In a recent study published in The Journal of Infectious Diseases, researchers identified that amongst healthy young adults, the event of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) triggered memory B cell and antibody reactions were equivalent following mildly symptomatic or asymptomatic infections.

Study: Memory B cell development after asymptomatic or mild symptomatic SARS-CoV-2 infection. Image Credit: Kateryna Kon/Shutterstock

Background

SARS-CoV-2, the etiologic agent of coronavirus disease of 2019 (COVID-19), could cause clinical outcomes starting from asymptomatic to severe respiratory failure. Amongst unvaccinated populations, 40 to 50% of SARS-CoV-2 infections are asymptomatic, with younger people more vulnerable to asymptomatic COVID-19.

Relative to symptomatic infection, the protective immunity and immune memory of individuals with asymptomatic COVID-19 should not well characterised. From the general public health standpoint, assessing immune memory following asymptomatic SARS-CoV-2 infection is critical.

Individuals with an asymptomatic SARS-CoV-2 infection could have a greater or lesser amount of SARS-CoV-2 immune memory than those that experienced a symptomatic COVID-19. While asymptomatic COVID-19 patients generate neutralizing and binding antibodies against the virus, they are sometimes of lesser quantity and could have distinct decay kinetics than those in symptomatic instances. As well as, data on adaptive immune memory amongst young adults and memory B cells across asymptomatic SARS-CoV-2 infections are scarce.

In regards to the study

The current work assessed antibody and memory B cell responses in COVID-19 Health Motion Response for Marines (CHARM) participants, aiming to shut knowledge gaps surrounding memory B cells in asymptomatic SARS-CoV-2 infections. The researchers sought to learn more about immune memory amongst young adults with COVID-19 who were asymptomatic versus those that were symptomatic. The study was performed before the emergence of SARS-CoV-2 variants of concern (VOCs) or the provision of COVID-19 vaccines.

The authors examined the SARS-CoV-2 memory B cell reactions, pseudovirus (PSV) neutralizing antibody titers, and spike (S) antibody titers in Marine recruits who were COVID-19 polymerase chain response (PCR) positive and experienced symptomatic or asymptomatic infection.

From May to November 2020, all subjects attended recruit training for the USA Marine Corps (USMC). Only study volunteers who were SARS-CoV-2 receptor-binding domain (RBD)-seronegative at trial enrollment time were included within the evaluations, independent of S immunoglobulin G (IgG) status.

Besides, the La Jolla Institute of Immunology’s Normal Blood Donor Program provided unexposed donors. All subjects submitted written informed consent before enrolling. In 24 hours, peripheral blood mononuclear cells (PBMCs) were extracted and kept in liquid nitrogen until they were used to measure antigen-specific memory B cells.

Results and conclusions

The study results illustrated that the topics were mainly young men, i.e., 93.9% male, with a median age of 18. While 4 of the 30 symptomatic participants reported one to 2 symptoms at a single timestamp, the rest reported a minimum of three symptoms or symptoms at two or more separate timestamps following infection, with the range of symptoms documented at each encounter various from one to 11. Not one of the symptomatic patients received any medicine besides symptomatic therapy, like acetaminophen or non-steroidal anti-inflammatory medications, and all were treated as outpatients.

The authors discovered that in young adults with asymptomatic SARS-CoV-2 infection, memory B cell reactions were much like those in patients with mild symptoms. Moreover, antigen-selective memory B cell reactions were linked with antibody responses.

The S IgG, IgA, and PSV neutralization titers of 36 asymptomatic COVID-19 patients were comparable to those of 30 symptomatic participants. Because of this, the team stated that SARS-CoV-2 infections that were mildly symptomatic or asymptomatic generated similar rapid IgA and IgG reactions that were effective at neutralizing the virus.

Significantly favorable associations were observed between PSV neutralization and S IgG titers and the proportion of memory B cells. SARS-CoV-2-infected young adults with asymptomatic infection demonstrated similar S- and RBD-attaching memory B cell percentages to those that were mildly symptomatic amongst the present cohort.

The reference to increased memory B cell or antibody responses typically indicated an extended or higher viral burden. Nonetheless, no link was present in the present investigation, suggesting antigen load may not be a major factor differentiating most asymptomatic patients from mildly symptomatic cases in young adults. Indeed, the team discovered no variations in the times of SARS-CoV-2 PCR positivity duration or the assessed SARS-CoV-2 PCR swab cycle threshold (Ct) values between mildly symptomatic and asymptomatic subjects. Hence, there have been no virological disparities between the 2 patient cohorts.

Conclusion

Based on the study findings, memory B cell and antibody responses brought on by asymptomatic SARS-CoV-2 infection in young adults were akin to mild symptomatic illness. Subsequently, within the case of asymptomatic or mildly symptomatic COVID-19, healthy young adults exhibit comparable antibody and memory B cell responses. Moreover, the present data shows that reported COVID-19 symptomology can’t be used to predict the range of each antibody and memory B cell responses amongst young adults.