Impact of Timing of Influenza Vaccination in Pregnancy on Transplacental Antibody Transfer, Influenza Incidence, and Birth Outcomes: A Randomized Trial in Rural Nepal

Maternal Immunizations: Past, Present, and Future

Maternal vaccines during pregnancy offer crucial protection against infections for both the pregnant person and their newborn. Vaccines against influenza, pertussis, coronavirus disease 2019, and respiratory syncytial virus are routinely recommended by the Centers for Disease Control and Prevention to safeguard pregnant women and their infants from potentially severe complications. Administering these vaccines during pregnancy helps transfer protective antibodies from the mother to the baby, enhancing immunity during the vulnerable early months of life. Extensive research supports the safety and efficacy of maternal vaccines, with numerous studies demonstrating their protective benefits for both pregnant people and newborns.

Effect of maternal vaccination on infant morbidity in Bangladesh

BACKGROUND

Risk factors of infant mortality in Africa and south Asian countries have been broadly discussed. However, infant morbidity is largely underestimated. We analyzed the data from a randomized vaccine trial in Bangladesh to identify and assess the effect of risk factors on infant morbidity.

METHODS

Pregnant women were randomly assigned to receive either inactivated influenza vaccine or pneumococcal polysaccharide vaccine and the infants were randomly assigned to receive 7-valent pneumococcal conjugate vaccine or Hib conjugate vaccine at week 6, 10 and 14. The data were collected from August 2004 through December 2005. Each pair of infant and mother were followed for 24 weeks after birth with weekly visits. Generalized estimating equations (GEE) for repeated measurements and Poisson regression models were used to identify the risk factors and evaluate their effect on the longitudinal incidence and total number of episodes of respiratory illness with fever (RIF), diarrhea disease, ear problem and pneumonia.

RESULTS

A total of 340 pregnant women were randomized with mean age of 25 years. The baseline mother and infant characteristics were similar between two treatment groups. Exclusive breastfeeding and higher paternal education level were common factors associated with lower infant morbidity of RIF (adjusted OR = 0.40 and 0.94 with p < 0.01 and p = 0.02, respectively), diarrhea disease (adjusted OR = 0.39 and 0.95 with p < 0.01 and p = 0.04, respectively), and ear problem (adjusted OR = 0.20 and 0.76 with p < 0.01 and p < 0.01, respectively). Maternal influenza vaccine significantly reduced the incidence of RIF (adjusted OR = 0.54; p < 0.01) but not diarrhea disease or ear problem (p > 0.05). Female infants had lower incidence of diarrhea disease (adjusted OR = 0.67; p = 0.01) and ear problem (adjusted OR = 0.12; p = 0.01).

CONCLUSIONS

Maternal influenza vaccination, exclusive breastfeeding, female children, and higher paternal education level significantly reduced the infant morbidity within the 24 weeks after birth in Bangladesh.

Maternal Vaccine Effectiveness Against Influenza-Associated Hospitalizations and Emergency Department Visits in Infants

Importance

Influenza virus infection during pregnancy is associated with severe maternal disease and may be associated with adverse birth outcomes. Inactivated influenza vaccine during pregnancy is safe and effective and can protect young infants, but recent evidence, particularly after the 2009 novel influenza A (H1N1) pandemic, is limited.

Objective

To evaluate the effectiveness of influenza vaccination during pregnancy against laboratory-confirmed influenza-associated hospitalizations and emergency department (ED) visits in infants younger than 6 months.

Design, Setting, and Participants

This was a prospective, test-negative case-control study using data from the New Vaccine Surveillance Network from the 2016 to 2017 through 2019 to 2020 influenza seasons. Infants younger than 6 months with an ED visit or hospitalization for acute respiratory illness were included from 7 pediatric medical institutions in US cities. Control infants with an influenza-negative molecular test were included for comparison. Data were analyzed from June 2022 to September 2023.

Exposure

Maternal influenza vaccination during pregnancy.

Main Outcomes and Measures

We estimated maternal vaccine effectiveness against hospitalizations or ED visits in infants younger than 6 months, those younger than 3 months, and by trimester of vaccination. Maternal vaccination status was determined using immunization information systems, medical records, or self-report. Vaccine effectiveness was estimated by comparing the odds of maternal influenza vaccination 14 days or more before delivery in infants with influenza vs those without.

Results

Of 3764 infants (223 with influenza and 3541 control infants), 2007 (53%) were born to mothers who were vaccinated during pregnancy. Overall vaccine effectiveness in infants was 34% (95% CI, 12 to 50), 39% (95% CI, 12 to 58) against influenza-associated hospitalizations, and 19% (95% CI, -24 to 48) against ED visits. Among infants younger than 3 months, effectiveness was 53% (95% CI, 30 to 68). Effectiveness was 52% (95% CI, 30 to 68) among infants with mothers who were vaccinated during the third trimester and 17% (95% CI, -15 to 40) among those with mothers who were vaccinated during the first or second trimesters.

Conclusions and Relevance

Maternal vaccination was associated with reduced odds of influenza-associated hospitalizations and ED visits in infants younger than 6 months. Effectiveness was greatest among infants younger than 3 months, for those born to mothers vaccinated during the third trimester, and against influenza-associated hospitalizations.

Transplacental transfer efficiency of maternal antibodies against influenza A(H1N1)pdm09 virus and dynamics of naturally acquired antibodies in Chinese children: a longitudinal, paired mother-neonate cohort study

BACKGROUND

The 2009 pandemic H1N1 influenza A virus (A(H1N1)pdm09 virus) evolves rapidly and has continued to cause severe infections in children since its emergence in 2009. We aimed to characterise the kinetics of maternally and naturally acquired antibodies against historical A(H1N1)pdm09 strains and to assess the extent to which the response to heterologous strains following infection or vaccination affects observed A(H1N1)pdm09 strain-specific antibody titres in a Chinese paediatric population.

METHODS

In this retrospective study, we used residual serum samples from 528 mother-neonate pairs from a non-interventional, longitudinal cohort study in southern China conducted from Sept 20, 2013, to Aug 24, 2018, from six local hospitals in Anhua County, Hunan Province, China. Mother-neonate pairs were eligible for inclusion if the neonates were born after Sept 20, 2013, and their mothers had resided in the study sites for at least 3 months. We tested samples with a haemagglutination inhibition (HAI) assay to measure antibody levels against three historical A(H1N1)pdm09 strains that were antigenically similar to the strains that circulated during the 2009 pandemic (A/Hunan-Kaifu/SWL4204/2009 [SWL4204/09 strain], A/Hunan-Daxiang/SWL1277/2016 [SWL1277/16 strain], and A/Hunan-Yanfeng/SWL185/2018 [SWL185/18 strain]). We also determined the seroprevalence, geometric mean titres (GMTs), transfer ratio of maternal antibodies, and the dynamics of maternally and naturally acquired antibodies in children, from birth to 3 years of age.

FINDINGS

1066 mother-neonate pairs were enrolled in the original cohort between Sept 20, 2013, and Oct 14, 2015. Of these, 528 pairs (523 mothers, 528 neonates) were selected for the present study. The median age of the mothers was 25 years (IQR 23 to 29). 291 (55%) of 528 children were boys and 237 (45%) were girls, and most children (452 [86%]) were breastfed before the age of 6 months. The GMTs and the seroprevalence for the SWL4204/09 strain were higher than those for the SWL1277/16 and SWL185/18 strains among mothers (GMTs: 10·4 [95% CI 9·8 to 11·1] vs 9·3 [8·7 to 9·8] vs 8·0 [7·5 to 8·4], p<0·0001; seroprevalence: 11·1% [95% CI 8·5 to 14·1] vs 6·9% [4·9 to 9·4] vs 4·6% [3·0 to 6·8], p=0·0003) and among neonates (GMTs: 10·7 [10·0 to 11·5] vs 9·4 [8·8 to 10·0] vs 8·1 [7·6 to 8·6], p<0·0001; seroprevalence: 13·4% [10·7 to 16·7] vs 8·7% [6·5 to 11·5] vs 6·1% [4·2 to 8·5], p=0·0002). Regardless of the A(H1N1)pdm09-specific strain, maternal antibodies could be transferred efficiently via the placenta (mean transfer ratios: 1·10 for SWL4204/09 vs 1·09 for SWL1277/16 vs 1·06 for SWL185/18; p=0·93). The A(H1N1)pdm09 strain-specific antibodies waned below the protective threshold of 1:40 within 2 months after birth. After maternal antibody waning, there were periodic increases and decreases in HAI antibody titres against three A(H1N1)pdm09 strains, and such increases were all significantly associated with a higher immune response to heterologous strains. Vaccination against the SWL4204/09 strain was associated with a poor response to the SWL185/18 strain (β-0·20, 95% CI -0·28 to -0·13; p<0·0001).

INTERPRETATION

Our findings suggest low pre-existing immunity against influenza A(H1N1)pdm09 virus among unvaccinated Chinese adult female and paediatric populations. This evidence, together with the rapid decay of maternal antibodies and the observed cross-reactivity among different A(H1N1)pdm09 strains, highlights the importance of accelerating maternal and paediatric influenza vaccination in China.

FUNDING

The Key Program of the National Natural Science Foundation of China.

TRANSLATION

For the Chinese translation of the abstract see Supplementary Materials section.

Safety of influenza vaccination during pregnancy: a systematic review

OBJECTIVE

We conducted a systematic review to evaluate associations between influenza vaccination during pregnancy and adverse birth outcomes and maternal non-obstetric serious adverse events (SAEs), taking into consideration confounding and temporal biases.

METHODS

Electronic databases (Ovid MEDLINE ALL, Embase Classic+Embase and the Cochrane Central Register of Controlled Trials) were searched to June 2021 for observational studies assessing associations between influenza vaccination during pregnancy and maternal non-obstetric SAEs and adverse birth outcomes, including preterm birth, spontaneous abortion, stillbirth, small-for-gestational-age birth and congenital anomalies. Studies of live attenuated vaccines, single-arm cohort studies and abstract-only publications were excluded. Records were screened using a liberal accelerated approach initially, followed by a dual independent approach for full-text screening, data extraction and risk of bias assessment. Pairwise meta-analyses were conducted, where two or more studies met methodological criteria for inclusion. The Grading of Recommendations, Assessment, Development and Evaluation approach was used to assess evidence certainty.

RESULTS

Of 9443 records screened, 63 studies were included. Twenty-nine studies (24 cohort and 5 case-control) evaluated seasonal influenza vaccination (trivalent and/or quadrivalent) versus no vaccination and were the focus of our prioritised syntheses; 34 studies of pandemic vaccines (2009 A/H1N1 and others), combinations of pandemic and seasonal vaccines, and seasonal versus seasonal vaccines were also reviewed. Control for confounding and temporal biases was inconsistent across studies, limiting pooling of data. Meta-analyses for preterm birth, spontaneous abortion and small-for-gestational-age birth demonstrated no significant associations with seasonal influenza vaccination. Immortal time bias was observed in a sensitivity analysis of meta-analysing risk-based preterm birth data. In descriptive summaries for stillbirth, congenital anomalies and maternal non-obstetric SAEs, no significant association with increased risk was found in any studies. All evidence was of very low certainty.

CONCLUSIONS

Evidence of very low certainty suggests that seasonal influenza vaccination during pregnancy is not associated with adverse birth outcomes or maternal non-obstetric SAEs. Appropriate control of confounding and temporal biases in future studies would improve the evidence base.

The association between maternal influenza vaccination during pregnancy and adverse birth outcomes in the United States: Pregnancy risk Assessment Monitoring System (PRAMS)

OBJECTIVE

Influenza vaccination during pregnancy is highly recommended. We examined the association between maternal influenza vaccination and adverse birth outcomes.

METHODS

This cross-sectional study used data from the Pregnancy Risk Assessment Monitoring System (PRAMS) during 2012-2017. The primary exposure was the receipt of influenza vaccination during pregnancy. Low birth weight (LBW), preterm birth (PTB), and small for gestational age (SGA) were the primary outcomes. We conducted multivariable logistic regression models to estimate the adjusted odds ratios (AOR) and 95% confidence intervals (CI). Covariates used to adjust confounding included maternal age, marital status, education, race and ethnicity, insurance status before pregnancy, and smoking status. For a subgroup in 2012-2015, we analyzed the association between influenza vaccination in each trimester and adverse birth outcomes.

RESULTS

During 2012-2017, compared with unvaccinated women, women vaccinated during pregnancy had a lower risk of LBW and PTB. During 2012-2015, maternal influenza vaccination in the 1st and 3rd trimesters was associated with a reduced risk of LBW and PTB, and vaccination in the 3rd trimester had a greater protective effect than in the 1st trimester. Influenza vaccination was not associated with SGA regardless of trimester.

CONCLUSIONS

Our findings suggest that influenza vaccination during pregnancy is a safe and effective way to protect newborns.

Dissecting Fc signatures of protection in neonates following maternal influenza vaccination in a placebo-controlled trial

Influenza is an important cause of illness and morbidity for infants. Seasonal influenza vaccination during pregnancy aims to provide protection to mothers, but it can also provide immunity to infants. The precise influence of maternal vaccination on immunity in infants and how vaccine-elicited antibodies provide protection in some but not all infants is incompletely understood. We comprehensively profiled the transfer of functional antibodies and defined humoral factors contributing to immunity against influenza in a clinical trial of maternal influenza vaccination. Influenza-specific antibody subclass levels, Fc ɣ receptor (FCGR) binding levels, and antibody-dependent innate immune functions were all profiled in the mothers during pregnancy and at birth, as well as in cord blood. Vaccination increased influenza-specific antibody levels, antibody binding to FCGR, and specific antibody-dependent innate immune functions in both maternal and cord blood, with FCGR binding most enhanced via vaccination. Influenza-specific FCGR binding levels were lower in cord blood of infants who subsequently developed influenza infection. Collectively these data suggest that in addition to increased antibody amounts, the selective transfer of FCGR-binding antibodies contributes to the protective immune response in infants against influenza.

The burden of influenza among Kenyan pregnant and postpartum women and their infants, 2015–2020

Background

In tropical Africa, data about influenza‐associated illness burden are needed to assess potential benefits of influenza vaccination among pregnant women. We estimated the incidence of influenza among pregnant women and their infants in Siaya County, Kenya.

Methods

We enrolled women at <31 weeks of gestation and conducted weekly follow‐up until 6‐month postpartum to identify acute respiratory illnesses (ARIs). We defined ARI among mothers as reported cough, rhinorrhoea or sore throat and among infants as maternal‐reported cough, difficulty breathing, rhinorrhoea or clinician diagnosis of respiratory illness. We collected nasal/nasopharyngeal and oropharyngeal swabs from mothers/infants with ARI and tested for influenza A and B using molecular assays. We calculated antenatal incidence of laboratory‐confirmed influenza among mothers and postnatal incidence among mothers and infants.

Results

During June 2015 to May 2020, we analysed data from 3,026 pregnant women at a median gestational age of 16 weeks (interquartile range [IQR], 13, 18) and followed 2,550 infants. Incidence of laboratory‐confirmed influenza during pregnancy (10.3 episodes per 1,000 person‐months [95% confidence interval {CI} 8.6–11.8]) was twofold higher than in the postpartum period (4.0 [95% CI 2.6–5.5]; p < 0.01). Incidence was significantly higher among human immunodeficiency virus (HIV)‐infected pregnant women (15.6 [95% CI 11.0–20.6] vs. 9.1 [95% CI 7.5–10.8]; p < 0.01). Incidence among young infants was 4.4 (95% CI 3.0–5.9) and similar among HIV‐exposed and HIV‐unexposed infants.

Conclusion

Our findings suggest a substantial burden of influenza illnesses during pregnancy, with a higher burden among HIV‐infected mothers. Kenyan authorities should consider the value of vaccinating pregnant women, especially if HIV infected.

Serological Responses to Influenza Vaccination during Pregnancy

Pregnant women, newborns, and infants under six months old are at the highest risk of developing severe and even fatal influenza. This risk is compounded by the inability to vaccinate infants under six months, highlighting the importance of vertically transferred immunity. This review identifies novel insights that have emerged from recent studies using animal models of pregnancy and vaccination. We also discuss the knowledge obtained using existing clinical trials that have evaluated influenza-specific serological responses in pregnant women and how these responses may impact early life immunity. We delineate the mechanisms involved in transferring specific maternal antibodies and discuss the consequences for early life immunity. Most importantly, we highlight the need for continued research using pregnant animal models and the inclusion of pregnant women, a commonly neglected population, when evaluating novel vaccine platforms to better serve and treat communicable diseases.

Safety, Immunogenicity, Efficacy and Effectiveness of Inactivated Influenza Vaccines in Healthy Pregnant Women and Children Under 5 Years: An Evidence-Based Clinical Review

Annual influenza vaccination is often recommended for pregnant women and young children to reduce the risk of severe influenza. However, most studies investigating the safety, immunogenicity, and efficacy or effectiveness of influenza vaccines are conducted in healthy adults. In this evidence-based clinical review, we provide an update on the safety profile, immunogenicity, and efficacy/effectiveness of inactivated influenza vaccines (IIVs) in healthy pregnant women and children <5 years old. Six electronic databases were searched until May 27, 2021. We identified 3,731 articles, of which 93 met the eligibility criteria and were included. The IIVs were generally well tolerated in pregnant women and young children, with low frequencies of adverse events following IIV administration; however, continuous vaccine safety monitoring systems are necessary to detect rare adverse events. IIVs generated good antibody responses, and the seroprotection rates after IIVs were moderate to high in pregnant women (range = 65%-96%) and young children (range = 50%-100%), varying between the different influenza types/subtypes and seasons. Studies show vaccine efficacy/effectiveness values of 50%-70% in pregnant women and 20%-90% in young children against lab-confirmed influenza, although the efficacy/effectiveness depended on the study design, host factors, vaccine type, manufacturing practices, and the antigenic match/mismatch between the influenza vaccine strains and the circulating strains. Current evidence suggests that the benefits of IIVs far outweigh the potential risks and that IIVs should be recommended for pregnant women and young children.

Review of Influenza Virus Vaccines: The Qualitative Nature of Immune Responses to Infection and Vaccination Is a Critical Consideration

Influenza viruses have affected the world for over a century, causing multiple pandemics. Throughout the years, many prophylactic vaccines have been developed for influenza; however, these viruses are still a global issue and take many lives. In this paper, we review influenza viruses, associated immunological mechanisms, current influenza vaccine platforms, and influenza infection, in the context of immunocompromised populations. This review focuses on the qualitative nature of immune responses against influenza viruses, with an emphasis on trained immunity and an assessment of the characteristics of the host–pathogen that compromise the effectiveness of immunization. We also highlight innovative immunological concepts that are important considerations for the development of the next generation of vaccines against influenza viruses.

Efficacy and safety of influenza vaccination during pregnancy: realizing the potential of maternal influenza immunization

Introduction: Pregnant women are at higher risk of severe complications following influenza infection compared to the general population. Influenza vaccination during pregnancy can offer direct protection to pregnant women and passive immunity to infants up to 6 months of age via maternal antibodies. Pregnant women are a high priority group for influenza immunization.Areas covered: This review provides an overview of the basis for recommending influenza vaccine to pregnant women, current immunization policies, the evidence supporting the safety and effectiveness of maternal vaccination, and future research needed. We conducted a search of PubMed for articles describing the safety or efficacy of influenza vaccines administered during pregnancy. Published articles from inception to 17 November 2020 were reviewed.Expert opinion: Experimental and observational evidence support the efficacy, effectiveness and safety of influenza immunization during pregnancy. These data support the continued provision of inactivated influenza vaccine to pregnant women, as recommended by global immunization policies. To achieve success with maternal influenza immunization programs, further work is needed to inform policy development in low- and middle-income settings and implementation and promotion in high-income settings.

Transplacental Respiratory Syncytial Virus and Influenza Virus Antibody Transfer in Alaska Native and Seattle Mother-Infant Pairs

BACKGROUND

Alaska Native (AN) infants are at risk for severe disease due to respiratory syncytial virus (RSV) and influenza. Maternal immunization protects young infants through transplacental antibody transfer. RSV- and influenza-specific transplacental antibody transfer in mother-infant pairs has not previously been evaluated in the AN population.

METHODS

Serum samples collected during pregnancy and at birth from AN mother-infant pairs in the Yukon-Kuskokwim Delta region (YKD) of Alaska (2000-2011; n = 75) and predominantly white pairs in Seattle, Washington (2014-2016; n = 57), were tested for RSV and influenza antibody using a microneutralization and hemagglutination inhibition assay, respectively, and compared between sites.

RESULTS

Mean RSV antibody concentrations in pregnant women in YKD and Seattle were similar (log2 RSV antibody 10.6 vs 10.7, P = .86), but cord blood RSV antibody concentrations were significantly lower in infants born to mothers in YKD compared with Seattle (log2 RSV antibody 11.0 vs 12.2, P < .001). Maternal and cord blood influenza antibody concentrations were lower for women and infants in YKD compared with Seattle for all 4 influenza antigens tested (all P < .05). The mean cord to maternal RSV antibody transfer ratio was 1.15 (standard deviation [SD], 0.13) in mother-infant pairs in Seattle compared with 1.04 (SD, 0.08) in YKD. Mean cord blood to maternal antibody transfer ratios for influenza antigens ranged from 1.22 to 1.42 in Seattle and from 1.05 to 1.59 in YKD.

CONCLUSIONS

Though the transplacental antibody transfer ratio was high (>1.0) for both groups, transfer ratios for RSV antibody were significantly lower in AN mother-infant pairs. Further studies are needed to elucidate the impact of lower transplacental antibody transfer on infant disease risk in rural Alaska.Alaska Native and continental US mother-infant pairs have high transplacental antibody transfer ratios (>1.0) for influenza and respiratory syncytial virus, but anti-respiratory syncytial virus antibody levels are significantly lower in Alaska Native pairs than in those from the continental US.

A Retrospective observational cohort study of the effect of antenatal influenza vaccination on birth outcomes in Cape Town, South Africa, 2015-2016

Background

There are conflicting data concerning the impact of antenatal influenza vaccination on birth outcomes including low birthweight (LBW), preterm birth, small for gestational age (SGA), and stillbirth.

Methods

We conducted a retrospective observational cohort study of infants born to women residing in Mitchells Plain, Cape Town. Infants were born at 4 health facilities during May 28 – December 31, 2015 and April 15 – December 31, 2016. We performed crude and multivariable logistic regression, propensity score (PS) matching logistic regression, and inverse probability of treatment weighted (IPTW) regression to assess vaccine effectiveness (VE) against LBW, preterm birth, SGA, and stillbirth adjusting for measured confounders.

Results

Maternal vaccination status, antenatal history, and ≥1 birth outcome(s) were available for 4084/5333 (76.6%) pregnancies, 2109 (51.6%) vaccinated, and 1975 (48.4%) unvaccinated. The proportion LBW was lower in vaccinated (6.9%) vs. unvaccinated (12.5%) in multivariable [VE 0.27 (95% CI 0.07‐0.42)], PS [VE 0.30 (95% CI 0.09‐0.51)], and IPTW [VE 0.24 (95% CI 0.04‐0.45)]. Preterm birth was less frequent in vaccinated (8.6%) than unvaccinated (16.4%) in multivariable [VE 0.26 (0.09‐0.40)], PS [VE 0.25 (95% CI 0.09‐0.41)], and IPTW [VE 0.34 (95% CI 0.18‐0.51)]. The proportion SGA was lower in vaccinated (6.0%) than unvaccinated (8.8%) but not in adjusted models. There were few stillbirths in our study population, 30/4084 (0.7%).

Conclusions

Using multiple analytic approaches, we found that influenza vaccination was associated with lower prevalence of LBW (24‐30%) and preterm birth (25‐34%) in Cape Town during 2015‐2016.

The Protective Role of Maternal Immunization in Early Life

With birth, the newborn is transferred from a quasi-sterile environment to the outside world. At this time, the neonatal immune system is inexperienced and continuously subject to a process of development as it encounters different antigenic stimuli after birth. It is initially characterized by a bias toward T helper 2 phenotype, reduced T helper 1, and cytotoxic responses to microbial stimuli, low levels of memory, and effector T and B cells and a high production of suppressive T regulatory cells. The aim of this setting, during fetal life, is to maintain an anti-inflammatory state and immune-tolerance. Maternal antibodies are transferred during pregnancy through the placenta and, in the first weeks of life of the newborn, they represent a powerful tool for protection. Thus, optimization of vaccination in pregnancy represents an important strategy to reduce the burden of neonatal infections and sepsis. Beneficial effects of maternal immunization are universally recognized, although the optimal timing of vaccination in pregnancy remains to be defined. Interestingly, the dynamic exchange that takes place at the fetal-maternal interface allows the transfer not only of antibodies, but also of maternal antigen presenting cells, probably in order to stimulate the developing fetal immune system in a harmless way. There are still controversial effects related to maternal immunization including the so called "immunology blunting," i.e., a dampened antibody production following infant's vaccination in those infants who received placentally transferred maternal immunity. However, clinical relevance of this phenomenon is still not clear. This review will provide an overview of the evolution of the immune system in early life and discuss the benefits of maternal vaccination. Current maternal vaccination policies and their rationale will be summarized on the road to promising approaches to enhance immunity in the neonate.

The Power of First Impressions: Can Influenza Imprinting during Infancy Inform Vaccine Design?

Influenza virus infection causes severe respiratory illness in people worldwide, disproportionately affecting infants. The immature respiratory tract coupled with the developing immune system, and lack of previous exposure to the virus is thought to synergistically play a role in the increased disease severity in younger age groups. No influenza vaccines are available for those under six months, although maternal influenza immunization is recommended. In children aged six months to two years, vaccine immunogenicity is dampened compared to older children and adults. Unlike older children and adults, the infant immune system has fewer antigen-presenting cells and soluble immune factors. Paradoxically, we know that a person's first infection with the influenza virus during infancy or childhood leads to the establishment of life-long immunity toward that particular virus strain. This is called influenza imprinting. We contend that by understanding the influenza imprinting event in the context of the infant immune system, we will be able to design more effective influenza vaccines for both infants and adults. Working through the lens of imprinting, using infant influenza animal models such as mice and ferrets which have proven useful for infant immunity studies, we will gain a better understanding of imprinting and its implications regarding vaccine design. This review examines literature regarding infant immune and respiratory development, current vaccine strategies, and highlights the importance of research into the imprinting event in infant animal models to develop more effective and protective vaccines for all including young children.

Immunogenicity of influenza vaccines administered to pregnant women in randomized clinical trials in Mali and South Africa

Background

A key consideration for expanding recommendations for influenza vaccination is a robust assessment of immunogenicity and efficiency of transplacental antibody transfer after maternal vaccination.

Methods

We pooled data from two trials of maternal influenza vaccination to analyze vaccine immunogenicity with more power than either trial had alone. We compared hemagglutination-inhibition (HAI) titers and titer factor change for women and their infants between trial arms using t-tests; maternal and infant putative seroprotective titers (HAI ≥ 1:40) within each trial arm and maternal seroconversion between trial arms using exact tests; and transplacental antibody transfer between trial arms using t-tests. We used marginal linear models and generalized estimating equations to examine the impact of time between maternal vaccination and delivery on transplacental antibody transfer, infant titers, and infant seroprotection.

Results

For all vaccine components (A/H1N1, A/H3N2, and Type B), >80% of vaccinated women had seroprotective titers, >60% of them seroconverted, and >50% of their infants were born with seroprotective titers. These immunogenicity outcomes occurred more often in vaccine recipients and their infants than in controls. No difference in efficiency of transplacental antibody transfer was observed between vaccine recipients and controls.

Conclusions

Our results provide robust support for further expansion of maternal influenza vaccination recommendations.

Clinical Trials Registration: NCT01430689 and NCT01306669.

Efficacy, duration of protection, birth outcomes, and infant growth associated with influenza vaccination in pregnancy: a pooled analysis of three randomised controlled trials

Background

Maternal influenza immunisation can reduce morbidity and mortality associated with influenza infection in pregnant women and young infants. We aimed to determine the vaccine efficacy of maternal influenza immunisation against maternal and infant PCR-confirmed influenza, duration of protection, and the effect of gestational age at vaccination on vaccine efficacy, birth outcomes, and infant growth up to 6 months of age.

Methods

We did a pooled analysis of three randomised controlled trials done in Nepal (2011–2014), Mali (2011–2014), and South Africa (2011–2013). Pregnant women, gestational age 17–34 weeks in Nepal, 28 weeks or more in Mali, and 20–36 weeks in South Africa, were enrolled. Women were randomly assigned 1:1 to a study group, in which they received trivalent inactivated influenza vaccine (IIV) in all three trials, or a control group, in which they received saline placebo in Nepal and South Africa or quadrivalent meningococcal conjugate vaccine in Mali. Enrolment at all sites was complete by April 24, 2013. Infants and women were assessed for respiratory illness, and samples from those that met the case definition were tested for influenza by PCR testing. Growth measurements, including length and weight, were obtained at birth at all sites, at 24 weeks in South Africa, and at 6 months in Nepal and Mali. The three trials are registered with ClinicalTrials.gov, numbers NCT01430689, NCT01034254, and NCT02465190.

Findings

10 002 women and 9800 liveborn infants were included. Pooled efficacy of maternal vaccination to prevent infant PCR-confirmed influenza up to 6 months of age was 35% (95% CI 19 to 47). The pooled estimate was 56% (28 to 73) within the first 2 months of life, 39% (11 to 58) between 2 and 4 months, and 19% (–9 to 40) between 4 and 6 months. In women, from enrolment during pregnancy to the end of follow-up at 6 months postpartum, the vaccine was 50% (95% CI 32–63) efficacious against PCR-confirmed influenza. Efficacy was 42% (12 to 61) during pregnancy and 60% (36 to 75) postpartum. In women vaccinated before 29 weeks gestational age, the estimated efficacy was 30% (–2 to 52), and in women vaccinated at or after 29 weeks, efficacy was 71% (50 to 83). Efficacy was similar in infants born to mothers vaccinated before or after 29 weeks gestation (34% [95% CI 12 to 51] vs 35% [11 to 52]). There was no overall association between maternal vaccination and low birthweight, stillbirth, preterm birth, and small for gestational age. At 6 months of age, the intervention and control groups were similar in terms of underweight (weight-for-age), stunted (length-for-age), and wasted (weight-for-length). Median centile change from birth to 6 months of age was similar between the intervention and the control groups for both weight and length.

Interpretation

The assessment of efficacy for women vaccinated before 29 weeks gestational age might have been underpowered, because the point estimate suggests that there might be efficacy despite wide CIs. Estimates of efficacy against PCR-confirmed influenza and safety in terms of adverse birth outcomes should be incorporated into any further consideration of maternal influenza immunisation recommendations.

Funding

Bill & Melinda Gates Foundation.

Safety and immunogenicity of three seasonal inactivated influenza vaccines among pregnant women and antibody persistence in their infants

OBJECTIVE

Inactivated influenza virus vaccines (IIVs) are recommended for all pregnant women in the United States. We conducted a prospective, randomized, double blind study of three licensed seasonal trivalent IIVs (IIV3s) to assess their safety and immunogenicity in pregnant women and determine the level and persistence of passively transferred maternal antibody in infants.

STUDY DESIGN

139 pregnant women ages 18-39 years and 14-33 weeks' gestation, and 44 non-pregnant women, were randomized 1:1:1 to receive a single intramuscular dose of one of three licensed IIV3s (Agriflu®, Fluzone®, or Fluarix®) prior to the 2010-2011 influenza season. Reactogenicity, adverse events (AEs) and pregnancy outcomes were documented. Serum samples for hemagglutination inhibition (HAI) and neutralization antibody assays were collected prior to and 28 and 180 days after immunization. Maternal sera and cord blood were collected at the time of delivery and sera were obtained from 44 infants at 6 weeks of age.

RESULTS

Pregnant and non-pregnant women experienced similar frequency of injection site (92% and 86%, respectively) and systemic (95% and 87%, respectively) reactions, the majority of which were mild. There were no vaccine-associated maternal or infant serious AEs. Antibody responses to the three vaccine antigens were not different between pregnant and non-pregnant women. The ratios of cord blood (infant) to maternal HAI antibody titers at delivery ranged between 1.1 and 1.7 for each of the vaccine antigens. Influenza antibody concentrations in infants were 70-40% of the birth titer by 6 weeks of age.

CONCLUSIONS

The three IIV3s were well tolerated in pregnant women. Antibody responses were comparable in pregnant and non-pregnant women, and after second or third trimester vaccination. Transplacental transfer of maternal antibodies to the infant was efficient. However, antibody titers decline rapidly in the first 6 weeks of life.

Global Perspectives on Immunization During Pregnancy and Priorities for Future Research and Development: An International Consensus Statement

Immunization during pregnancy has been recommended in an increasing number of countries. The aim of this strategy is to protect pregnant women and infants from severe infectious disease, morbidity and mortality and is currently limited to tetanus, inactivated influenza, and pertussis-containing vaccines. There have been recent advancements in the development of vaccines designed primarily for use in pregnant women (respiratory syncytial virus and group B Streptococcus vaccines). Although there is increasing evidence to support vaccination in pregnancy, important gaps in knowledge still exist and need to be addressed by future studies. This collaborative consensus paper provides a review of the current literature on immunization during pregnancy and highlights the gaps in knowledge and a consensus of priorities for future research initiatives, in order to optimize protection for both the mother and the infant.

Are all vaccines safe for the pregnant traveller? A systematic review and meta-analysis

Pregnant travellers and their offspring are vulnerable to severe outcomes following a wide range of infections. Vaccine-preventable diseases can have a particularly severe course in pregnant women, but little is known about the safety of travel vaccines in pregnant women. We performed a systematic review of all published literature concerning the safety of vaccines frequently given to travellers such as yellow fever, MMR (mumps, measles and rubella), influenza, Tdap (tetanus, diphtheria and pertussis), meningococcus, hepatitis A and B, rabies, polio, typhoid fever, tick-borne encephalitis and Japanese encephalitis vaccines. We included case series, cohort studies and randomized controlled trials (RCTs). For the meta-analysis, we included only RCTs that compared the administration of a vaccine to placebo or to no vaccine. Outcome measures included severe systemic adverse events, maternal outcomes related to the course of pregnancy, neonatal outcomes and local adverse events. We calculated the risk ratio and its 95% confidence interval as the summary measure. The safety of influenza vaccine is supported by high-quality evidence. For Tdap vaccine, no evidence of any harm was found in the meta-analysis of RCTs. A slight increase in chorioamnionitis rate was reported in 3 out of 12 observational studies. However, this small possible risk is far outweighed by a much larger benefit in terms of infant morbidity and mortality. Meningococcal vaccines are probably safe during pregnancy, as supported by RCTs comparing meningococcal vaccines to other vaccines. Data from observational studies support the safety of hepatitis A, hepatitis B and rabies vaccines, as well as that of the live attenuated yellow fever vaccine. We found little or no data about the safety of polio, typhoid, Japanese encephalitis, tick-borne encephalitis and MMR vaccines during pregnancy.

Immunobiological aspects of vaccines in pregnancy: Maternal perspective

Immunization during pregnancy is an efficient strategy to protect both the mother and the newborn infant against infectious pathogens. Pregnant women have an increased susceptibility to severe infections caused by some pathogens, but the mechanisms involved remain poorly understood. Pregnancy is associated with dynamic changes in maternal immune system that are critical for tolerance of the fetus. These changes could also play an important role in shaping maternal immune components that are transferred to the newborn infant following natural infection or vaccination to prevent infectious diseases in early life. As the momentum for maternal immunization is growing, there is a need to increase our understanding of the immunobiology of maternal immunization in order to better prevent infectious diseases in the pregnant women and the young infant.

Vaccinating pregnant women against influenza needs to be a priority for all countries: An expert commentary

BACKGROUND

In 2012, the World Health Organization recommended influenza vaccination for all pregnant women worldwide and the prioritisation of pregnant women in national influenza vaccination programmes. Nevertheless, vaccination rates in pregnant women often remain much lower than national targets.

OBJECTIVES

To assess the benefits and risks associated with influenza infection and vaccination during pregnancy, and to consider obstacles that work against influenza vaccine uptake during pregnancy.

RESULTS

There is strong evidence that maternal and foetal outcomes can be compromised if women develop influenza infections during pregnancy. Influenza vaccines have been administered to millions of pregnant women and have demonstrated benefits in terms of disease prevention in mothers and their infants. There is a consensus amongst several recommending authorities that influenza vaccines may be safely administered during all stages of pregnancy. Healthcare professionals are recognised as the most important influencers of vaccine uptake, being well placed to recommend vaccination and directly address safety concerns.

CONCLUSIONS

Despite data supporting the value of influenza vaccination during pregnancy, vaccine uptake remains low globally. Low uptake appears to be largely due to ineffective communication with pregnant women about the risks and benefits of influenza vaccination. A graphical abstract is available online.

Sepsis: Precision-Based Medicine for Pregnancy and the Puerperium

Sepsis contributes significantly to global morbidity and mortality, particularly in vulnerable populations. Pregnant and recently pregnant women are particularly prone to rapid progression to sepsis and septic shock, with 11% of maternal deaths worldwide being attributed to sepsis. The impact on the neonate is considerable, with 1 million neonatal deaths annually attributed to maternal infection or sepsis. Pregnancy specific physiological and immunological adaptations are likely to contribute to a greater impact of infection, but current approaches to the management of sepsis are based on those developed for the non-pregnant population. Pregnancy-specific strategies are required to optimise recognition and management of these patients. We review current knowledge of the physiology and immunology of pregnancy and propose areas of research, which may advance the development of pregnancy-specific diagnostic and therapeutic approaches to optimise the care of pregnant women and their babies.

Pregnancy-related immune suppression leads to altered influenza vaccine recall responses

Pregnancy is a risk factor for severe influenza infection. Despite achieving seroprotective antibody titres post immunisation fewer pregnant women experience a reduction in influenza-like illness compared to non-pregnant cohorts. This may be due to the effects that immune-modulation in pregnancy has on vaccine efficacy leading to a less favourable immunologic response. To understand this, we investigated the antigen-specific cellular responses and leukocyte phenotype in pregnant and non-pregnant women who achieved seroprotection post immunisation. We show that pregnancy is associated with better antigen-specific inflammatory (IFN-γ) responses and an expansion of central memory T cells (Tcm) post immunisation, but low-level pregnancy-related immune regulation (HLA-G, PIBF) and associated reduced B-cell antibody maintenance (TGF-β) suggest poor immunologic responses compared to the non-pregnant. Thus far, studies of influenza vaccine immunogenicity have focused on the induction of antibodies but understanding additional vaccine-related cellular responses is needed to fully appreciate how pregnancy impacts on vaccine effectiveness.

Why Should We Advocate Maternal Immunization?

Maternal vaccination provides a method for protecting the pregnant woman, fetus and neonate during a period when there is increased susceptibility to infectious diseases. A dynamic state of immune tolerance during pregnancy and the need to develop adaptive memory to a new foreign antigen-rich environment lead to windows of vulnerability to infection for the mother and neonate, respectively. Passive transfer of humoral immunity through the placenta and breast milk from the mother can bridge the gap in immunity for the neonate. Studies on boosting this natural process of antibody transfer have led to the recommendation for administering inactivated influenza, diphtheria, tetanus toxoid and acellular pertussis vaccines during pregnancy. Several new maternal vaccine candidates are on the horizon.

Burden, effectiveness and safety of influenza vaccines in elderly, paediatric and pregnant populations

Vaccination is the most practical means available for preventing influenza. Influenza vaccines require frequent updates to keep pace with antigenic drift of the virus, and the effectiveness, and sometimes the safety, of the vaccine can therefore vary from season to season. Three key populations that the World Health Organization recommends should be prioritized for influenza vaccination are pregnant women, children younger than 5 years of age and the elderly. This review discusses the burden of influenza and the safety and effectiveness profile of influenza vaccines recommended for these groups.