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Background Despite the heightened risk of serious influenza during infancy, vaccination is not recommended in infants younger than 6 months. We aimed to assess the safety, immunogenicity, and efficacy of maternal immunisation with trivalent inactivated influenza vaccine for protection of infants against a first episode of laboratory-confirmed influenza. Methods We did this prospective, active-controlled, observer-blind, randomised phase 4 trial at six referral centres and community health centres in Bamako, Mali. Third-trimester pregnant women (≥28 weeks’ gestation) were randomly assigned (1:1), via a computer-generated, centre-specific list with alternate block sizes of six or 12, to receive either trivalent inactivated influenza vaccine or quadrivalent meningococcal vaccine. Study personnel administering vaccines were not masked to treatment allocation, but allocation was concealed from clinicians, laboratory personnel, and participants. Infants were visited weekly until age 6 months to detect influenza-like illness; laboratory-confirmed influenza diagnosed with RT-PCR. We assessed two coprimary objectives: vaccine efficacy against laboratory-confirmed influenza in infants born to women immunised any time prepartum (intention-to-treat population), and vaccine efficacy in infants born to women immunised at least 14 days prepartum (per-protocol population). The primary outcome was the occurrence of a first case of laboratory-confirmed influenza by age 6 months. This trial is registered with ClinicalTrials.gov, number NCT01430689. Findings We did this trial from Sept 12, 2011, to Jan 28, 2014. Between Sept 12, 2011, and April 18, 2013, we randomly assigned 4193 women to receive trivalent inactivated influenza vaccine (n=2108) or quadrivalent meningococcal vaccine (n=2085). There were 4105 livebirths; 1797 (87%) of 2064 infants in the trivalent inactivated influenza vaccine group and 1793 (88%) of 2041 infants in the quadrivalent meningococcal vaccine group were followed up until age 6 months. We recorded 5279 influenza-like illness episodes in 2789 (68%) infants, of which 131 (2%) episodes were laboratory-confirmed influenza. 129 (98%) cases of laboratory-confirmed influenza were first episodes (n=77 in the quadrivalent meningococcal vaccine group vs n=52 in the trivalent inactivated influenza vaccine group). In the intention-to-treat population, overall infant vaccine efficacy was 33·1% (95% CI 3·7–53·9); in the per-protocol population, vaccine efficacy was 37·3% (7·6–57·8). Vaccine efficacy remained robust during the first 4 months of follow-up (67·9% [95% CI 35·1–85·3] by intention to treat and 70·2% [35·7–87·6] by per protocol), before diminishing during the fifth month (57·3% [30·6–74·4] and 60·7 [33·8–77·5], respectively). Adverse event rates in women and infants were similar among groups. Pain at the injection site was more common in women given quadrivalent meningococcal vaccine than in those given trivalent inactivated influenza vaccine (n=253 vs n=132; p<0·0001), although 354 [92%] reactions were mild. Obstetrical and non-obstetrical serious adverse events were reported in 60 (3%) women in the quadrivalent meningococcal vaccine group and 61 (3%) women in the trivalent inactivated influenza vaccine group. Presumed neonatal infection was more common in infants in the trivalent inactivated influenza vaccine group than in those in the quadrivalent meningococcal vaccine group (n=60 vs n=37; p=0·02). No serious adverse events were related to vaccination. Interpretation Vaccination of pregnant women with trivalent inactivated influenza vaccine in Mali—a poorly resourced country with high infant mortality—was technically and logistically feasible and protected infants from laboratory-confirmed influenza for 4 months. With adequate financing to procure the vaccine, implementation will parallel the access to antenatal care and immunisation coverage of pregnant women with tetanus toxoid. Funding Bill & Melinda Gates Foundation.
We did this prospective, active-controlled, observer-blind, randomised phase 4 trial at six referral centres and community health centres in Bamako, Mali. In the year before starting the trial, influenza activity occurred from September to April, with peaks in October and February. Third-trimester pregnant women (≥28 weeks' gestation based on last menstrual period, ultrasound, or uterine height) presenting to participating health centres for prenatal care were eligible for inclusion. Participants had to be able to understand and comply with planned study procedures, provide written informed consent before initiation of any study procedures, and intend to reside in the study area until their newborn infants were 6 months old. Participants could not be members of a household that already had a woman who was participating or had participated in this study. Other exclusion criteria were a history of severe reactions following previous immunisation with influenza or meningococcal vaccines; Guillain–Barré syndrome; known allergy or hypersensitivity to eggs, egg proteins, latex, diphtheria toxoid, or any other components of trivalent inactivated influenza vaccine (Vaxigrip) and quadrivalent meningococcal conjugate vaccine (Menactra); known chronic medical disorder that, in the judgment of the investigator, could compromise assessment of the study vaccine or put the participant at risk; known active infection with HIV, hepatitis B virus, or hepatitis C virus; complications with the ongoing pregnancy, including preterm labour (with cervical change), placental abruption, premature rupture of membranes, known major congenital anomaly, or pre-eclampsia; acute illness or an oral temperature greater than or equal to 37·8°C within 72 h of vaccination (resulted in a temporary delay of vaccination); receipt of any other vaccine, excluding tetanus toxoid, within 2 weeks (for inactivated vaccines) or 4 weeks (for live vaccines and meningococcal A conjugate vaccine) before vaccination in this study; receipt of immunoglobulins or any blood products within 30 days before administration of study vaccines; chronic administration of immunosuppressants or other immune-modifying drugs within 90 days before administration of study vaccines; or any disorder that, in the opinion of the investigator, might compromise the wellbeing of the participant or compliance with study procedures, or interfere with the assessment of study vaccines. We additionally excluded women who intended to travel out of the study area in the 40 days after delivery. Enrolment continued until the requisite number of laboratory-confirmed influenza cases was detected in infants born to vaccinated women. Approval for the research was obtained from the University of Maryland, Baltimore Institutional Review Board; the ethics committee of the Faculté de Médecine, Pharmacie et Odonto-Stomatologie of Mali; and the Ministry of Health of Mali. Community sensitisation was achieved through community leaders, health centre representatives and community members who attended community-wide meetings. All participants provided informed consent. If the participant was illiterate, consent was obtained in the presence of a literate witness after listening to the audiotaped version of the consent form in Bambara, the local language. Participants were randomly allocated (1:1), via a computer-generated, centre-specific list with alternate block sizes of six or 12, to receive trivalent inactivated influenza vaccine (Vaxigrip, Sanofi Pasteur, Lyon, France) or quadrivalent meningococcal conjugate vaccine (Menactra, Sanofi Pasteur, Lyon, France). At enrolment, consenting participants were assigned an identification number, which at vaccination was referenced on the randomisation list and the allocated treatment given. The identification numbers for ineligible participants or those who withdrew before vaccination were not reassigned. Study personnel who administered study vaccines and were aware of treatment allocation had no contact with participants after vaccination and were instructed not to reveal the identity of the study vaccines either to participants or to personnel masked to treatment allocation. Although the syringes used to administer the vaccines were different in appearance, participants were instructed to look away from the vaccinator and were unaware of the assigned intervention. Quadrivalent meningococcal conjugate vaccine, rather than placebo, was given to controls to provide a potential benefit for all participants in this poor, mostly illiterate, vulnerable population of pregnant women. Moreover, that vaccine was unlikely to interfere with the primary outcome of the trial, yet would provide protection against meningococcal disease. Although disease due to serogroup A has largely disappeared from the region, other serogroups continue to cause epidemics in Mali. The composition of trivalent inactivated influenza vaccine, supplied in prefilled syringes, changed during the trial. From September, 2011, to November, 2012, A/California/7/2009(H1N1[pandemic]-like), A/Perth/16/2009(H3N2)-like, and B/Brisbane/60/2008-like (2011 northern hemisphere formulation and then 2012 southern hemisphere formulation) were administered. From December, 2012, to April, 2013, A/California/7/2009 (H1N1[pandemic]-like), A/Victoria/361/2011(H3N2)-like strain, and B/Wisconsin/1/2010-like (2012 northern hemisphere formulation) were administered. Quadrivalent meningococcal conjugate vaccine, composed of 4 μg each of Neisseria meningitidis serogroup A, C, Y, and W-135 polysaccharides conjugated to diphtheria toxoid protein, was supplied in single-dose vials. A single 0·5 mL dose of trivalent inactivated influenza vaccine or quadrivalent meningococcal conjugate vaccine was injected into the deltoid muscle. Study vaccines were stored in secure, temperature-monitored refrigerators or cold rooms at 2–8°C. After vaccination, women were observed for 30 min. 7 days after vaccination, field personnel interviewed the women about any local and systemic reactions. 28 days after vaccination, participants were clinically evaluated. Additional visits to evaluate safety and immunogenicity in women and infants were done at delivery and when the infant was 3 months and 6 months old. Each evaluation included a physical examination and blood specimen collection. When available, the infant birth sample was cord blood; otherwise, the birth sample was collected within 7 days after birth. To determine gestational age at birth, the New Ballard Score was measured at delivery or within 7 days after birth.19 Serious adverse events were recorded throughout study participation. Besides safety follow-up visits, from enrolment to when the infant reached age 6 months, field personnel undertook weekly visits to detect influenza-like illness and severe acute respiratory infection. During each visit, the participating woman and infant (if already born) had their temperatures measured and were examined for influenza-like illness; women were additionally examined for severe acute respiratory infection. When case definitions for either disease were met (appendix p 5), nasopharyngeal and oropharyngeal swabs and a malaria blood smear were obtained. If influenza was detected by RT-PCR, the case was deemed to be laboratory-confirmed influenza. Standard-of-care treatment was offered. Because the primary objective was to measure the efficacy of maternal immunisation for prevention of laboratory-confirmed influenza in their infants younger than 6 months, women were withdrawn from weekly surveillance of influenza-like illness following stillbirth, fetal death, infant death, or other events that precluded infant surveillance. Nevertheless, safety follow-up of women continued until 6 months after delivery. Appendix p 6 describes methods for sample collection, RT-PCR to detect influenza virus, virus subtyping, and haemagglutination inhibition antibody measurement. We assessed two coprimary objectives: vaccine efficacy in infants born to women vaccinated any time prepartum (intention-to-treat analysis), and vaccine efficacy in infants born to women vaccinated at least 14 days prepartum (per-protocol analysis). The primary outcome was the occurrence of a first case of laboratory-confirmed influenza by age 6 months. Secondary outcomes were the occurrence of a first case of laboratory-confirmed influenza in women (prepartum and post partum); occurrence of a first case of febrile influenza-like illness by age 6 months in infants; occurrence of a first case of febrile influenza-like illness in women (prepartum and post partum); occurrence of local and systemic reactogenicity after injection, related serious adverse events for the entire follow-up period, and all pregnancy complications; levels of influenza virus antibodies by haemagglutination inhibition before and 4 weeks after vaccination, at delivery, and 3 and 6 months after delivery. Tertiary outcomes included the frequency of each influenza virus type circulating in the study population, the levels of maternally derived influenza virus haemagglutination inhibition antibodies present in infants at birth and at ages 3 and 6 months, birthweights of infants born at a health centre, and the occurrence of severe acute respiratory infection in pregnant women. Appendix p 6 lists additional outcomes not included in the manuscript. We calculated vaccine efficacy with the formula: where VE is vaccine efficacy, h is the ratio of follow-up time up to age 6 months in infants born to recipients of quadrivalent meningococcal vaccine to the follow-up time in infants born to recipients of trivalent inactivated influenza vaccine, and P is the proportion of all cases of laboratory-confirmed influenza occurring by age 6 months in infants whose mothers received trivalent inactivated influenza vaccine. This calculation is equivalent to estimating vaccine efficacy as 1–R, where R is the ratio of laboratory-confirmed influenza incidence rates. We used the ratio of incidence rates, rather than the ratio of proportions of participants who had laboratory-confirmed influenza to account for infants lost to follow-up before age 6 months. We estimated vaccine efficacy in both the intention-to-treat and the per-protocol populations. Only infants' first laboratory-confirmed influenza episodes were counted. Follow-up time was time from birth to first case of laboratory-confirmed influenza, infants reaching age 6 months, or exiting the study. We calculated vaccine efficacy for each month of age (0–5 months) and cumulative to each month of age. For safety outcomes, we used Fisher's exact tests and Student's t tests to compare the proportion of participants who had each event per vaccine group. We did time-to-event analysis using Cox proportional hazards regression with laboratory-confirmed influenza as the outcome to establish whether year of vaccination or timing of vaccination relative to delivery had an effect on efficacy. Birthweight analysis was limited to weights that were either 500 g and more or 5000 g and less. We compared birthweight between vaccine groups both overall and within influenza seasons, defined as months with higher-than-average rates of laboratory-confirmed illness (February to April, September to October). Sample-size calculations were based on a comparison of the expected proportion, P, of all cases of laboratory-confirmed influenza that occurred by age 6 months in infants whose mothers received trivalent inactivated influenza vaccine to the null value, P0, using exact binomial calculations and assuming equal total follow-up time in each vaccine group (h=1). For the intention-to-treat analysis, we assumed a laboratory-confirmed influenza attack rate of 2·2% by age 6 months in infants born to recipients of quadrivalent meningococcal vaccine and a 55% reduction in the attack rate in infants of recipients of trivalent inactivated influenza vaccine, to 0·99%; therefore, p=0·31034 and P0=0·5. For a one-sided α of 0·025, 77 cases of laboratory-confirmed influenza were needed to ensure 90% power for the intention-to-treat analysis, implying a need for about 4828 participants. Allowing for a 10% loss to follow-up, the sample size calculated became about 5370 participants. For the per-protocol analysis, we assumed vaccine efficacy to be 60%—ie, a laboratory-confirmed influenza attack rate of 0·88% by age 6 months in infants born to recipients of trivalent inactivated influenza vaccine. To ensure 90% power to show a vaccine efficacy of more than 5%, 67 cases of laboratory-confirmed influenza were needed, implying a sample size of 4352 participants. Allowing for a 20% loss to follow-up, or for the mother receiving vaccine less than 14 days before delivery, the sample-size requirement became about 5440 participants. Enrolment was closed once 77 cases of infant laboratory-confirmed influenza were recorded, but surveillance continued until the infants reached 6 months of age. A Data Safety Monitoring Board oversaw the study and reviewed data on a regular basis. We did analyses with Stata (version 14.0) and NCSS (version 10). We did power calculations with PASS (version 12). This trial is registered with ClinicalTrials.gov, number {"type":"clinical-trial","attrs":{"text":"NCT01430689","term_id":"NCT01430689"}}NCT01430689. The funder of the study had no role in the study design, data collection, data analysis, data interpretation, or writing of the report. The corresponding author had full access to all the data in the study and had final responsibility for the decision to submit for publication.
