From May 23 to August 13, 2021, the Institut Pasteur, in partnership with the French National Health Insurance Fund (CNAM), Santé publique France and the Ipsos institute, conducted the fourth part of the ComCor epidemiological study on circumstances and places of infection with the SARS-CoV-2 virus in France. The aim of the study was to identify the socio-demographic factors, places visited and behaviors associated with a higher risk of infection with SARS-CoV-2. The study was also used to carry out a real-world analysis of the effect of previous infection with COVID-19 and the efficacy of messenger RNA vaccines against symptomatic infection with the Delta variant. The findings were published in The Lancet Regional Health Europe on November 26, 2021.

In the fourth part of the ComCor study, the data of 12,634 individuals who tested positive for SARS-CoV-2 between May 23 and August 13, 2021 and 5,560 non-infected controls recruited over the same period and matched by age, sex, population density, region of residence and week of exposure were analyzed. The recruited individuals completed a self-administered questionnaire. The period under study coincided with the gradual reopening of public places and the emergence of the Delta variant in mainland France. The results describe the circumstances of infection for diagnosed cases of SARS-CoV-2. Of the 12,634 people who tested positive, 8,644 (68%) were diagnosed as having been infected with the Delta variant.

The study also enabled real-life estimations to be made, for France, about the protective effect of previous infection with COVID-19 and the effectiveness of vaccines against symptomatic SARS-CoV-2 infection caused by the Delta variant.

The period under study, from May 23 to August 13, was characterized by the following events:

May 19th: Reopening of museums, cultural places and outdoor facilities (bars & restaurants, sports centers)

June 9th: Reopening of nightclubs ("Health pass" required)

August 9th: "Health pass" extended to all public settings

Circumstances and places of SARS-CoV-2 transmission

The analysis identified indoor bars and private parties as places of transmission of the Delta variant in people aged under 40 between June 9 and July 9, with a higher risk for men than for women. This period coincided with the Euro 2020 football championship, suggesting that supporters gathering together for matches may have played a role in spreading the virus, as was also observed in the United Kingdom. When nightclubs opened, they were also places of transmission.

In people aged over 40, those with children in their household or close circle were at greater risk of infection, with the risk ranging from +30% for middle school children to +90% for very young children (under the age of three).

Certain modes of transport were also associated with a moderate increase in the risk of infection: sharing a car with family and friends (+30%) (excluding car-sharing platforms, for which no increased risk of infection was observed), taxis (+50%), metro (+20%), train (+30%) and flying (+70%).

No additional risk of infection was recorded for cultural venues, shops (excluding local shops), restaurants (it was a period during which many restaurants mainly used outdoor seating areas or well ventilated indoor areas), places of worship, sports activities and family gatherings (excluding weddings, for which an increased risk was recorded).

These results emphasize the importance of compliance with hygiene and distancing measures, especially wearing masks and airing closed premises.

Real-world analysis of the protective effect of previous infection with COVID-19 and the effectiveness of vaccines against symptomatic infection with the Delta variant

The study revealed that protection conferred by previous infection is 95% if the infection occurred within the past 6 months; this rate falls to 74% if the infection occurred more than 6 months ago.

As documented in other international studies, two doses of mRNA vaccine confer less protection against symptomatic forms of the Delta variant, with efficacy waning over time after the second dose; the level of protection was estimated at 67% in the ComCor study. The protection level is similar (61%) for people vaccinated with a first dose of adenovirus vector vaccine (AstraZeneca) and a second dose of mRNA vaccine (Pfizer or Moderna). It should be noted that the ComCor study does not enable estimates to be made about vaccine protection against severe forms of disease, which according to other studies remains above 90%.

France is one of the few countries in which people previously infected with the virus are given just one dose of mRNA vaccine. The study demonstrates that this single dose confers a higher level of protection than that observed with two doses of mRNA vaccine in the absence of previous infection (85% compared with 67%). The level of protection is similar to that in individuals who were previously infected and receive two doses of mRNA vaccine (96%; the difference compared with those previously infected and receiving one dose is not statistically significant).

Incubation period for the Delta variant

An analysis of the 651 subjects who only had a single contact with the person who transmitted the Delta variant to them revealed an incubation period of 4.3 days, which is shorter than the 5 days observed for the 8,442 subjects who were infected by a different variant (not Delta) following a single contact. This shorter incubation period for infections with the Delta variant could partly explain why the dynamics of outbreaks associated with this variant have been more severe.

This study, like any observational study, contains limitations which are explained in the document Results and critical analysis of the study on places and circumstances of SARS-CoV-2 infection (April 19, 2021).

The ComCor study will continue and will be used to reassess how places and circumstances of transmission are affected by new control measures introduced for the epidemic and by the progress of vaccination among the population.

Rebecca Grant, Tiffany Charmet, Laura Schaeffer, Simon Galmiche, Yoann Madec, Cassandre Von Platen, Olivia Chény, Fa?za Omar, Christophe David, Alexandra Rogoff, Juliette Paireau, Simon Cauchemez, Fabrice Carrat, Alexandra Septfons, Daniel Levy-Bruhl, Alexandra Mailles, Arnaud Fontanet.
Impact of SARS-CoV-2 Delta variant on incubation, transmission settings and vaccine effectiveness: Results from a nationwide case-control study in France.
The Lancet Regional Health - Europe, 2021. doi: 10.1016/j.lanepe.2021.100278.