For more than two years after the emergence of COVID-19 (Coronavirus Disease-2019), significant regional differences in morbidity persist. These differences clearly show lower incidence rates in several regions of the African and Asian continents. The work reported here aimed to test the hypothesis of a pre-pandemic natural immunity acquired by some human populations in central and western Africa, which would, therefore, pose the hypothesis of an original antigenic sin with a virus antigenically close to the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). To identify such pre-existing immunity, sera samples collected before the emergence of COVID-19 were tested to detect the presence of IgG reacting antibodies against SARS-CoV-2 proteins of major significance. Sera samples from French blood donors collected before the pandemic served as a control. The results showed a statistically significant difference of antibodies prevalence between the collected samples in Africa and the control samples collected in France. Given the novelty of our results, our next step consists in highlighting neutralizing antibodies to evaluate their potential for pre-pandemic protective acquired immunity against SARS-CoV-2. In conclusion, our results suggest that, in the investigated African sub-regions, the tested populations could have been potentially and partially pre-exposed, before the COVID-19 pandemic, to the antigens of a yet non-identified Coronaviruses.

Introduction: The Republic of the Congo detected its first case of coronavirus disease 2019 (COVID-19) on March 14, 2020, and within several weeks, the country had introduced protective measures that were still in force in July 2020. Over the course of time, the progression in the number of clinical cases has appeared to be lower than expected, although reverse transcription polymerase chain reaction (RT-PCR) testing has been somewhat limited. In order to evaluate the incidence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) within the Congolese population, a seroprevalence study was conducted on healthy individuals from different districts of Brazzaville who were willing to know their COVID-19 infection status. Methods: Oropharyngeal swab and blood samples were collected from 754 healthy volunteers between April 2020 and July 2020. The samples were analyzed for SARS-CoV-2 using a qualitative RT-PCR assay, and Immunoglobulin G (IgG) and Immunoglobulin M (IgM) antibodies were detected using two different rapid tests. Results: A total of 56 participants (7.4%) tested positive for SARS-CoV-2. The remaining 698 participants (92.6%) had negative RT-PCR results; of these, 117 were found to have anti-SARS-CoV-2 antibodies using serological tests. For these RT-PCR-negative subjects, the seroprevalence of IgG and IgM was found to increase over time: from 1.7% and 2.5% in April, up to 14.2% and 17.6% in July, respectively. In April 2020, 5% of the women were found to have IgG or IgM antibodies, whereas the antibodies were not detected in any of the men. The seroprevalence in RT-PCR negative subjects was significantly higher in women within IgG (P = 0.012) and IgM (P = 0.045) over the first three months. Conclusion: The proportion of the population who seroconvert over the course of the first wave is an important data to predict the risk of future COVID-19 waves and this will facilitate the efficient use of limited resources in a low income country like the Republic of the Congo.

Objectives : As limited data are available from Central Africa, the aim of the present study was to evaluate the anti-SARS-CoV-2 Ab prevalence in indigenous residents, in Bomassa, a village located in the Sangha Region in the Republic of Congo. Methods : Plasma and oropharyngeal swabs samples were collected from 304 healthy adult individuals, randomly recruited in May 2021 before vaccine introduction in the area. In addition, 82 plasma samples from the same area in 2019 were included as controls for cross-reaction investigation against other Coronaviruses. The SARS-CoV-2 virus was detected by qRT-PCR and sequenced using Next-Generation Sequencing. ELISA method was used for detecting IgG, IgM and neutralizing Ab against SARS-CoV-2 antigens. Results : About 4.9% (15/304) of the participants were SARS-CoV-2 positive and B.1.631 was the only variant identified. Of 109 individuals harboring anti-SARS-CoV-2 IgG and/or IgM Ab, 45.9 % (50/109) had anti-SARS-CoV-2 neutralizing Ab. A proportion of 3.7% (3/82) of control samples collected before the pandemic were positive to IgG, but negative for neutralizing Ab. Conclusions : While seroprevalence against SARS-CoV-2 represented 25% in indigenous population, almost 50% of seropositive participants had neutralizing antibodies. This finding highlights that the spread of the SARS-COV2 infection is under-estimate in the country.

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