Antibody responses to known and unknown SARS-CoV-2 infections after mRNA vaccine booster

We followed a fully-vaccinated (two mRNA vaccine doses) community cohort (n=41) without prior COVID-19 diagnosis from September 2021 through March 2022 through the Omicron wave following a booster mRNA vaccination. 19.5% of participants reported a known SARS-CoV-2 infection during the Omicron wave, which was confirmed by anti-nucleocapsid IgG. An additional 36.5% also developed anti-nucleocapsid IgG after the boost, consistent with unknown, asymptomatic SARS-CoV-2 infection during this period. Infection defined by anti-nucleocapsid IgG, whether known to participant or not, increased anti-spike IgG levels, relative to those lacking anti-nucleocapsid IgG, at 120 days post-booster.

Low levels of protective humoral immunity following mild or asymptomatic infection with SARS-CoV-2 in a community-based serological study

The degree of protective humoral immunity after mild or asymptomatic SARS-CoV-2 infection is not known. We measured antibody-mediated neutralization of spike protein-ACE2 receptor binding--a surrogate measure of protection against SARS-CoV-2 infection--in a large and diverse community-based seroprevalence study. Comparisons were made across three groups of seropositive participants that differed in the severity of infection and engagement with clinical care (N=790). The clinical group was seropositive for prior infection, symptomatic, and diagnosed with COVID-19 by a healthcare provider. The symptomatic group was seropositive and reported one or more symptoms of infection but received no clinical care. The asymptomatic group was seropositive but reported no symptoms. 86.2% of all infections were mild or asymptomatic; 13.8% received clinical care. Of the clinical cases, 96.3% were outpatient; only 3.7% required hospitalization. Moderate or high levels of neutralizing activity were detected following 27.5% of clinical infections, in comparison with 5.4% of symptomatic and 1.5% of asymptomatic infections. The majority of infections in the general population are mild or asymptomatic and likely result in low levels of antibody-mediated protective immunity.

Comparison of IgG and neutralizing antibody responses after one or two doses of COVID-19 mRNA vaccine in previously infected and uninfected persons

ObjectiveTo compare anti-SARS-CoV-2 spike receptor binding domain (RBD) IgG antibody concentrations and antibody-mediated neutralization of spike-ACE2 receptor binding in vitro following vaccination of non-hospitalized participants by sero-status and acute virus diagnosis history. MethodsParticipants were studied before and after mRNA vaccination in a community-based, home-collected, longitudinal serosurvey; none reported hospitalization for COVID-19. Prior to vaccination, some reported prior positive acute viral diagnostic testing and were seropositive (COVID-19+). Participants who did not report acute viral diagnostic testing were categorized as seropositive or seronegative based on anti-spike RBD IgG test results. Primary measures were anti-spike RBD IgG concentration and percent antibody-mediated neutralization of spike protein-ACE2 interaction prior to vaccination, and after one or two doses of vaccine. ResultsOf 290 unique vaccine recipients, 42 reported a prior COVID-19 diagnosis and were seropositive (COVID-19+). Of the 248 with no history of acute viral diagnostic testing, 105 were seropositive and 143 seronegative before vaccination. The median age was 38yrs (range 21-83) with 65% female and 35% male; 40% were non-white. Responses were evaluated after one (n=140) or two (n=170) doses of BNT162b2/Pfizer or mRNA-1273/Moderna vaccine. After one dose, median post-vaccine IgG concentration and percent neutralization were each significantly higher among the COVID-19+ group (median 47.7 {micro}g/ml, IgG; >99.9% neutralization) compared to the seropositives (3.4 {micro}g /ml IgG; 62.8% neutralization) and seronegatives (2.2 {micro}g /ml IgG; 39.5% neutralization). The latter two groups reached >95% neutralization after the second vaccine dose. ConclusionsA prior outpatient COVID-19 diagnosis was associated with strong anti-spike RBD IgG and in vitro neutralizing responses after one vaccine dose. Persons seropositive for anti-spike RBD IgG in the absence of acute viral diagnostic testing, and those who were seronegative, required two doses to achieve equivalently high levels of IgG and neutralization activity. One mRNA vaccine dose is not sufficient to generate in vitro evidence of strong protection against COVID-19 among most persons previously infected with SARS-CoV-2, nor among seronegative persons.

A surrogate virus neutralization test to quantify antibody-mediated inhibition of SARS-CoV-2 in finger stick dried blood spot samples

BackgroundThe spike protein of SARS-CoV-2 engages the human angiotensin-converting enzyme 2 (ACE2) receptor to enter host cells, and neutralizing antibodies are effective at blocking this interaction to prevent infection. Widespread application of this important marker of protective immunity is limited by logistical and technical challenges associated with live virus methods and venous blood collection. To address this gap, we validated an immunoassay-based method for quantifying neutralization of the spike-ACE2 interaction in a single drop of capillary whole blood, collected on filter paper as a dried blood spot (DBS) sample. MethodsSamples are eluted overnight and incubated in the presence of spike antigen and ACE2 in a 96-well solid phase plate. Competitive immunoassay with electrochemiluminescent label is used to quantify neutralizing activity. The following measures of assay performance were evaluated: dilution series of confirmed positive and negative samples, agreement with results from matched DBS-serum samples, analysis of results from DBS samples with known COVID-19 status, and precision (intra-assay percent coefficient of variation; %CV) and reliability (inter-assay; %CV). ResultsDilution series produced the expected pattern of dose-response. Agreement between results from serum and DBS samples was high, with concordance correlation = 0.991. Analysis of three control samples across the measurement range indicated acceptable levels of precision and reliability. Median % neutralization was 46.9 for PCR confirmed convalescent COVID-19 samples and 0.1 for negative samples. ConclusionsLarge-scale testing is important for quantifying neutralizing antibodies that can provide protection against COVID-19 in order to estimate the level of immunity in the general population. DBS provides a minimally-invasive, low cost alternative to venous blood collection, and this scalable immunoassay-based method for quantifying neutralization of the spike-ACE2 interaction can be used as a surrogate for virus-based assays to expand testing across a wide range of settings and populations.