As we improve our understanding of immune responses and correlates of protection against the severe acute respiratory syndrome coronavirus (SARS-CoV-2), two UK researchers emphasize that we will probably utilize different assays to tackle specific questions. Their commentary is published in the prestigious journal Lancet Infectious Diseases.
The global death toll from the ongoing coronavirus disease (COVID-19) pandemic could reach two million before a vaccine is available, and could be even higher without arduous efforts to prevent the spread of the SARS-CoV-2, warns the World Health Organization (WHO).
The immune status of the population may play a role, but our comprehension of both individual and population-level immunity to SARS-CoV-2 remains deficient. Therefore, the development of reliable and robust serological assays for detecting previous infections with the virus has been one of the salient goals in this global scientific effort.
Adequate public health planning necessitates scalable assays that are validated against large sample banks taken from individuals with proven seasonal (i.e., non-severe acute respiratory syndrome) coronaviruses, as well as those with well-characterized symptomatic and asymptomatic COVID-19. It is vital to avoid false-positive results due to cross-reactivity with seasonal viral strains – especially when seropositive-individuals consider themselves immune.
Transmission electron micrograph of a SARS-CoV-2 virus particle, isolated from a patient. Image captured and color-enhanced at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID
A head-to-head benchmark comparison
A recent study published in The Lancet Infectious Diseases journal by the UK National SARS-CoV-2 Serology Assay Evaluation Group provided the first large-scale comparative study that aimed to appraise how four widely available, high-throughput commercial assays and a novel, in-house 384-well enzyme-linked immunosorbent assay are actually performing.
These authors, led by Dr. Mark Ainsworth from the Oxford University Hospitals NHS Foundation Trust, compared the performance of the assay on detecting total IgG to a trimeric SARS-CoV-2 spike glycoprotein and used all five assays on 976 pre-pandemic samples presumed to be negative, as well as 536 serum samples from patients with laboratory-confirmed COVID-19.
In a nutshell, the study reported that all assays had a relatively high sensitivity (between 92.7-99.1%) and specificity (reaching 99.9%). While the most sensitive test appraised was the aforementioned in-house ELISA, the Roche, Abbott, and Siemens assays were the most specific ones. This means that a combination of assays should be used to refine testing strategies as much as possible.
Addressing study limitations
However, Dr. Catherine F Houlihan and Dr. Rupert Beale from the University College London in the United Kingdom were quick to notice certain limitations of this particular study. First and foremost, what caught their attention was a small number of asymptomatic and mildly symptomatic cases included in the study. Antibody responses are expectedly lower in these individuals, which may translate to lower sensitivity of all assays.
Furthermore, data on age, sex, and immune status were incomplete, which means that the results could be limited when you try to apply them to specific patient groups. This may be particularly important in children, who are more likely to have had a recent infection with seasonal coronavirus strains when compared to adults.
"The expectation is that the best predictor of antibody-mediated protection will come from neutralization assays, in which the ability of patient serum to prevent live virus infecting cell cultures is measured," say Dr. Houlihan and Dr. Beale. "However, these assays are impractical to deploy at scale", they add.
The need for many assays
As our knowledge regarding cellular and humoral correlates of protection against SARS-CoV-2 increases, and the armamentarium of immunoassays multiplies, we will probably make use of different assays to answer specific questions. For example, a majority of current vaccine candidates elicit responses to spike glycoprotein rather than nucleocapsid protein.
"Measuring antibodies to spike will therefore indicate whether there has been a good response, whereas measuring antibodies to nucleocapsid would help identify whether the individual had nonetheless become infected," explained Dr. Houlihan and Dr. Beale.
In any case, additional studies are needed to elucidate what titer of neutralizing antibodies is adequate for protection, how long such neutralization activity persists, and which assay is the best for predicting it.
In conclusion, finding an optimal assay will be pivotal for assessing vaccine responses and for appraising potential risk of reinfection that has already been demonstrated for seasonal coronaviruses, but not yet for SARS-CoV-2.