The Future of Testing for COVID Immunity
Why and when do vaccine breakthrough infections occur? How will we know whether boosters are required?
To address this question, it is informative to review 1) the immune factors that correlate with resolution of COVID-19 infection; 2) the immune responses elicited by the different vaccines currently in use and the differences between each vaccine; and 3) the most informative targets for testing that define effective protection from COVID-19 infection, predict potential for breakthrough infections in vaccinated individuals and inform strategies for booster injections. As you will see, neutralizing antibodies may not be the full answer.
What immune factors are seen with resolution of COVID-19?
The focus on protective immunity from COVID-19 has been critical in defining outcomes for vaccine development. Early studies looking at the development of immune protection after SARS-CoV-2 infection in patients revealed two important factors (Tan et al. Front. Mol 2020; 14(6):746-751). The first is humoral immune responses as evidenced by the development of high levels of IgG neutralizing antibodies within three weeks that persist until at least seven months. The second is T cell antiviral responses as shown by an increase in interferon γ-producing CD4+ and CD8+ cells following SARS-CoV-2 antigen stimulation. Whether both humoral and T cell responses are needed has not been clear. Levels of neutralizing antibodies can be variable post COVID-19 infection. Patients with mild to moderate COVID-19 disease and low viral loads had lower levels of neutralizing antibodies while patients with severe disease and high viral load had high levels of neutralizing antibodies, suggesting there may be a correlation between antibody production and viral load. Since high viral load and more severe disease occurs despite high antibody levels, it has been proposed that T cell antiviral responses play a more critical role. In support of this, low CD4 T cell levels predict a higher risk of death (Wang F. JCI Insight. (2020) 5:e137799). Further studies are needed to determine if testing for humoral and/or T cell responses predicts risk for developing COVID-19 infection, severity of illness, or potential breakthrough infection in a vaccinated individual. Also, the titers of antibodies that are needed to prevent reinfection are not known.
Which immune responses are induced with the different vaccines?
CD4 and CD8 T cell responses following COVID-19 infection are generated to a broad range of COVID-19 epitopes, with spike protein reactive cells comprising only about 26% (Grifoni A, et al. Cell 181 (7) (2020), 1489- 1501). Whether vaccines that are directed specifically toward spike proteins will be fully effective against variants and prevent breakthrough infections remains to be determined. Furthermore, the adenovirus serotype 26 (Ad26)-vector-based vaccine expressing an optimized SARS-CoV-2 spike (Ad26.COV2.S) may have a different immune activation profile compared to mRNA-based vaccines (Grigoryan L and Pulendran B. Semin Immunol 2020;50:101422). As an example, the Moderna vaccine does not elicit CD8 responses in non-human primates while the Pfizer vaccine does in humans. No T cell responses were observed with the vaccine from Sinovac (China) in their preclinical studies with mice and non-human primates. In Rhesus Macaques, CD8 T cell responses play a critical role in COVID-19 eradication, especially in the setting of suboptimal antibody levels (McMahon K et al Nature. 2021 Feb;590(7847):630-634). Similar data are observed with HIV-based vaccines where CD8 T cells lower the levels of neutralizing antibody needed to prevent reinfection. Collier et al (JAMA 2021; 325(23):2370-2380) showed that pregnant and non-pregnant women receiving mRNA-based COVID-19 vaccines had reduced titers of neutralizing antibodies to SARS-CoV-2 variants from the U.K. and South Africa but preserved T cell responses. Whether that translates into more effective protection remains to be determined. A potential advantage of mRNA vaccines is that antigens are synthesized in the cell cytoplasm and then expressed bound to MHC-class I molecules on the cell surface for recognition by CD8 T-cells (Hellerstein M Vaccine X, 2020; 6:100076).
Taken together, T cell response may play a critical role not only to kill virus infected cells, but also orchestrate a multiprong approach to virus eradication through induction of neutralizing antibodies and leukocyte recruitment. Thus, reactivation of T cells following reinfection can induce a robust activation and recruitment of other white blood cells, orchestrating a robust and synergistic response.
What should be targets for testing?
It is not known whether the absence of SARS-CoV-2 antigens such as M or N (matrix and nucleocapsid proteins) will mimic the functionality of T-cell responses from natural infection or what the durability of induced T-cells will be. Do ineffective T cell responses explain the recent breakthrough infections in 75% of vaccinated individuals in Provincetown, MA? Testing neutralizing antibodies in parallel with T cell responses in those individuals would be very informative.
Finally, there appear to be differences among the different vaccines in their ability to protect against lower respiratory tract infection, upper respiratory tract infection or both. Can this be ascribed to T cell responses? No data exists at this time, but this may be an important differentiator to test for, especially given the high viral load in the nasopharynx of individuals infected with the delta variant, responsible for the high transmission rate.
Testing neutralizing antibodies and T cell responses in all infected individuals, especially breakthrough cases, will help us understand the relative roles and dynamics of the components of the complex immune response to guide therapy and future vaccination policy.