Dengue disease is a mosquito-borne viral disease endemic throughout most tropical and subtropical countries across the world.1,2 It is caused by 4 closely related but antigenically distinct dengue virus serotypes. Infection with the virus can often be asymptomatic but may lead to undifferentiated fever, dengue fever or dengue hemorrhagic fever with plasma leakage that can result in hypovolemic shock (dengue shock syndrome).3 An estimated 50–100 million people develop dengue illness each year with half a million people requiring hospitalization and approximately 20,000 deaths.2,4 In the Philippines, from January to July 2012, over 51,000 dengue cases were reported resulting in 328 deaths, with over 40% of cases occurring early in life (1–10 year age group).5
There is no licensed vaccine for dengue disease, and management of the disease is currently limited to supportive therapy. A safe and effective dengue vaccine is needed for the prevention of dengue disease. A yellow fever-17D–dengue virus tetravalent dengue vaccine (CYD-TDV) is currently in development.6,7 Studies have shown that CYD-TDV is well tolerated and has a favorable safety profile across all age groups assessed so far.6–12 Although CYD-TDV provided satisfactory immunogenicity against all 4 dengue serotypes in a phase IIb proof-of-concept study and was shown to be partially protective against virologically confirmed, symptomatic dengue for 3 of the 4 serotypes (1, 3 and 4), this was not the case for serotype 2, which was the prevalent serotype in the context of this setting.7 Two large scale phase III studies have been undertaken in children in five countries in South East Asia (n=10,275) and five countries in Latin America (n=20,869).13,14 Continued surveillance of the participants in these two studies is currently on going to better define vaccine efficacy and safety in various epidemiological settings over the longer term.
It is anticipated that CYD-TDV may be introduced as part of the childhood immunization schedule in endemic areas. Vaccination against dengue during the first year of life might be hampered by the presence of maternal dengue antibodies that could interfere with vaccine immunogenicity and efficacy.15 Introduction of CYD-TDV into the childhood vaccination schedule from 1 year of age or older may, therefore, be preferable.16 Optimizing vaccination opportunities through concomitant administration of the CYD-TDV with other routine childhood vaccines may be desirable to ensure high-vaccine coverage.16 In this study, the primary objectives were to describe the safety of CYD-TDV administered alone or with the measles, mumps and rubella (MMR) vaccine in toddlers aged 12–15 months and to describe dengue vaccine viremia and biological safety following vaccination. Immunogenicity of both vaccines was a secondary descriptive objective.
MATERIALS AND METHODS
Study Design and Participants
This was a randomized, controlled, phase II trial conducted in 210 toddlers in 2 centers (San Pablo and Muntinlupa City) in the Philippines between January 18, 2010 and May 8, 2012. As this was the first clinical trial with CYD-TDV in toddlers, the study was designed with a stepwise (3 steps) enrolment process as a safety precaution, with a modified double-blind stage for the first vaccination and open-label for the second and third vaccinations. Before progressing to the next step, an internal safety review was undertaken to assess the safety results, and in particular, considered whether the following events occurred within 14 days after the first dengue injection: serious adverse events assessed as related to dengue vaccine; grade 3 solicited systemic reactions (see below) in more than 10% of toddlers and grade 3 biochemical or hematological parameter abnormalities (day 8 post-vaccination) in more than 10% of toddlers. All 3 study steps were conducted.
The study was conducted in accordance with the ethical principles of the Declaration of Helsinki and the International Conference on the Harmonization-Good Clinical Practice. In addition, the study protocol, including any amendments, was approved by each study site’s Institutional Review Board and Independent Ethics Committee. Written informed consent was obtained from the parents/guardians of all children before study entry. This trial has been registered at www.clinicaltrials.gov (identifier NCT01064141).
Toddlers aged 12–15 months were eligible if they were in good health and were born at full term (≥37 weeks pregnancy) with a birth weight ≥2.5 kg. All vaccinations in the national immunization schedule were to have been received, with the exception of measles vaccination. Exclusion criteria included history of varicella, MMR or hepatitis A infection or vaccination, or vaccination against flaviviruses; known or suspected congenital or acquired immunodeficiency; receipt of immunosuppressive therapy such as anticancer chemotherapy or radiation therapy within the preceding 6 months or long-term systemic corticosteroid therapy (for more than 2 consecutive weeks within the previous 3 months); receipt of blood or blood-derived products in the past 3 months; history of central nervous system disorder or disease, including seizures; thrombocytopenia, bleeding disorders or receipt of anticoagulants in the 3 weeks preceding inclusion; known systemic hypersensitivity to any of the components of the vaccines; seropositivity to human immunodeficiency virus and clinically significant laboratory abnormalities.
Randomization and Blinding
Children were recruited and screened up to 7 days before M −1 (ie, 1 month before administration of study injections) for eligibility to participate in the study (Fig. 1). Each eligible child was randomly assigned to 1 of the 4 groups (Fig. 1), via an interactive voice recognition system. A double randomization procedure was used. Toddlers were sequentially assigned an enrolment number and randomized to 1 of the 4 study groups (see below) according to a permuted block randomization schedule (using a block size of 6 for the first step and 4 for the second and third steps of enrolment). The vaccination nurse prepared and administered the vaccine(s) in a separate room, away from the investigator in charge of safety assessment. The investigator, sponsor and parents/legally acceptable representatives of the children did not know which vaccine was administered.
Vaccination and Vaccines
First Study Step
Ninty toddlers were randomized 2:1 to receive 3 doses of CYD-TDV (group 1; DV) or active control vaccines (group 2; control) at months (M) 0, 6 and 12 (Fig. 1). Control vaccines were varicella (OKAVAX; Sanofi Pasteur, France) and 2 doses of hepatitis A (AVAXIM 80; Sanofi Pasteur, France). All toddlers received MMR vaccination (TRIMOVAX; Sanofi Pasteur, France) 1 month earlier (M −1).
Second Study Step
Forty toddlers were randomized 1:1 to receive varicella (group 3; Co-ad) or MMR (group 4; Seq) at M −1, followed by CYD-TDV and MMR coadministration (group 3; Co-ad) or CYD-TDV and placebo (group 4; Seq) at M0, followed in both groups by CYD-TDV injections at M6 and M12. The placebo was a 0.9% NaCl solution.
Eighty toddlers were randomized and vaccinated identically to study step 2.
CYD-TDV was supplied as powder and solvent for suspension for injection. Each 0.5 mL dose of reconstituted vaccine contained 5 ± 1 log 10 cell-culture infectious dose 50% of each live, attenuated, recombinant dengue serotype 1–4 virus. The solvent for reconstitution consisted of 0.4% NaCl and 2.5% human serum albumin. CYD-TDV, MMR and varicella vaccines were administered as subcutaneous injections in the deltoid region of the upper arm. In the case of vaccine coadministration, injections were in opposite arms. The hepatitis A vaccine was administered as an intramuscular injection in the upper arm. All toddlers also received a diphtheria (D), tetanus (T), acellular pertussis (aP), inactivated poliovirus (IPV) and haemophilus influenzae type b (Hib) polysaccharide conjugated to tetanus protein (PRP~T) combined vaccine (DTaP–IPV//PRP~T; PENTAXIM; Sanofi Pasteur, France), approximately 9–10 months after MMR injection, in accordance with immunization schedule of the Philippines (Fig. 1).
Safety and Reactogencity
The children were kept under observation for 30 minutes after each trial vaccination to assess any immediate adverse events. Parents/guardians were requested to record daily any solicited injection site reactions (pain, erythema and swelling) during the 7-day period following each study vaccination. Solicited injection site reactions were recorded for each injection site in case of coadministration. Systemic reactions (fever, vomiting, abnormal crying, drowsiness, loss of appetite and irritability) were recorded for 14 days post-vaccination. Parents/guardians were provided with digital thermometers and flexible rulers to record body temperature and the diameter of any injection site erythema and/or swelling. In addition, they also recorded the intensity of the subjective adverse reactions (tenderness and for all systemic adverse events, except fever) experienced using a 3 point grading scale of increasing severity (grades 1–3) previously described.17 Pain at the injection site was graded as follows: grade 1, minor reaction when the injection site was touched; grade 2, the child cried and protested when the injection site was touched and grade 3, the child cried when the injected limb was moved, or when movement of the injected limb was reduced. Systemic reactions were graded as follows: vomiting (grade 1, 1 episode per 24 hours; grade 2, 2–5 episodes per 24 hours; grade 3, ≥ 6 episodes per 24 hours or requiring parenteral hydration); crying (grade 1, <1 hour; grade 2, 1–3 hours; grade 3, >3 hours); drowsiness (grade 1, sleepier than usual or less interested in surroundings; grade 2, not interested in surroundings or did not wake up for a feed/meal; grade 3, slept most of the time or difficult to wake up); loss of appetite (grade 1, ate less than normal; grade 2, refused 1 or 2 feeds/meals; grade 3, refused ≥3 feeds/meals or refused most feeds/meals) and irritability (grade 1: easily consolable; grade 2, required increased attention; grade 3, inconsolable). Measurable adverse reactions of erythema, swelling and fever were graded on the 3 point scale during statistical analysis. Injection site erythema and swelling were classified as follows: grade 1, where they were <2.5 cm in diameter; grade 2, ≥2.5 to <5 cm; and grade 3, ≥5 cm. Fever was defined as body temperature ≥38.0° and classified as grade 1, where the measured temperature ranged between ≥ 38.0 and ≤ 38.5°C, grade 2, > 38.5 to ≤ 39.5°C and grade 3, ≥39.5°C.
Unsolicited adverse events were recorded for 30 ± 2 days following each injection and assessed for causal relationship to vaccination by the investigator. Serious adverse events were recorded and followed-up throughout the trial according to standard clinical trial practice (up to 6 months after the last vaccination) and assessed for causal relationship to vaccination by the investigator and the sponsor. No safety data were collected after injection of the pediatric combined vaccine.
Dengue vaccine viremia was evaluated 8 days after the first vaccination by quantitative and serotype-specific reverse transcriptase polymerase chain reactions using methods similar to those described elsewhere.18 The lower limit of quantification was approximately 5 log10 genomic equivalents per milliliter across the serotypes. Biological safety was monitored by recording the occurrence and timing of laboratory test results (biochemistry: creatinine, alanine and aspartate aminotransferases, total bilirubin) and hematological parameters (hematocrit, hemoglobin and full blood cell counts) that were outside the normal range at baseline and 8 days after the first dengue vaccination. Detection of natural infections in the trial population exposed to dengue was set up in case of fever episodes (ie, temperature ≥38°C for at least 2 consecutive days), passively reported by the families. Dengue infection was confirmed by serotype-specific reverse transcription-polymerase chain reaction derived from a published method7 and dengue nonstructural protein 1 antigen (NS1 Ag) enzyme-linked immunosorbent assay (ELISA; Platelia Dengue NS1 Ag, BioRad, Marnes-la-Coquette, France).
Blood samples were collected from all toddlers at baseline and on day 30 (±2 days) after vaccinations at M6 and M12 and additionally from toddlers in groups 3 and 4 only on day 30 (±2 days) after vaccinations at M0 (Fig. 1). Dengue virus neutralizing antibody titers were determined using the 50% plaque reduction neutralization test against the parental dengue virus strains of the CYD-TDV constructs (Sanofi Pasteur GCI, Swiftwater, PA).19,20 The lower limit of quantification for the assay was 10 (1/dil). Antibody levels against measles, rubella and mumps were measured by ELISA at Pharmaceutical Product Development Laboratories, PA. Serum samples were added to wells of 3 separate measles, mumps or rubella antigen-coated ELISA microtiter test plates (Trinity Biotech., NY). Serum samples were tested in duplicate at a 1:100 starting dilution and were not serially diluted. Assay methods for the different antigens varied slightly and are summarized as follows. Test plates were incubated at 18–24°C for 20–30 minutes and then aspirated and washed with phosphate buffered saline/Tween 20. An optimized dilution of goat anti-human IgG horse radish peroxidase conjugate solution was added to all wells, and the plates were incubated at 18–24°C for 20–30 minutes. Plates were washed with phosphate buffered saline/Tween 20, and the enzyme substrate buffer was added to all wells. Plates were incubated at 18–24°C for 10–15 minutes. Following incubation, stop reagent was added to all wells and plates were read using an ELISA microtiter plate reader at 450 nm. The average optical density (OD) value of each sample was used to calculate the concentration in milli-international units per milliliter, endotoxin units per milliliter or international units per milliliter for measles, mumps and rubella, respectively, by comparison with a standard reference curve run on each assay plate. If the sample OD was not within previously established ranges of the OD standard curve, the sample was retested at the appropriate dilution (between 1:5 and 1:500) in a new assay run to obtain a valid antibody concentration. For measles and rubella antibody quantification, World Health Organization international reference standards and in-house controls were used, whereas only in-house controls were available for the mumps antibody quantification. The lower limit of quantification of the assay was 0.782 mIU/mL for measles, 4.04 EU/mL for mumps and 0.086 IU/mL for rubella. Antibody levels above 120 mIU/mL were considered seropositive for measles, and antibody levels above 10 IU/mL were considered seropositive for rubella. A 2-fold and 4-fold increase in mumps antibody levels relative to baseline levels was used to analyze the response to the mumps vaccine. Immune responses to varicella and pediatric combined vaccines were not evaluated.
This study was designed to assess the safety of the CYD-TDV administered alone or with the MMR vaccine in toddlers and to provide preliminary immunogenicity data on the study vaccine that will be useful in the planning of further pediatric studies. Consequently, there was no hypothesis testing, and as such, we did not conduct statistical comparisons between groups.
The sample size was set at 210 toddlers, so that there was a 95% probability of observing an adverse event with a true incidence of 1.7% in participants who received the dengue vaccine (n = 180) and of 5% in individual groups 1 (DV), 3 (Co-ad) and 4 (Seq; n = 60, respectively). Data were summarized in terms of point estimates and 95% confidence intervals using normal approximation for quantitative data and exact binomial distributions (Clopper–Pearson method) for proportions.
Safety and reactogenicity were assessed on the safety analysis sets, defined for each injection as the subset who received that injection, with the participants analyzed according to the study product received at that injection. For the analysis of safety after any injection, participants were analyzed according to the study product received at the first injection. The safety analysis set was also used for the description of vaccine viremia and biological safety.
The full analysis set was used for all immunogenicity analyses. The full analysis set consisted of all participants who received at least 1 injection of study vaccine and had at least 1 valid postinjection serology result. Analyses of dengue neutralizing antibody levels were performed according to each study group. Immunogenicity was described using geometric mean titers (GMTs), and seropositivity rates against measles and rubella (with threshold values of 120 mIU/mL and 10 IU/mL, respectively) and a 2-fold or 4-fold rise for mumps antibodies.
Of 222 toddlers who were screened, 210 were eligible and enrolled to the 4 study groups (Fig. 2). Of the randomized toddlers, 205 completed the study and 5 discontinued [2 in group 1 (DV) and 3 in group 3 (co-ad)]. Overall, the vaccine groups were balanced in terms of age, height, weight and body mass index, with slightly more males than females in all groups (Table 1). Flavivirus serological status (dengue and/or Japanese encephalitis) at screening visit was balanced between vaccine groups, ranging from 45% to 52% (Table 1).
Safety and Reactogenicity
Serious Adverse Events
In all, 10 toddlers (4.8%) experienced 13 serious adverse events during the study: 1 of 60 (1.7%) toddlers in group 1 (DV); 1 of 30 (3.3%) in group 2 (Control); 7 of 59 (11.9%) in group 3 (Co-ad) and 1 of 60 (1.7%) in group 4 (Seq). All serious adverse events were considered unrelated to vaccination by the study investigator. Febrile convulsions were experienced by 3 children in group 3 (Co-ad). One child experienced 2 episodes of febrile convulsion during the study, both of which resolved within 2 days: the first occurred 10 days after CYD-TDV and MMR coadministration and was assessed as unrelated by the study investigator (because it occurred in the context of a systemic viral infection) but was assessed as possibly related to both CYD-TDV and MMR vaccines by the study sponsor; the second occurred 25 days after vaccination with the DTaP–IPV//PRP~T vaccine, corresponding to 117 days after the second CYD-TDV injection. This child was discontinued from subsequent vaccinations as a result. A second child experienced a benign febrile convulsion 85 days after CYD-TDV and MMR coadministration, and a third child experienced acute gastroenteritis and a simple febrile seizure 25 days after the last dose of CYD-TDV. In both of these children, febrile convulsions were not considered as related to the study vaccine by the study investigator because they both occurred beyond the risk window associated with live vaccines (>5 to 12 days).21,22
Reactogenicity and Unsolicited Adverse Events
Step 1: CYD-TDV compared with active control vaccines—There were no immediate unsolicited adverse events after either CYD-TDV vaccination [group 1 (DV)] or vaccination with the active control vaccines [group 2 (control)]. Solicited injection site reactions within 7 days of any vaccination were reported in 25.0% of children in group 1 (DV) and 26.7% in group 2 (control). Solicited systemic reactions within 14 days of any vaccination were reported in 48.3% of children in group 1 (DV) versus 76.7% in group 2 (Control). There were no grade 3 solicited reactions reported in either group. The rates of unsolicited adverse events within 30 ± 2 days after any vaccination were 65.0% in group 1 (DV) and 73.3% in group 2 (control). All unsolicited adverse events were systemic in nature and none were considered related to vaccination.
After the first vaccination, CYD-TDV appeared less reactogenic at the injection site than the varicella vaccine at M0 (Fig. 3A). Pain and erythema of grade 1 intensity were the most frequently reported injection site reactions (Fig. 3B). There were no grade 3 injection site reactions, and only one of grade 2: erythema following varicella vaccination. The frequency of injection site reactions did not increase with subsequent CYD-TDV injections and did not exceed 5% (safety data after vaccinations 2 and 3 not shown). Fever, abnormal crying and irritability of grade 1 to 2 intensity were reported for 20.0–26.7% children after CYD-TDV vaccination, these rates were similar to, and no higher than, the rates reported after varicella vaccination (Fig. 3C). Vomiting appeared less frequent after the first injection of CYD-TDV compared with varicella vaccination (11.7% vs. 30.0%). Systemic reactogenicity did not increase with subsequent CYD-TDV (45.0%, 6.7% and 12.1% after the first, second and third injections, respectively) or control vaccine injections (60.0%, 20.0% and 10.0% after varicella and the 2 hepatitis A injections, respectively). There were no solicited systemic reactions of grade 3 intensity.
Rates of unsolicited adverse events were similar between both study groups. Unsolicited adverse events occurred at a higher rate after the first injection (with either CYD-TDV or varicella) than with subsequent vaccinations in groups 1 (DV) or 2 (control; ie, with subsequent hepatitis A injections).
No major differences were observed in reactogenicity and unsolicited adverse events reported according to the baseline flavivirus status in groups 1 (DV) and 2 (control).
Steps 2 and 3: CYD-TDV Coadministration with MMR or Placebo—There were no immediate unsolicited adverse events following any injection in the CYD-TDV and MMR coadministration group [group 3 (Co-ad)] or the CYD-TDV and placebo group [group 4 (Seq)]. Overall, coadministration of CYD-TDV with MMR appeared slightly more reactogenic than CYD-TDV with placebo at M0 (Fig. 4A).
Injection site reactions were similarly low with each injected product, that is, CYD-TDV, MMR and placebo (Fig. 4B), and all were of Grade 1 intensity, except for one grade 2 erythema at each injection site in a child in group 3 (Co-ad). Injection site reactions occurred in less than 5% of children following subsequent injections with CYD-TDV (safety data after vaccinations 2 and 3 not shown).
Figure 4C summarizes the percentage of children who experienced solicited systemic reactions following CYD-TDV and MMR coadministration and CYD-TDV and placebo injections. The most notable difference between coadministration compared with CYD-TDV administered alone (with placebo) was the higher rate grade 1 to 2 fever after coadministration. Reporting rates of the other solicited systemic adverse events were less than 20%. Systemic reactogenicity did not increase with subsequent injections with CYD-TDV. There were no solicited systemic reactions of grade 3 intensity.
Overall, 56% in group 3 (Co-ad) and 48% in group 4 (Seq) reported unsolicited adverse events within 30 ± 2 days after any vaccination. All were systemic events, and none were assessed as related to the vaccines. The rates of unsolicited adverse events tended to decrease after the second and third vaccinations with CYD-TDV.
No major differences were observed in reactogenicity and unsolicited adverse events reported according to the baseline flavivirus status in groups 3 (Co-ad) and 4 (Seq).
There were no grade 3 abnormalities in laboratory parameters at baseline. The rates for biological parameter abnormalities were not different 8 days after the first injection compared with baseline in each study group. Five children had grade 3 abnormalities 8 days after the first injection: 1 child following CYD-TDV and MMR vaccination in group 3 (Co-ad) and 3 children following CYD-TDV injection in group 4 (Seq) were found to have grade 3 alanine transaminase increases (4-fold greater than the upper limit of the normal range for age) and 1 child following CYD-TDV injection in group 4 (Seq) had a grade 3 hemoglobin decrease. All reported biological abnormalities were assessed as unrelated to the study vaccine.
Three children had quantifiable levels of dengue serotype 4 vaccinal viremia (close to the lower limit of quantification; 5.55–5.98 log10 Geq/mL): 2 children after the first CYD-TDV dose in groups 1 (DV) and 4 (Seq), and 1 after CYD-TDV coadministered with MMR in group 3 (Co-ad). None of these children experienced clinical symptoms of dengue or had concomitant adverse events or serious adverse events.
A total of 55 febrile episodes were reported during the entire trial period; of these, 3 dengue infections were virologically confirmed, 2 in group 1 (DV) more than 28 days after the second injection and 1 in group 2 (Control) more than 28 days after the first injection. All 3 cases were positive for dengue NS1 Ag, 2 of which were identified as serotypes 1 and 3 by PCR. None of these cases required hospitalization and none were considered as severe dengue by an independent data monitoring committee.
Dengue Antibody Levels
After the first vaccination, GMTs for serotypes 1 and 2 increased slightly (remaining close to baseline values) in groups 3 (Co-ad) and 4 (Seq). However, relatively higher GMTs were observed in group 4 (Seq) compared with group 3 (Co-ad) for serotypes 3 and 4. In group 3 (Co-ad), GMTs increased 2-fold and 4-fold from baseline to 30.8 (1/dil) and 20.5 (1/dil) for serotypes 3 and 4, respectively. In group 4 (Seq), GMTs (1/dil) increased 7-fold and 13-fold from baseline to 96.0 (1/dil) and 89.1 (1/dil) for serotypes 3 and 4, respectively. GMTs for each of the 4 dengue virus serotypes at baseline and 30 days after the third vaccination are summarized by study group in Figure 5. Following the third vaccination, there was a marked increase in GMTs for all serotypes and a balanced immune response across serotypes after 3 vaccinations with CYD-TDV administered alone or when coadministered with the MMR vaccine. GMTs remained at baseline levels following active control vaccines in group 2 (Control). MMR coadministration does not appear to affect CYD-TDV immunogenicity after completion of the vaccination schedule.
Measles, Mumps and Rubella Antibody Levels
The immunogenicity against the MMR antigens following concomitant administration of the MMR vaccine with CYD-TDV [group 3 (Co-ad)] was similar to that achieved when MMR was administered alone, before CYD-TDV vaccination [group 4 (Seq); Table 2]. After MMR vaccination, all toddlers in group 3 (Co-ad) and 96.7% in group 4 (Seq) were seropositive for measles, at about 95% of the toddlers in both groups 3 (Co-ad) and 4 (Seq) displayed a significant (2-or 4-fold) increase in mumps antibodies and 98.3% in both groups 3 (Co-ad) and 4 (Seq) were seropositive for rubella.
Our study, the first in children younger than 2 years, has demonstrated that CYD-TDV has a satisfactory safety profile with no safety issues in this population, including when coadministered with a licensed MMR vaccine. Describing the safety, immunogenicity and lack of interference between coadministered vaccines is important as the possibility of providing several vaccines at a single office or clinic visit can facilitate immunization practices and improve overall coverage. In this study, coadministering these 2 live attenuated vaccines did not appear to affect adversely the safety or immunogenicity profile of either vaccine following completion of the vaccination schedule.
The safety profile of the CYD-TDV in our study appears similar to that observed in 2 phase I studies conducted in the Philippines11 and in Mexico8 and a phase II study in Singapore6 in older participants (aged 2–45 years) with broadly similar rates to our study for both injection site and systemic reactions (mainly grade 1 or 2 in severity) and a trend to decreased reactogenicity with subsequent CYD-TDV vaccinations. As in previous trials, the safety profile of CYD-TDV in toddlers in our study was not adversely affected by previous Flavivirus exposure.7,11 Serious adverse events (mainly febrile convulsions with or without concomitant infections, nearly all of which were unrelated to vaccination) were reported slightly more frequently in our study compared with other studies that included participants aged 2–45 years.6,8,11 This was expected considering that the peak incidence for febrile convulsions is in toddlers around 18 months.
In previous trials with CYD-TDV, vaccine viremia was evaluated as an indirect indicator of vaccine safety as dengue viremia has been shown to be associated with virulence and dengue disease severity.23 In our study, viremia was assessed at a single time point (eight days) after the first vaccination only, as previous studies have shown that CYD-TDV viremia peaks around day 8 after the first dose of CYD-TDV. After the first injection of CYD-TDV, low levels of vaccine viremia, predominantly serotype 4, were observed in all groups, without clinical significance and consistent with a previous study also conducted in the Philippines in which low levels of serotype 4 vaccine viremia occurred across a wide age range.11 The few grade 3 elevations of hepatic enzyme in the study groups 3 and 4 were assessed as not related to the study vaccination, but rather to viral infections that are frequent during the rainy season in the Philippines.
Overall in our study, the safety profile of CYD-TDV appears similar to other commercial vaccines usually administered during the second year of life (varicella and hepatitis A vaccines in the control group). The first injection of CYD-TDV, which elicits more reactions than subsequent injections, was even less reactogenic than the licensed varicella vaccine control. Coadministration of CYD-TDV and MMR tended to be slightly more reactogenic than CYD-TVD alone, with more mild to moderate fever reactions that occurred mostly within 8–14 days after injection and lasted between 1 and 3 days. Unfortunately, the reactogenicity after MMR vaccination alone was not assessed in our study, and it is not clear whether coadministration is more reactogenic than following MMR vaccination alone. Nonetheless, only 1 serious event occurred within 28 days after the concomitant administration of MMR and CYD-TDV vaccines. In other toddler studies where the MMR vaccine was coadministered with either a yellow fever vaccine24 or with the Japanese encephalitis vaccine engineered with the same yellow fever virus backbone as CYD-TDV,25 concomitant administration tended to be more reactogenic than when the 2 vaccines were administered 4 or 6 weeks apart.
After completion of the vaccination schedule in our study, the GMTs increased from baseline levels by a factor >10 for all dengue virus serotypes and across the study groups, except in group 2 (control) where a slight elevation of GMTs was observed as a result of natural exposure to dengue (one dengue case was confirmed in group 2). The GMTs following 3 doses of CYD-TDV ranged by factor of about 3 across both serotypes and study groups. This narrow range between the lowest and highest GMTs is indicative of a balanced immunity across serotypes in toddlers, and is consistent with the balanced immunogenicity achieved with CYD-TDV in older age groups (>2 years).8,11 The GMTs achieved in toddlers are also consistent with those reported in young children (2–5 years) in the phase I study in the Philippines, where the baseline serostatus for flaviviruses was similar.11 Coadministration with the MMR vaccine in group 3 (Co-ad) does not appear to affect the overall immunogenicity of CYD-TDV. Although a slight decrease in immune response was observed after the first injection of CYD-TDV coadministered with MMR in group 3 compared with group 4 (Seq), the immune response was similar after subsequent vaccinations.
Coadministration of CYD-TDV with the MMR vaccine, a total of 7 live virus vaccines, does not appear to affect the overall immunogenicity of the MMR vaccine. Seropositivity rates were ≥95% against measles and rubella, and the rise in mumps antibody were similar between groups 3 (Co-ad) and 4 (Seq). Overall, immune responses for MMR appear to be comparable with those previously reported after MMR vaccination in toddlers and young children.26–28 In addition, the GMTs for MMR antibodies reached similar levels 28 days post-MMR vaccination in group 3 (Co-ad) and 4 (Seq). The lack of CYD-TDV and MMR vaccine interaction on the immunogenicity of both vaccines in our study is consistent with lack of interaction between coadministrated MMR and live attenuated Japanese encephalitis chimeric virus vaccine in another study (unpublished data). However, coadministration of yellow fever and MMR vaccines in Brazilian toddlers was shown to reduce immunogenicity to yellow fever and for mumps and rubella components, compared with responses following separate MMR and yellow fever vaccinations 30 days apart.24 It should be noted that the primary aim of our study was to provide an assessment of the safety of CYD-TDV in children below 2 years of age. In particular, the study was not specifically intended to test a statistical hypothesis of noninferiority of immune response to the CYD-TDV and MMR vaccines alone compared with coadministration.
In conclusion, consistent with previous trials conducted in older age groups, the CYD-TDV has a satisfactory safety profile and is immunogenic in toddlers. The concomitant administration with MMR vaccine does not seem to adversely affect the safety profile of CYD-TDV and the immunogenicity of MMR or CYD-TDV. These data are encouraging and support larger phase III trials to assess coadministration of CYD-TDV with other live or killed vaccines in the immunization calendar for toddlers. Larger phase III vaccine trials with long-term follow-up are needed before CYD-TDV can be widely recommended—such studies are currently ongoing.
Editorial assistance with the preparation of the manuscript was provided by professional medical writers, Lorraine Ralph and Richard Glover of inScience Communications, Springer Healthcare, funded by Sanofi Pasteur.
The authors thank all the children and their parents/guardian’s for their participation in this research and the study investigators and coordinators for all their hard work.
In addition, thanks are due to the following people within Sanofi Pasteur: Grenville Marsh for his support in the preparation of this manuscript. Mark Boaz and the Global Clinical Immunology Department for conducting the immunological assays; Karina Abalos for study management and logistics; Christophe Carre for the statistical analyses; Rémi Forrat, Jean Lang, Alain Bouckenooghe and Melanie Saville for their contributions to the study design and development.
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Keywords:Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
vaccine; safety; reactogenicity; immunogenicity