Dynamics of anti-SARS-CoV-2 seroconversion in individual patients and at the population level

Presence of SARS-CoV-2-specific T cells before vaccination in the Mexican population

The immune response to SARS-CoV-2 has been extensively studied following the pandemic outbreak in 2020; however, the presence of specific T cells against SARS-CoV-2 before vaccination has not been evaluated in Mexico. In this study, we estimated the frequency of T CD4+ and T CD8+ cells that exhibit a specific response to S (spike) and N (nucleocapsid) proteins in a Mexican population. We collected 78 peripheral blood samples from unvaccinated subjects, and the presence of antibodies against spike (RBD) and N protein was determined. Peripheral blood mononuclear cells were isolated and stimulated with a pool of S or N protein peptides (Wuhan-Hu-1 strain). IL-1β, IL-4, IL-6, IL-10, IL-2, IL-8, TNF-α, IFN-γ, and GM-CSF levels were quantified in the supernatant of the activated cells, and the cells were stained to assess the activation and memory phenotypes. Differential activation frequency dependent on serological status was observed in CD4+ cells but not in CD8+ cells. The predominantly activated population was the central memory T CD4+ cells. Only 10% of the population exhibited the same phenotype with respect to the response to nucleocapsid peptides. The cytokine profile differed between the S and N responses. S peptides induced a more proinflammatory response compared with the N peptides. In conclusion, in a Mexican cohort before vaccination, there was a significant response to the S and N SARS-CoV-2 proteins resulting from previous infections with seasonal coronaviruses or previous undetected exposure to SARS-CoV-2.

Nasopharyngeal Viral Load Is the Major Driver of Incident Antibody Immune Response to SARS-CoV-2 Infection

Background

Virologic determinants of seroconversion to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection were defined in a post hoc analysis of prospectively studied vaccine- and infection-naïve individuals at high risk for coronavirus disease 2019 (COVID-19).

Methods

This phase 3 COVID-19 prevention trial (NCT04452318) with casirivimab and imdevimab was conducted in July 2020-February 2021, before widespread vaccine availability. Placebo-treated participants who were uninfected (SARS-CoV-2 quantitative reverse transcription polymerase chain reaction [RT-qPCR] negative) and seronegative were assessed weekly for 28 days (efficacy assessment period [EAP]) for COVID-19 symptoms and SARS-CoV-2 infection by RT-qPCR of nasopharyngeal swab samples and for serostatus by antinucleocapsid immunoglobulin (Ig) G. Regression-based modeling, including causal mediation analysis, estimated the effects of viral load on seroconversion.

Results

Of 157/1069 (14.7%) uninfected and seronegative (for antispike IgG, antispike IgA, and antinucleocapsid IgG) participants who became infected during the EAP, 105 (65%) seroconverted. The mean (SD) maximum viral load of seroconverters was 7.23 (1.68) log10 copies/mL vs 4.8 (2.2) log10 copies/mL in those who remained seronegative; viral loads of ∼6.0 log10 copies/mL better predicted seroconversion. The mean of the maximum viral load was 7.11 log10 copies/mL in symptomatic participants vs 5.58 log10 copies/mL in asymptomatic participants. The mean duration of detectable viral load was longer in seroconverted vs seronegative participants: 3.24 vs 1.63 weeks.

Conclusions

Maximum SARS-CoV-2 viral load is a major driver of seroconversion and symptomatic COVID-19, with high viral loads (∼6.0 log10 copies/mL) better predicting seroconversion. Serology underestimates infection rates, incidence, and prevalence of SARS-CoV-2 infection.

A Highly Sensitive Immunoassay for Determination of Immune Response to SARS-CoV-2 in Capillary Blood Samples

Throughout the pandemic, serological assays have been revealed as crucial for detecting previous exposures to the virus and determining the timing of antibody maintenance after vaccination or natural infection. This study aimed to develop an optimized enzyme-linked immunosorbent assay (ELISA)-based serology, which could be used in case of reagent shortages, such as that occurred in the beginning of this health emergency. As a result, we present a high-sensitive immunoassay for the determination of IgG levels in venous serum samples, using 2 μg/mL antigen (receptor-binding domain of the spike protein S1) for coating the plate and utilizing human samples at a dilution 1:1000. This method showed non-inferiority features versus a commercial kit, is less expensive, and has a higher spectrophotometric range that allows for a better quantification of the antibody titers. The optical density values before and after heating venous serum samples at 56 °C during 30 min was quite similar, showing that heat inactivation can be used to reduce the biohazardous risks while handling samples. Furthermore, we show that finger-stick capillary blood samples can also serve as a suitable source for IgG detection, bypassing the need for serum isolation and being suitable for point-of-care application (Pearson's coefficient correlation with capillary serum was 0.95, being statistically significant).