Postjab nAb assessment at one’s fingertips

A group of researchers from Singapore has recently developed a finger stick blood test that can rapidly and semi-quantitatively assess the neutralizing antibody (nAb) response of people who have been vaccinated against COVID-19.

“It captures a clinically relevant range of nAb levels and effectively differentiates prevaccination, postfirst dose, and postsecond dose vaccination samples within 10 minutes,” the researchers said. “This test is highly suited for point-of-care (PoC) settings and provides an insightful [assessment of] nAb response in a postvaccinated population.”

Blood samples were collected via finger prick or venous draw from 170 healthy adults (aged 21–65 years) who had been scheduled for COVID-19 vaccination under Singapore’s national programme. Participants were divided into four groups according to vaccination status: prevaccination (prevac; n=36), 1–2 weeks after first dose (P1 1–2W; n=10), 3–6 weeks after first dose (P1 3–6W; n=50), and 3–16 weeks after second dose (P2; n=74).

Compared with negative controls and prevac samples, nAb response, expressed as percent blocking, improved after vaccination. The prevac median was 1.96 percent, which jumped to 14.3 percent and 36.5 percent in the P1 1–2W and P1 3–6W groups, respectively. P2 participants, on the other hand, showed the highest median percent blocking at 89.1 percent. [Bioeng Transl Med 2022;7:e10293]

Aside from simple detection of nAb response, the novel finger stick test was also able to match the WHO international standards for antibody detection. Compared with a commercially available test, the novel finger-prick assay had an 81.5-percent sensitivity and 100-percent specificity. Corresponding values were 100 percent and 71.4 percent when compared against a lab-based test.

In addition, the researchers noted a significant and highly linear correlation in nAb detection between venous and finger-prick blood samples (r, 0.9758; p<0.001), strongly suggesting that the novel test in question is suitable for PoC deployment.

Paper-based test

The novel finger stick assay uses the cellulose pulldown virus neutralization test (cpVNT), a rapid, paper-based method of testing antibody neutralization activity against SARS-CoV-2 and which can return results in 10 minutes.

CpVNT relies on the interactions between the virus’ receptor-binding domain (RBD) and the host cell’s angiotensin-converting enzyme II (ACE2). Through a cellulose binding domain (CBD), the paper-based test captures the RBD-ACE2 complex, then attaches reporter molecules to ACE2 in order to generate a detectable signal. The presence of nAbs in a blood sample then disrupts the overall CBD-RBD-ACE2 complex and quenches the measurable signal as a result.

“Our data also shows the strength of the modified cpVNT as a PoC test to provide insights on the deteriorating vaccine efficacy observed globally against the delta variant and the climbing breakthrough infections among vaccinated population,” the researchers said, pointing out that the finger stick test has indeed observed a significant drop in nAb activity against the delta variant >3 months after vaccination.

“Thus, our test's ability to detect variant specific nAb waning effects among a vaccinated population provides an extremely valuable tool to pre-emptively test nAb responses against emerging variants and through this inform booster planning and public health management,” they added.