The CoVacc Trial: Single vs. Two-Dose BNT162b2 Vaccination in Children

The CoVacc trial emerged during a period of shifting global health priorities, specifically addressing the optimal dosing strategy for BNT162b2 vaccination in children aged 5 to 11 who had already experienced a SARS-CoV-2 infection. As the pandemic evolved into an endemic state, the question of whether a single dose could provide sufficient immune protection compared to the standard two-dose regimen became paramount for clinicians. This randomized phase 2 study aimed to evaluate both the immunogenicity and the safety of these two distinct primary vaccination schedules. By focusing on children with prior infections, the researchers aimed to clarify the role of hybrid immunity in shaping antibody responses. Furthermore, the study sought to determine if a reduced-dose regimen could offer a superior safety profile while remaining non-inferior in terms of the neutralizing antibody response. Understanding these dynamics is essential for refining pediatric vaccination policies and ensuring that children receive adequate protection without unnecessary exposure to potential reactogenicity associated with multiple doses.

Methodology of the CoVacc Study

To investigate the efficacy of varied dosing schedules, the researchers employed an open-label, randomized phase 2 trial design. The study enrolled children aged 5 to 11 years from three European countries, all of whom had confirmed evidence of a previous SARS-CoV-2 infection. Researchers randomly assigned participants in a 1:1 ratio to receive either a single dose or two doses of the BNT162b2 vaccine. For those in the two-dose cohort, the interval between the first and second doses ranged from three to twelve weeks, reflecting real-world flexibility in clinical practice. The primary objective of the methodology was to assess the geometric mean ratio (GMR) of neutralizing antibodies against the wild-type virus at 28 days post-vaccination. Additionally, the trial was designed to monitor secondary endpoints, including the long-term persistence of antibody titers up to 12 months and the safety profile of each regimen. Consequently, this robust framework aimed to provide data on the immunological benefits and potential side effects of the vaccine in this specific demographic.

Immunogenicity Results: Comparing Neutralizing Antibody Titers

The immunogenicity analysis revealed that both the single and two-dose regimens elicited significant antibody responses in the study participants. At 28 days after the completion of the assigned vaccination schedule, the geometric mean titer (GMT) of neutralizing antibodies against wild-type SARS-CoV-2 reached 1801.1 IU/mL in the two-dose arm. Similarly, the single-dose arm showed a robust response with a GMT of 1715.5 IU/mL. However, despite these impressive numbers, the study could not demonstrate the statistical non-inferiority of the single-dose regimen. The geometric mean ratio was calculated at 0.9, which fell outside the predefined non-inferiority margin of a 1.5-fold change. Interestingly, the researchers observed that anti-RBD IgG titers against the wild-type virus and neutralizing titers against the BA.5 variant remained high in both groups at the 28-day mark. Moreover, antibody levels stayed above 100 IU/mL even after 6 and 12 months, suggesting that the initial dose effectively acted as a potent booster for the pre-existing hybrid immunity. These findings highlight the enduring nature of the immune response.

Safety and Reactogenicity Profile of BNT162b2 Vaccination in Children

When examining the safety and reactogenicity profile of BNT162b2 vaccination in children, the trial noted distinct differences between the two dosing groups. Systemic adverse events of grade 2 or higher were more frequently reported among those who received two doses compared to those who received only one. Specifically, 53% of participants in the two-dose arm experienced such events within seven days of vaccination, whereas the rate was lower at 31% in the single-dose cohort. Although the p-value of 0.18 indicated that this difference was not statistically significant, the trend suggests that a single-dose regimen might offer a more favorable tolerability profile for younger children. Most reported symptoms were mild to moderate and transient, resolving within a few days. Importantly, no serious adverse events or unexpected safety signals were identified during the study period. This well-tolerated profile supports the continued use of BNT162b2 in this age group, though the possibility of reduced side effects with a single-dose strategy remains an area of clinical interest for pediatricians.

Impact of Premature Trial Termination on Statistical Power

A critical aspect of the CoVacc trial was its premature termination, which occurred in January 2024. This early closure was primarily driven by a declining interest in COVID-19 vaccination among parents of age-eligible children as the perceived threat of the virus diminished over time. Consequently, the study was only able to enroll 31 participants, falling far short of the number required to achieve full statistical power. This limitation directly impacted the ability of the researchers to draw definitive conclusions regarding the non-inferiority of the single-dose regimen. Although the observed antibody titers were robust in both arms, the wide confidence intervals resulting from the small sample size prevented a clear statistical demonstration of equivalence. Furthermore, the termination underscores the challenges of conducting clinical research in a rapidly changing public health landscape where social factors can abruptly halt progress. Despite these obstacles, the data collected provides valuable insights into the immunological trajectory of children with hybrid immunity, even with a smaller cohort than originally intended by the study design.

Clinical Implications for Pediatric Vaccination Strategies

From a clinical perspective, the results of the CoVacc trial offer important guidance for pediatricians, particularly in regions where a large proportion of children have already been exposed to SARS-CoV-2. The study supports the hypothesis that, for children with prior infection, a single dose of BNT162b2 effectively functions as an immunological booster, leveraging pre-existing memory cells to produce a rapid humoral response. While the trial could not confirm non-inferiority due to its small size, the raw immunogenicity data remains compelling. Clinicians should consider these findings when discussing vaccination schedules with parents, as a single dose may provide an optimal balance between protection and reactogenicity. Additionally, the sustained antibody levels observed at 12 months suggest that frequent boosting may not be necessary for children with hybrid immunity. As local and national guidelines continue to evolve, integrating these findings into practice can help tailor vaccination efforts to the specific needs of individual patients, ensuring that pediatric care remains evidence-based.

Frequently Asked Questions

Does a single dose provide enough protection in previously infected children?

Studies indicate that a single dose of BNT162b2 in children who have previously recovered from COVID-19 effectively triggers a robust memory response. This phenomenon, often referred to as hybrid immunity, results in neutralizing antibody levels that are comparable to those seen in individuals receiving two doses. Therefore, for children with confirmed prior infection, a single dose may serve as a sufficient booster to enhance their existing immunity against various SARS-CoV-2 variants without requiring a second primary dose.

What are the most common adverse events observed in this age group?

The most frequently reported systemic adverse events in children receiving BNT162b2 include fatigue, headache, and muscle pain. In the CoVacc trial, approximately 53% of participants in the two-dose group reported grade 2 or higher systemic events, compared to only 31% in the single-dose group. Most symptoms are mild to moderate, transient, and typically resolve within 48 hours. Significantly, no serious safety signals or occurrences of myocarditis were reported within this small, prematurely terminated study cohort.

Why was the CoVacc trial terminated prematurely, and how does this affect the results?

The CoVacc trial was stopped early due to a significant decrease in interest regarding COVID-19 vaccination for children within the eligible age range. Consequently, the study only enrolled 31 participants, which was far fewer than the target needed for full statistical power. Because the sample size was small, researchers were unable to statistically demonstrate the non-inferiority of the single-dose regimen compared to the two-dose schedule, despite observing robust and sustained antibody titers in both study arms.

Disclaimer: This content is for informational and educational purposes only. It does not constitute medical advice or establish a doctor-patient relationship. Always seek the advice of a qualified healthcare provider for any questions regarding a medical condition. Refer to the latest local and national guidelines for clinical practice.

References

Prat-Aymerich C et al. A prematurely terminated phase 2, randomised trial to evaluate immunogenicity and reactogenicity of a single versus two-dose primary vaccination regimen of the mRNA vaccine BNT162b2 in previously SARS-CoV-2 infected children 5-11 years old (CoVacc trial). Vaccine. 2026 Jun 27. doi: undefined. PMID: 42364277.

Walter EB et al. Evaluation of the BNT162b2 Covid-19 Vaccine in Children 5 to 11 Years of Age. N Engl J Med. 2022 Jan 6;386(1):35-46. doi: 10.1056/NEJMoa2116298.

Tsampalieros A et al. Hybrid immunity after BNT162b2 Covid-19 vaccine administration in children aged 5 to 11 years. J Pediatric Infect Dis Soc. 2024 Aug 13. doi: 10.1093/jpids/piae082.