The central problem, according to Dadgar and Rostami, is heterogeneity—not merely statistical heterogeneity, but the deeper clinical and methodological diversity baked into the studies being pooled. Homologous and heterologous booster strategies are not single interventions. Each label conceals a matrix of variables: which priming vaccine was used, which booster platform followed, how long the dosing interval was, which age groups were enrolled, and what epidemiological backdrop the trial or observational study operated within. Some studies measured immune responses weeks after the booster; others measured at different time points entirely, and many spanned different periods of variant circulation. When such dissimilar comparisons are folded into a single summary estimate, the result may be a number that is difficult to apply to any specific vaccine sequence or any specific population.

This concern echoes a broader methodological literature. A 2023 review and case study published in Science Advances by Meah and colleagues examined design and analysis heterogeneity in observational studies of COVID-19 booster effectiveness and documented how widely these studies diverge in population definitions, outcome measures, and analytic choices. The Dadgar and Rostami letter leans on such work to make its point: a pooled effect estimate drawn from heterogeneous sources can obscure as much as it reveals. For clinicians deciding whether a particular patient who received an inactivated-virus primary series should receive an mRNA booster, a summary statistic averaged across dozens of different combinations offers little actionable guidance.

The second major critique concerns the evidence base itself. The letter notes that the reviewed studies rely heavily on immunogenicity outcomes—neutralizing antibody titers and cellular immune responses—rather than clinical endpoints such as infection, hospitalization, or death. Immunogenicity measures are indispensable tools. They allow rapid comparison of vaccine strategies without waiting for disease events to accumulate, and they provide mechanistic insight into how different platforms prime and reshape immunity. But antibody titers do not always translate directly into proportional reductions in clinically meaningful outcomes, particularly when the circulating variant changes between the time of measurement and the time of exposure. A higher neutralizing titer against one variant may not predict protection against the next. The authors argue, citing a 2024 systematic review with trial sequential analysis by Asante and colleagues in BMC Medicine, that conclusions about the immunological superiority of heterologous boosting should be carefully distinguished from evidence of improved clinical protection.

Third, the letter highlights prior SARS-CoV-2 infection as a critical and often under-controlled confounder. Previous infection substantially modifies baseline immunity and reshapes the immune response to a booster dose, a phenomenon documented in detail by Wachter and colleagues in a 2025 study in the Journal of Allergy and Clinical Immunology showing that prior infection affects adaptive immune responses to Omicron BA.4/BA.5 mRNA boosters. If included studies differed in how they excluded, measured, or stratified participants by prior infection status, then pooled estimates may partly reflect differences in baseline serostatus rather than the independent effect of homologous versus heterologous boosting. The problem is especially acute in observational studies, where individuals who receive different booster regimens may also differ systematically in age, comorbidity, exposure risk, and access to vaccines—differences that can masquerade as treatment effects in a pooled analysis.

Follow-up duration is the fourth pillar of the critique. Many booster studies report immune responses shortly after vaccination, often within a few weeks, when the post-boost antibody peak is at its highest. The letter questions whether an early immunological advantage necessarily implies sustained superiority over months. A higher peak antibody response may fade faster than a more modest one, and neither pattern fully characterizes protection against severe disease, which depends on memory B cells, T cell responses, and mucosal immunity as much as on circulating antibodies. Recent work on immune durability, including a one-year follow-up study by Awadalla and colleagues in Frontiers in Immunology examining humoral and cellular durability across vaccine platforms after homologous and heterologous boosters, illustrates that the durability question is now being addressed directly—but the letter’s point stands: longer follow-up is needed to determine whether early differences between booster strategies persist and remain clinically meaningful.

Perhaps the most granular objection is that the broad category of heterologous boosting may obscure clinically important differences between specific vaccine combinations. Viral-vector-to-mRNA, inactivated-to-mRNA, mRNA-to-mRNA, and protein-subunit-based booster strategies are immunologically distinct interventions. They engage innate immune pathways differently, present antigen in different formats, and carry different safety profiles and practical implications for cold-chain logistics and programmatic delivery. A single pooled comparison of heterologous versus homologous boosting, the authors argue, loses exactly the granularity that clinicians and immunization program managers need. Subgroup analyses, or more formally network meta-analytic approaches that can rank specific sequences against one another, would better inform decisions about which particular booster combination to deploy in which setting.

None of this amounts to a rejection of the underlying review. Dadgar and Rostami are explicit that Pial and colleagues have addressed an important question and provided a useful overview of the available evidence. Their critique is methodological, not adversarial—a familiar genre in epidemiology, where the gap between a statistically significant pooled estimate and a clinically actionable recommendation is often wide. The letter’s summary is measured: heterogeneity in vaccine platforms, booster combinations, dosing intervals, variant periods, prior infection status, and outcome definitions limits the interpretation of a single pooled estimate. Heterologous boosting may generate higher short-term immune responses in some settings, but its comparative clinical benefit, durability, and applicability to specific vaccine sequences require cautious interpretation.

The practical stakes remain high. Booster policy decisions—whether to offer an mRNA booster after a viral-vector primary series, or a protein-subunit booster after inactivated vaccine—are made under real-world constraints of supply, logistics, and variant evolution. If heterologous strategies do confer durable advantages, mixing platforms could become standard practice; if the apparent advantage is an artifact of pooling heterogeneous studies with unbalanced baseline immunity, resources might be better spent elsewhere. The letter’s recommendations for future evidence syntheses are correspondingly concrete: separate immunogenicity from clinical outcomes, stratify by prior infection status and variant period, and evaluate specific booster combinations rather than treating heterologous boosting as a uniform strategy. As SARS-CoV-2 continues to evolve, the quality of the evidence behind booster sequencing will matter as much as the vaccines themselves.

Subject of Research: Methodological critique of a systematic review and meta-analysis comparing homologous and heterologous COVID-19 vaccine booster strategies

Article Title: Comments on " Homologous and heterologous booster of COVID-19 vaccines: A systematic review and meta-analysis"

Article References: Dadgar, F., & Rostami, M. (2026). Comments on " Homologous and heterologous booster of COVID-19 vaccines: A systematic review and meta-analysis". New Microbes and New Infections, Article 101858. https://doi.org/10.1016/j.nmni.2026.101858

Image Credits: AI Generated

DOI: 10.1016/j.nmni.2026.101858

Keywords: COVID-19 vaccines, heterologous boosting, homologous boosting, systematic review, meta-analysis, immunogenicity, SARS-CoV-2 variants, prior infection, booster durability, vaccine platforms, methodology, Comments