The multi-center, prospective study recruited CCP and childhood healthy control (CHC) to receive the standard dose SARS-CoV-2 inactivated vaccines (CoronaVac, 4 μg per 0.5 ml per shot) from the pediatric hematology/oncology cancer database (pediatric Vacan cohort, Shan 2021-137) of the five tertiary hospitals (see Supplementary Methods ) between 13 December 2021 and 18 July 2022. An independent group of ACP was also retrospectively included to match the CCP by cancer-related treatment and time of receiving vaccines from 1 March 2021 to 1 July 2022. Ethical approval has been gained at the Second Affiliated Hospital of Shantou University Medical College (SAHSU). The study was performed according to the ethical principles of the Declaration of Helsinki and Good Clinical Practice, and all parents of the CCP and CHC provided informed consent before participation.

Inclusion criteria for CCP were as follows (1): age between 3 and 18 years old (2); diagnosis of solid cancers in the past 3 years, irrespective of the current status of disease activities or treatment; and (3) willingness to and ability to have the clinical samples tested by the researchers, including blood, feces, and urine. Key exclusion criteria were as follows (1): contraindications to receiving vaccines, including hypersensitive reactions to the adjuvants of the CoronaVac vaccines (2); the prognosis of the CCP was less than 6 months or equivalent to hospice settings (3); history of severe autoimmune, genetic, or hematological diseases (4); prior infection of SARS-CoV-2 (5); diagnosis of hematological malignancies (which will be published elsewhere); and (6) participants with positive SARS-CoV-2 antibody titers before receiving vaccines.

The bio-specimen in the study were per-protocol blood and routine samples of serum from clinical settings collected for downstream analysis. All participant data and samples were de-identified and stored with the analytic ID number in the current investigation. The electronic data capture systems of the SAHSU were applied to save and monitor the de-identified profiles.

The anti-SARS-CoV-2 antibody serology testing was performed prior to receiving vaccines, and 3 months after the second dose, respectively. After collection of the whole blood in Ethylene Diamine Tetraacetic Acid (EDTA) tubes, the serum was carefully collected and centrifuged for 10 min and stored at −80°C until testing. Times of blood drawing, processing, and refrigeration were recorded for each sample. The time from blood drawing to refrigeration was kept to be less than 24 h, and the process was done by the lab technicians blinded to the data. The sera were analyzed in the third-party laboratory in Guangzhou Province for antibody testing. For qualitative and quantitative analysis, the S-specific immunoglobulin G (IgG) was tested using the chemiluminescence kit (Bioscience Technology, Guangzhou, China).

Demographic and therapeutic data were extracted from the Case Record Form of the five tertiary referral hospitals. The demographic information included the gender and the age of participants (CCP, CHC, and ACP). The nutrition status of the CCP was defined with the body mass index of the corresponding growth milestones of the Chinese children and was thus divided into underweight, normal, and overweight by the treating oncologists blinded to the participant data. In the ACP group, the nutrition status was defined by the treating oncologists at follow-up. The treatment details included the cumulative chemotherapy time, steroid use history, tyrosine kinase inhibitor use, and radiotherapy history. The survival time was calculated from the time of cancer diagnosis to the time of receiving vaccines, and the chemotherapy status was also recorded as active or inactive. The pathological types of the CCP and ACP were determined by both histological and radiological evidence from the patient database of the five hospitals.

Included CCP participants received standard, two-dose CoronaVac SARS-CoV-2 vaccines at registered vaccination sites of the Centers for Disease Control and Prevention, with an intervening period of 5 to 7 weeks between the two doses. Participants with severe adverse events after the first doses of SARS-CoV-2 vaccines would not receive the second dose as required by the institutional board.

The primary outcome of the study was to report humoral responses against variants of concern (VOCs) after the COVID-19 vaccination in each group. The secondary outcome was to illustrate follow-up adverse events, or safety, of CCP during 3 months following SARS-CoV-2 vaccination. Solicited adverse events were recorded according to China National Medical Products Administration guidelines (16). Both efficacy and safety outcomes would be compared with ACP and CHC groups by propensity score-matched (PSM) analysis to minimize selection bias that may occur during the study process. Participant data, serology, and neutralization test methods are shown in supplementary details (see Supplementary Methods ).

The primary research goal was the humoral response against each variant, including serology conversion, serology conversion rate, and neutralization ability. The secondary goal was adverse events, including local and systemic events graded according to Common Terminology Criteria for Adverse Events. Categorical variables were recorded as numbers and percentages, and continuous variables were recorded as means ± standard deviations (SD) and median (25th–75th quartile). The power (1 − β) of each statistical test was calculated according to the sample size on the PASS software (V.15.0), and each test was based on a pre-specified statistical hypothesis with a type I error of 0.05.

To minimize potential bias, the propensity scores were calculated in the CCP, ACP, and CHC groups. The score was calculated with a logistic conditional regression model (17). Variables included in the regression model included treatment details and baseline demographics. For comparison between CCP and CHC groups, variables included gender and age. After calculation, the nearest neighbor head-to-head (1:1) method was applied to match each comparable participant with the upper acceptable caliper width of 0.2 without replacement (18). To find potential imbalance and to evaluate matching performance, standardized mean difference (SMD) was calculated in both unmatched and matched data (19, 20). According to Austin et al., an SMD over ( √ ( ( n 1 + n 2 ) / ( n 1 * n 2 ) ) ) * 1.96 is regarded as imbalanced pairs in the matched samples, where n1 and n2 stand for sample sizes of matching pairs (21). Thus, in matched samples, statistical tests of difference were McNemar test for categorical variables and Wilcoxon signed-rank test for continuous variables (22). Statistics used were carried out in SPSS V.23.0 software.

The study enrolled 121 CCP and 153 gender-matched CHC in a prospective cohort. No participant had ever received SARS-CoV-2 vaccines or been infected by SARS-CoV-2 vaccines, and all patients had negative antibody titers at recruitment. Ten CCP and 19 CHC did not finish the second dose due to concerns about future adverse events. Thus, a total of 111 CCP (73 male and 38 female patients) and 134 CHC (82 male and 52 female patients) were included in the final analysis ( Table 1 ). A total of 163 (88 male and 75 female patients) ACP were enrolled that match the treatment and vaccination details of CCP ( Supplementary Table 1 ). The median age of CCP was 8 (quartile, 5.5–15), and the median age of CHC was 9.5 (quartile, 6–13).

As for pathology types of CCP, 10 CCP (9.0%) had primitive neuroectodermal tumors (PNET), 30 CCP (27.0%) had neuroblastoma, 26 CCP (27.0%) had nasopharyngeal carcinoma, 10 CCP (9.0%) had rhabdomyosarcoma, 21 CCP (18.9%) had osteosarcoma or Ewing sarcoma, 4 CCP (3.6%) had hepatic blastoma, and 10 CCP (9.0%) had germ cell tumors.

Forty-six CCP (41.4%) received chemotherapy for less than 6 months, 21 patients (19.0%) over 1 year, and 44 patients (39.6%) between 6 months and a year. Median chemotherapy time was 7 (quartile, 5–11) months. Twenty CCP were on active chemotherapy when vaccinated. Seventy-eight patients (70.3%) had received radiotherapy in the past year, and 26 patients (23.3%) had received steroid therapies during the past 6 months. The median time of survival of the CCP was 27 (quartile, 20–37) months.

Overall, after receiving two-dose vaccines, 76 of 111 CCP (68.4%) were seropositive and 35 patients had a negative response. Mean antibody titers were 281.8 ± 315.5 U/ml, and 43 (38.7%) participants had antibody levels over 300 U/ml. Univariate analysis of seroconversion failure showed a significant association with the following variables: age, pathology types, nutrition status, and chemotherapy time. Multivariate regression identified the following two variables to be independently significant ( Table 2 ): age (OR = 0.84, 95% CI = 0.71–0.98, p = 0.03) and chemotherapy time (OR = 3.70, 95% CI = 1.71–7.98, p< 0.01). To quantify risk prediction of the serology failure, a nomogram was built that incorporated the two variables ( Supplementary Figure 1A ). A calibration curve was drawn to compare the actual and predicted risk of seroconversion failure ( Supplementary Figure 1B ). The concordance index of the nomogram was 0.79.

In the CHC group, 116 (86.5%) participants had positive serologic responses. Mean serologic titers were 1,210.75 ± 905.04 U/ml, and 100 (74.6%) CHC participants had an adequate response. By PSM comparison (see Supplementary Table 2 for imbalance evaluation results), the rate of seroconversion in the CCP group was significantly less than that of the CHC group (p< 0.01 by paired McNemar test), and the serologic level of the CCP group was significantly less than that of the CHC group (p< 0.01 by Wilcoxon signed-rank test, Figures 1A, B ). Antibody response and adverse event comparison are illustrated in Table 3 .

In the ACP group, 129 patients had positive serologic responses. Mean serologic titers were 414.41 ± 338.77 U/ml, and 88 patients had an adequate serologic response (53.9%). By PSM comparison between ACP and CCP groups (see Supplementary Table 3 for imbalance evaluation results), there were significantly fewer patients in the CCP group who had positive responses than in the ACP group (p< 0.01 by paired McNemar test), and the titer levels were significantly lower in the CCP group as well (p< 0.01 by Wilcoxon signed-rank test, Figures 1C, D ). Antibody response and adverse event comparison are illustrated in Table 4 .

Neutralization test was performed with the spike protein of wild type and VOCs. In the CCP group, percent inhibition was 41.75 ± 25.76 for wild types, 35.68 ± 21.50 for B.1.1.7 (Alfa), 36.32 ± 23.65 for B1.351 (Beta), 34.83 ± 21.13 for P.1 (Gamma), 35.93 ± 24.14 for B.1.617.2 (Delta), and 36.15 ± 24.19 for B.1.1.529 (Omicron). Analysis of variance (ANOVA) showed no significant difference among the six variants (p = 0.28, see Supplementary Figure 3 ). The Pearson correlation test showed that all subtypes were correlated with serologic titers of the CCP group ( Figure 2 ), and the one-way ANOVA test showed no significance in neutralization ability among VOCs. Longer chemotherapy time was associated with a lower rate of inhibition for vaccines against all variants (p< 0.01, Supplementary Figure 2 ). In the PSM comparison, there was a significant difference in all VOCs (Alfa, Beta, Gamma, Delta, and Omicron) between the CCP and ACP group, and between the CCP and CHC group (p< 0.05, see Figure 3 ).

Solicited severe adverse events of CCP reported during follow-up were graded as I and II, and all adverse events were either self-limited or medically well-managed. Thirty-two patients had local reactions following receiving vaccines, including tenderness, rash, and itchiness. Twenty-nine patients had systemic adverse events, including fever (n = 9), rash (n = 20), headache (n = 3), fatigue (n = 11), and myalgia (n = 15). None of the patients had adverse events associated with solid organs or the tumor itself.

Solicited severe adverse events of the CHC group were graded as I and II and were medically managed as well. Thirty-four participants (25.3%) had local reactions, and systemic adverse events were encountered in 31 patients (23.1%). Systemic adverse events included fever (n = 17), rash (n = 16), headache (n = 18), fatigue (n = 19), and myalgia (n = 15).

In the retrospective ACP group, all adverse events were also graded I or II, which were medically managed. Thirty-nine patients (23.9%) reported local reactions, and systemic events were seen in 40 patients (24.5%). Systemic events included fever (n = 22), rash (n = 27), headache (n = 14), fatigue (n = 19), and myalgia (n = 28).