The investigation of antibodies raised against SARS-CoV-2 can help to determine the extent of previous SARS-CoV-2 infections and track the humoral response to vaccination. Here we describe a time series anti-nucleocapsid, anti-spike IgG serological survey in the city of Matinhos, PR, Brazil during the year of 2021.

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The investigation of antibodies raised against different SARS-CoV-2 antigens can help to determine the extent of previous SARS-CoV-2 infections in the population and track the humoral response to vaccination. Therefore, serological surveys can provide key information to better manage the pandemic and/or to implement the most effective vaccination program. Here we describe a time series anti-Nucleocapsid, anti-Spike IgG serological survey analysis in the city of Matinhos, PR, Brazil during the year of 2021. Seroconversion rates to the Nucleocapsid antigen was not influenced by gender or age. Comparison of the serological data with official COVID-19 cases in the city suggest that case sub notification is higher than 47%. Furthermore, by applying serological data, the corrected infection fatality rate was estimated to be lower than 2.4 % in contrast with the official estimative of 3.6 %. The rates of IgG reactive to Spike antigen resembled the curve of the fraction the population that had taken the second vaccine dose. Up to 82% of Spike seroconversion was detected in the end of 2021 confirming the effective of the COVID-19 vaccination program in the city. This SARS-CoV-2 serological study unraveled the SARS-CoV-2 infection rates and the response to vaccination in the city of Matinhos, it is likely that the numbers reported here may be similar in other cities in Brazil.

(1) Background: COVID-19 vaccination in Brazil has been performed mostly with CoronaVac (Sinovac), ChAdOx1-S (AstraZeneca-University of Oxford) and BNT162b2 (Pfizer-BioNTech) vaccines. The titers of IgG antibodies reactive to the SARS-CoV-2 spike protein correlate with vaccine efficacy. Studies comparing vaccine immunogenicity in a real-world scenario are lacking. (2) Methods: We performed a population-based study to analyze the immunoglobulin G response to different COVID-19 vaccines. Citizens older than 18 years (n = 2376) provided personal data, a self-declaration of any previous COVID-19 positive tests and information regarding COVID-19 vaccination: the vaccine popular name and the date of each dose. Blood samples were collected and the levels of IgG reactive to SARS-CoV-2 antigens were determined and compared between different vaccine groups. (3) Results: The seroconversion for anti-spike IgG achieved > 95% by February 2022 and maintained stable until June 2022. Higher anti-spike IgG titers were detected in individuals vaccinated with BNT162b2, followed by ChAdOx1-S and CoronaVac. The anti-spike IgG response was negatively correlated with age and interval after the second dose for the BNT162b2 vaccine. Natural infections boosted anti-spike IgG in those individuals who completed primary vaccination with ChAdOx1-S and CoronaVac, but not with BNT162b2. The levels of anti-spike IgG increased with the number of vaccine doses administered. The application of BNT162b2 as a 3rd booster dose resulted in high anti-spike IgG antibody titers, despite the type of vaccine used during primary vaccination. (4) Conclusions: Our data confirmed the effectiveness of the Brazilian vaccination program. Of the vaccines used in Brazil, BNT162b2 performed better to elicit anti-spike protein IgG after primary vaccination and as a booster dose and thus should be recommended as a booster whenever available. A continuous COVID-19 vaccination program will be required to sustain anti-spike IgG antibodies in the population.

As SARS-CoV-2 has reportedly infected companion, livestock and wildlife species, animals may play a role as virus reservoirs and reverse zoonosis may become a significant risk. Since these host species may serve as ground for new variants, SARS-CoV-2 surveillance in animal species should be considered as crucial for pandemics prevention. Accordingly, the present study has aimed to validate a high-throughput ultrafast magnetic bead immunoassay to detect anti- SARS-CoV-2 IgG antibodies in dog serum samples. The assay was validated using serum from eleven positive dogs to SARS-CoV-2 by RT-qPCR and 160 pre-pandemic samples, reaching 72.7% to 81.8% sensitivity and 97.5 to 98.1% specificity. Thus, this novel assay has shown adequate performance of anti-SARS-CoV-2 antibodies detection and may be used for surveillance and research purposes.