PREreview of Pre-existing antibodies and age shape the immune response following EV-A71 vaccine: a prospective serological study among the Chinese pediatric population

The article “Pre-existing antibodies and age shape the immune response following EV-A71 vaccine: a prospective serological study among the Chinese pediatric population” examines how age and baseline immunity influence the immune response to the EV-A71 vaccine in children. The study uses serological data from 1,585 children aged 6 to 35 months in China who received two doses of the vaccine, and antibody titers were measured before and after immunization. The authors conclude that both age and pre-existing antibodies significantly affect immune responses following vaccination, and they suggest that children should be vaccinated early in life to maximize protection. The manuscript is well-organized, clearly written, and addresses an important public health topic, as EV-A71 remains a leading cause of severe hand, foot, and mouth disease in young children. However, while the study is scientifically relevant and uses a large dataset, there are major methodological issues and interpretational flaws that must be addressed before the conclusions can be considered valid.

A major problem with this study is that it does not control for natural EV-A71 infection during the study period. Since EV-A71 is endemic in China and often spreads among children through asymptomatic infection, some participants may have naturally acquired EV-A71 during the follow-up period. Because the study did not monitor participants for infection symptoms or test for viral exposure between vaccine doses, it is impossible to know whether antibody increases were caused by the vaccine or by natural infection. This is a serious confounding variable that weakens the study’s internal validity. The authors briefly acknowledge cross-reactivity and natural exposure as limitations but do not attempt to estimate or adjust for this effect. Including regional infection data or conducting sensitivity analyses would make the findings more credible.

Another major issue is the combination of two fundamentally different populations in the analysis: children from a Phase III clinical trial and children from an observational cohort study. These two groups differ in important ways, such as age, potential exposure to infection, and health monitoring, which introduces bias when the results are pooled together. Although the authors statistically adjust for “data source” in their model, this does not fully correct for structural differences in study design. For example, clinical trial participants were younger and had lower baseline antibody levels than cohort children, who were older and more likely to be naturally exposed to EV-A71 before vaccination. Because of this, vaccine responses might appear different between the two groups for reasons unrelated to age or immune history. The authors should have analyzed these groups separately to show whether the trends they observed were consistent in both populations.

A third significant weakness in the paper is the authors’ interpretation of the fold increase in antibody titers. They conclude that children with higher levels of pre-existing antibodies have weaker vaccine responses because their immune systems are already primed. While a lower fold increase in seropositive children is expected, this result does not necessarily mean that pre-existing antibodies inhibit vaccine response. Instead, it may reflect a natural “ceiling effect,” where individuals with already high antibody levels have limited ability to increase further. Another possible explanation is technical; neutralizing antibody assays tend to be less precise at very high titers, compressing the range of detectable increases. The authors do not discuss these alternative explanations, which makes their conclusion feel overstated. They imply a biological mechanism that they did not test experimentally.

In addition to these major problems, the paper has several minor weaknesses. Some figures lack clear axis labels and statistical descriptions, especially Figures 2 and 3. The authors also do not fully justify their choice of seropositivity cutoffs for classifying antibody levels, which weakens their claims about immune categories. The description of statistical models in the Methods section lacks detail, and it is unclear how model assumptions were checked. Finally, the conclusion states that children should be vaccinated “as early as possible,” yet the data show that older children produced stronger antibody responses. This contradiction should be addressed by clarifying whether early vaccination is recommended for protection before exposure, rather than for enhanced immunogenicity.

In conclusion, this study provides valuable insights into EV-A71 serology in children and highlights a significant relationship between age, pre-existing immunity, and vaccine response. However, the lack of control for natural infection, the flawed pooling of two distinct study populations, and the overinterpretation of immunological findings significantly weaken the reliability of the conclusions. With major revisions that address these methodological and interpretational issues, the study has potential for publication. At its current stage, it requires substantial improvement to ensure scientific accuracy and credibility.

Competing interests

The author declares that they have no competing interests.

Use of Artificial Intelligence (AI)

The author declares that they used generative AI to come up with new ideas for their review.