This study was conducted at a time when the town of Nice had one of the highest SARS-CoV-2 incidence rates in France. It shows that the incidence rate was significantly higher in the most deprived census blocks regardless of the age group. We also report a more rapid weekly progression among the most deprived population. (FDep5). Lastly, we observed a decrease in the number of cases in the over-80 age group, starting in the 5th week of 2021, which could reflect an early effect of the administration of the vaccine that was initiated in this population at the end of December 2020.
In the GAM model, a statistically significant association was found between the number of cases and the various socio-economic indicators that characterize the most disadvantaged populations, such as crowding, type of housing, living environment or occupation. These results are in line with studies showing a correlation between population density and the spread of the epidemic (19) (20) (21) (22), and with European studies highlighting an increase in the number of SARS-CoV-2 cases among low-income earners (7) (23).
We also showed the living environment to be an important determinant of viral spread: census blocks most affected by the virus were those where cases lived in appartments and overcrowded dwellings. This has also been reported in a study on SARS-CoV-2 seroprevalence on a representative sample of the French population (24). The major role of the urban model and housing type had already been reported for other infectious diseases (5).
The number of cases was also found to be associated with the type of occupation, with more cases among artisans, shopkeepers, company managers, employees and workers and fewer cases among executives and professions requiring a higher level of education. The first category includes most « essential » professions, i.e. people who could not work remotely and who were therefore in direct, unavoidable, and repeated contact with the rest of the population (25) (26). These jobs are more frequent among residents of the most deprived census blocks and may have thus favoured the spread of SARS-CoV-2 in these areas. Farming was also a profession associated with the number of cases; in Nice, there are very few and they mainly reside in one of the deprived census blocks. There was no correlation with case numbers according to the GAM model for so-called intermediate professions which include non-medical health porfessionals; this may be due to an early effect of the vaccine which became available on January 3rd, along with masks, gloves and personal protective equipment (27). A study conducted in New York in March 2020, and another in the United States as a whole, also found a link between the type of occupation and the risk of contamination (28) (29).
Our results on the incidence rate of SARS CoV-2 in deprived areas are also in line with the study on Covid-19-related mortality conducted at the start of the epidemic by the Health office for the Ile de France region (Office Régionalde Santé Ile-de-France), in which excess mortality was closely linked to the social and urban geography of the population (30).
The degree of social deprivation would thus contribute to the increase in contaminations related to overcrowded living conditions, the cohabitation of generations in the same dwelling and the greater frequency of housing in apartments, particularly the resulting over-occupation. Limited possibilities of working remotely, inherent in the type of profession of this section of the population, would also be among the factors favouring contamination (23). By forcing people living in high-incidence areas such as the town of Nice to remain at home from 6:00 pm to 6:00 am, the curfew may have contributed to a greater risk of SARS-CoV-2 transmission among such most deprived populations (25).
Complying with hygiene measures, conforming to a strict lockdown or isolating oneself in an apartment without outdoor spaces is undoubtedly more complicated than in a house with a garden, the more so when it comes to over-crowded apartments or limited surfaces. Our results could warrant surveillance at a smaller scale than a department, even within municipalities, to accurately chart the progression of Covid-19 or any other epidemic. This could thus lead to the rapid implementation of intervention strategies targeting the areas that need them most.
The methodology chosen for this study may have induced certain limitations regarding our results.
The SIDEP database which we used offers the advantage of providing all cases recorded in a given geographical area. Nevertheless, among the 10 712 recorded cases and checked with a verified zipcode for the town of Nice, 1 525 (12%) lacked an address allowing geolocation (missing number, collective accommodation lacking an address, missing addresses). Following further verification, the number of missing addresses was reduced to 634 (6%). This may have biased our results. However, in a similar study, focusing on France as a whole, for which geolocation was essential, the authors also reported a missing address in 20.5% of cases. To compensate for these missing data, they chose to allocate cases to census block on a probability basis (13). Thanks to the local presence of our team and a number of cases allowing manual verification, we were able to improve the accuracy and completeness of the data, achieving geolocation without having to resort to an estimate. In addition to regional or national surveillance, it appears useful to maintain territorial management of epidemiological surveillance.
Although the study was conducted in 2021, we chose to use the 2017 INSEE data, which was the most recent year for which distribution per age group and census block was available, as well as the related socio-economic data. The delay between the study period and the one for which socio-economic data are available has already been mentioned in other French studies (13). Between 2017 and 2019, the latest year for which the overall population in Nice is known, the number of inhabitants increased by 1.6%, from 340 017 to 345 528. We thus considered that this bias did not compromise the validity of our results, despite potential minor changes in socio-economic data over the 4-year period.
In our view, the FDep was the most suitable social deprivation index for our population and provided the most complete results for the town of Nice. Indeed, in contrast to the Townsend index for the United Kingdom population in 1987 (31), this index was specifically developed and validated for the French population. As for the Ecological Deprivation Index (EDI), developed for the French and European population, it included 10 weighted variables (32), while the INSEE data per census block in Nice were not complete for these 10 variables. The Fdep, with its 4 variables, could be computed almost completely for the whole of Nice.
The missing data for certain variables, such as the poverty index in 5 of the 144 studied census blocks, could have biased our results. However, since two of these census blocks were in the most deprived category, two in the most affluent category, and one in the middle category, we considered that these missing data had little impact on our results.
To classify the deprivation index values into categories, we chose to use the Jenks natural breaks classification method, rather than a classification based on quintiles as used in other studies. Indeed, this method does not classify data in an arbitrary fashion subdividing the population in groups of 20% as with quintiles, but according to their uniformity, with thresholds that we feel are more in line with reality. When applied to a transmissible disease, the Jenks algorithm can limit within-class variance while maximising variance between classes (17).
Until individual specificities are taken into account, the Fdep, which is a simple composite index, can provides the basis for a synthetic approach at the level of a particular territory. Considering that the socio-economic status is a multidimensional and complex concept, and with Khalatbari-Soltani (33), we believe that surveillance of infectious diseases should take socio-economic data into account. This would allow precise identification of at-risk groups in order to implement targeted and equitable public health policies (24).