 7C antecedents and vaccination behaviours, with differences between effect sizes of models with and without personality traits being < 5%.

CONCLUSIONS

Our results suggest that the 7C psychological antecedents of vaccination are not the mere expression of personality and that their impact on vaccine behaviours is independent of personality traits. As such, the 7C antecedents may be modifiable by appropriate information and vaccine promotion.

TRIAL REGISTRATION

The "ComCor" study received ethical approval by the Comité de Protection des Personnes Sud Ouest et Outre Mer 1 on 21/09/2020. The addition of the "Cognitiv" questionnaire received ethical approval by the Comité de Protection des Personnes Sud Ouest et Outre Mer 1 on 01/02/2022. The data protection authority Commission Nationale de l'Informatique et des Libertés (CNIL) authorised the processing of data on 21/10/2020. The study is registered with ClinicalTrials.gov under the identifier NCT04607941.

Are the 7C psychological antecedents associated with COVID-19 vaccine behaviours beyond intentions? A cross-sectional study on at-least-one-dose and up-to-date vaccination status, and uptake speed among adults in France

BACKGROUND

Widely documented psychological antecedents of vaccination are confidence in vaccines, complacency, convenience, calculation, collective responsibility (5C model) with the recent addition of confidence in the wider system and social conformism. While the capacity of these seven antecedents (7C) to explain variance in COVID-19 vaccine intentions has been previously documented, we study whether these factors also are associated with vaccine behaviours, beyond intentions.

METHODS

From February to June 2022, we recruited a sample of adults in France, including persons with notified recent SARS-CoV-2 infection, along with relatives and randomly selected non-infected persons. Participants completed self-administered questionnaires assessing COVID-19 vaccination history and the 7C antecedents. We defined vaccination behaviours as three outcomes: at-least-one-dose vaccine status by 2022 (N = 49,019), up-to-date vaccination status (N = 46,566), and uptake speed of first dose (N = 25,998). We conducted multivariable logistic regressions and Cox models.

RESULTS

Among the 49,019 participants, 95.0% reported receipt of at least one dose and 89.8% were up to date with recommendations. All 7C antecedents were significantly associated with the outcomes, although effects were weaker for up-to-date vaccination status and uptake speed. The strongest effects (most vs. least vaccine-favourable attitude level, at-least-one-dose vaccination status) were observed for collective responsibility (OR: 14.44; 95%CI: 10.72-19.45), calculation (OR: 10.29; 95%CI: 7.53-14.05), and confidence in the wider system (OR: 8.94; 95%CI: 6.51-12.27).

CONCLUSION

This study demonstrates that the 7C not only explain vaccine intention, but also vaccine behaviours, and underpins the importance of developing vaccine promotion strategies considering the 7C antecedents.

Reduction of SARS-CoV-2 intra-household child-to-parent transmission associated with ventilation: results from a case-control study

PURPOSE

Our objective was to describe circumstances of SARS-CoV-2 household transmission and to identify factors associated with a lower risk of transmission in a nationwide case-control study in France.

METHODS

In a descriptive analysis, we analysed cases reporting transmission from someone in the household (source case). Index cases could invite a non-infected household member to participate as a related control. In such situations, we compared the exposures of the index case and related control to the source case by conditional logistic regression matched for household, restricted to households in which the source case was a child, and the index case and related control were the infected child's parents.

RESULTS

From October 27, 2020 to May 16, 2022, we included 104 373 cases for the descriptive analysis with a documented infection from another household member. The source case was mostly the index case's child (46.9%) or partner (45.7%). In total, 1026 index cases invited a related control to participate in the study. In the case-control analysis, we included 611 parental pairs of cases and controls exposed to the same infected child. COVID-19 vaccination with 3 + doses versus no vaccination (OR 0.1, 95%CI: 0.04-0.4), isolation from the source case (OR 0.6, 95%CI: 0.4-0.97) and the ventilation of indoor areas (OR 0.6, 95%CI: 0.4-0.9) were associated with lower risk of infection.

CONCLUSION

Household transmission was common during the SARS-CoV-2 pandemic in France. Mitigation strategies, including isolation and ventilation, decreased the risk of secondary transmission within the household.

TRIAL REGISTRATION

ClinicalTrials.gov registration number: NCT04607941.

SARS-CoV-2 self-test uptake and factors associated with self-testing during Omicron BA.1 and BA.2 waves in France, January to May 2022

BackgroundFollowing the SARS-CoV-2 Omicron variant spread, the use of unsupervised antigenic rapid diagnostic tests (self-tests) increased.AimThis study aimed to measure self-test uptake and factors associated with self-testing.MethodsIn this cross-sectional study from 20 January to 2 May 2022, the case series from a case-control study on factors associated with SARS-CoV-2 infection were used to analyse self-testing habits in France. A multivariable quasi-Poisson regression was used to explore the variables associated with self-testing among symptomatic cases who were not contacts of another infected individual. The control series from the same study was used as a proxy for the self-test background rate in the non-infected population of France.ResultsDuring the study period, 179,165 cases who tested positive through supervised tests were recruited. Of these, 64.7% had performed a self-test in the 3 days preceding this supervised test, of which 79,038 (68.2%) were positive. The most frequently reported reason for self-testing was the presence of symptoms (64.6%). Among symptomatic cases who were not aware of being contacts of another case, self-testing was positively associated with being female, higher education, household size, being a teacher and negatively associated with older age, not French by birth, healthcare-related work and immunosuppression. Among the control series, 12% self-tested during the 8 days preceding questionnaire filling, with temporal heterogeneity.ConclusionThe analysis showed high self-test uptake in France with some inequalities which must be addressed through education and facilitated access (cost and availability) for making it a more efficient epidemic control tool.