While previous research has described this maturation process in healthy infants, there has been a significant knowledge gap regarding how the infant immune system responds to infections, particularly to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes COVID-19.
Robust and Durable Antibody Responses
One of the primary findings of our study is the observation of robust and durable antibody responses against SARS-CoV-2 in infants and young children. This is in contrast to adults, where previous studies have shown a decay of antibody responses after COVID-19 infection. In adults, the half-life of antibodies is approximately 120 days.
However, in infants and young children, we found that the antibody responses were more sustained. This suggests that this age group may have a unique ability to maintain antibody levels over an extended period.
Vulnerability to Emerging Variants of Concern
While the antibody responses in infants and young children were robust, our data also indicated a potential vulnerability to emerging variants of concern (VOCs). Serum neutralization titers against Omicron variants were diminished in infants and young children previously infected with the original SARS-CoV-2 or Delta variants, and vice versa.
This highlights the importance of ongoing surveillance and vaccine development to address the evolving landscape of SARS-CoV-2 variants.
Memory B and T Cell Responses
Memory B and T cell responses are critical components of long-term immunity to infections. In infants and young children, we observed that memory B cell responses, responsible for producing affinity-matured, cross-specific antibodies, were of limited duration and magnitude.
Memory T cell responses, which provide broader immunity and are less susceptible to mutational changes in the virus, were also reduced compared to adults. This raises questions about the long-term immunity conferred by SARS-CoV-2 infection in this age group.
Innate Immune Response Differences
Our study highlighted stark differences between the mucosal and systemic immune responses in infants and young children compared to adults. In the first five days of infection, infants displayed high levels of type I and II interferons (IFNs), inflammatory cytokines, and various chemokines in their nasal mucosa.
Interestingly, these cytokines were not elevated in the blood of infants, which contrasts with findings in older children and adults with COVID-19. This unique mucosal immune response may contribute to the milder course of the disease in infants and young children.
The mucosal immune response in this age group was characterized by cytokines and chemokines associated with a Th17 response, and these levels correlated with activated CD4 T cells and neutralizing antibodies in the blood. This suggests that Th17 cells and neutrophils play a vital role in coordinating innate and adaptive immunity to SARS-CoV-2 in infants.
Rapid Activation of Innate Immunity
Another intriguing finding was the rapid activation of innate immunity in infants and young children following SARS-CoV-2 infection. In contrast, adults, especially those with severe COVID-19, exhibited major defects in myeloid cells and plasmacytoid dendritic cells during acute infection. The swift induction of mucosal immunity in the nasal tract in infants may be a key factor in containing viral replication early in the course of infection, contributing to the mild disease seen in this age group.
Our analysis also revealed epigenetic changes in infants and young children during convalescence. These changes were heterogeneous between individuals and differed from the more consistent epigenomic changes observed in adults following COVID-19 infection. One potential explanation is that frequent immune activation through vaccination in infants may obscure the epigenomic imprint of COVID-19.
This comprehensive study provides valuable insights into the dynamics of immunity to SARS-CoV-2 in infants and young children during the early stages of life. It reveals a surprisingly robust and durable antibody response in this age group, along with a potent mucosal immune activation characterized by Th17 and neutrophil-associated markers.
The disconnect between adaptive immune responses and pro-inflammatory cytokine responses in infants suggests the involvement of non-canonical pathways of innate activation. This discovery opens the possibility of developing vaccine adjuvants that target these pathways to stimulate persistent antibody responses without causing unwanted inflammation.
While this study contributes significantly to our understanding of infant immunity to SARS-CoV-2, it is not without limitations. Factors such as infections, vaccinations, microbiota, and maternal influence could impact the results. Nevertheless, the findings underscore the complexity of the infant immune response and emphasize the importance of further research in this area to inform vaccine development and public health strategies for this vulnerable population.