Accurate assays for the detection of antibodies to SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) are essential for the control of the COVID-19 (coronavirus disease 2019) pandemic. Here, we report antibody and antibody-avidity assays, relying on near-infrared-fluorescence amplification by nanostructured plasmonic gold substrates, for the simultaneous detection of antibodies to the S1 subunit of the spike protein and to the receptor binding domain of SARS-CoV-2 in human serum and saliva, and for quantifying immunoglobulin avidities against coronavirus antigens from SARS-CoV-2, SARS-CoV-1 and the common-cold viruses OC43, HKU1, NL63 and 229E. The antibody assay detected immunoglobulin M in 87% (52 of 60) COVID-19-positive serum samples collected 6 or more days after symptom onset (and the immunoglobulins M and G in all 33 samples collected at least 15 days after symptom onset), and correctly classified 456 out of the 457 COVID-19-negative serum samples tested (424 of them collected before the pandemic, including 73 that were positive for other viruses). We used the antibody-avidity assay to study antibody-maturation patterns, anamnestic responses, and cross-immunity to the common-cold coronaviruses.

Messenger RNA (mRNA) based vaccines (Pfizer/BioNTech and Moderna) are highly effective at providing immunity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, there is uncertainty about the duration of immunity, evolution of IgG antibody levels and IgG avidity (an index of antibody-antigen binding strength), and differences in the immune responses between vaccines. Here we performed a prospective pilot study of 71 previously COVID-19 free subjects upon receiving both doses of either the Pfizer (n = 54) or Moderna (n = 17) mRNA vaccine. Anti-spike protein receptor binding domain (RBD) IgG antibodies were measured longitudinally using a qualitative finger stick MidaSpot rapid test at the point-of-care for initial screening and a quantitative dry blood spot-based pGOLD laboratory test over ~ four months post-vaccination. The average anti-RBD IgG antibody levels peaked at ~ two weeks after the second dose vaccine and declined thereafter, while antibody avidity increased, suggesting antibody maturation. Moderna vaccine recipients compared to Pfizer vaccine recipients exhibited higher side effect severity, higher peak anti-RBD IgG antibody levels, and higher avidity up to the 90 days period. Differences in antibody levels diminished at ~ 120 days post-vaccination, in line with the similar efficacy observed in the two vaccines. The MidaSpot rapid test detected 100% anti-SARS-CoV-2 RBD positivity for fully vaccinated subjects in both Pfizer and Moderna cohorts post full vaccination but turned negative greater than 90 days post-vaccination for 5.4% of subjects in the Pfizer cohort, whose quantitative anti-IgG were near the minimum levels of the group. Immune responses were found to vary greatly among vaccinees. Personalized longitudinal monitoring of antibodies could be necessary to assessing the immunity duration of vaccinated individuals.

Sensitive detection of IgG antibodies against SARS-CoV-2 is important to assessing immune responses to viral infection or vaccination and immunity duration. Antibody assays using non-invasive body fluids such as saliva could facilitate mass testing including young children, elderly and those who resist blood draws, and easily allowing longitudinal testing/monitoring of antibodies over time. Here, we developed a new lateral flow (nLF) assay that sensitively detects SARS-CoV-2 IgG antibodies in the saliva samples of vaccinated individuals and previous COVID-19 patients. The 25-minute nLF assay detected anti-spike protein (anti-S1) IgG in saliva samples with 100% specificity and high sensitivity from both vaccinated (99.51% for samples ≥ 19 days post 1st Pfizer/BioNTech or Moderna mRNA vaccine dose) and infected individuals. Antibodies against nucleocapsid protein (anti-NCP) was detected only in the saliva samples of COVID-19 patients and not in vaccinated samples, allowing facile differentiation of vaccination from infection. SARS-CoV-2 anti-S1 IgG antibody in saliva measured by nLF demonstrated similar evolution trends post vaccination to that in matching dried blood spot (DBS) samples measured by a quantitative pGOLD lab-test, enabling the nLF to be a valid tool for non-invasive personalized monitoring of SARS-CoV-2 antibody persistence. The new salivary rapid test platform can be applied for non-invasive detection of antibodies against infection and vaccination in a wide range of diseases.