Chaouki Benabdessalem

Seroprevalence studies are essential to get an accurate estimate of the actual SARS-CoV-2 diffusion within populations. We report on the findings of the first serosurvey conducted in Tunis prior to the implementation of mass vaccination and analyzed factors associated with seropositivity. A household cross sectional survey was conducted (March-April 2021) in Tunis, spanning the end of the second wave and the beginning of the third wave of COVID-19. SARS-CoV-2 specific immunoglobulin G (IgG) antibodies to the spike (S-RBD) or the nucleocapsid (N) proteins were detected by in-house ELISA tests. The survey included 1676 individuals from 431 households. The mean age and sex ratio were 43.3 ± 20.9 years and 0.6, respectively. The weighted seroprevalence of anti-N and/or anti-S-RBD IgG antibodies was equal to 38.0% (34.6-41.5). In multivariate analysis, age under 10, no tobacco use, previous diagnosis of COVID-19, a history of COVID-19 related symptoms and contact with a COVID-19 case within the household, were independently associated with higher SARS-CoV-2 seroprevalence. More than one third of people living in Tunis obtained antibodies to SARS-CoV-2. Further studies are needed to monitor changes in these figures as Tunisian population is confronted to the subsequent epidemic waves and to guide the vaccine strategy.

Nearly two billion people are latently infected with Mtb (LTBI). Detection of LTBI with high risk to develop active tuberculosis (aTB) is considered the cornerstone to control the disease. The current challenge is to identify markers that better classify LTBI versus aTB. It has been previously shown that Rv0140, a reactivation-associated antigen of Mtb, induces significantly higher IFN-γ production in LTBI individuals as compared to aTB patients. Herein, we show that Rv0140 induces high granzyme B level by PBMCs derived from LTBI (n = 34) as compared to aTB (n = 18). Receiving operator characteristic (ROC) curves were used to evaluate the capacity of Rv0140 to discriminate between LTBI and aTB by measuring IFN-γ and granzyme B secretion. Our results show that, in response to Rv0140, granzyme B seems to allow better discrimination of LTBI from aTB with areas under the curve (AUC) of 0.88 (95% CI 0.79-0.98) as compared to IFN-γ with AUC of 0.85 (95% CI 0.74-0.96) even though CI overlap. Intracellular staining (ICS) experiments and the use of anti-MHC I antibody showed that granzyme B is mainly produced by CD8+ T cells in response to Rv0140. Thus, we propose granzyme B as a host marker to help identify LTBI individuals.

Management of the COVID-19 pandemic relies on molecular diagnostic methods supported by serological tools. Herein, we developed S-RBD- and N- based ELISA assays useful for infection rate surveillance as well as the follow-up of acquired protective immunity against SARS-CoV-2. ELISA assays were optimized using COVID-19 Tunisian patients' sera and prepandemic controls. Assays were further validated in 3 African countries with variable endemic settings. The receiver operating curve was used to evaluate the assay performances. The N- and S-RBD-based ELISA assays performances, in Tunisia, were very high (AUC: 0.966 and 0.98, respectively, p < 0.0001). Cross-validation analysis showed similar performances in different settings. Cross-reactivity, with malaria infection, against viral antigens, was noticed. In head-to-head comparisons with different commercial assays, the developed assays showed high agreement. This study demonstrates, the added value of the developed serological assays in low-income countries, particularly in ethnically diverse populations with variable exposure to local endemic infectious diseases.