The Efficacy of Immunoadjuvant-Containing Influenza Vaccines in Pregnancy

The aim of the work was to determine the clinical safety and immunogenicity of immunoadjuvant vaccines against influenza (MonoGripol Plus and Grippol® Plus) in 182 pregnant women in the II and III trimesters of gestation , and further assessment of fetal conditions and infants of the first 6 months of life. Results: It was shown that immunoadjuvant vaccines do not have a negative effect on the physiological course of pregnancy and the functional state of the fetoplacental complex. In the early postpartum period, the rates of physical and neuro-psychological development and the nature of feeding of children did not differ from the control group. In pregnant women vaccinated with Grippol® plus, the levels of seroprotection to strains of A/H1N1/v are 82.0%, A/H3N2/—88.0%, B—88.3% that measure the CPMP criteria and last more than a year . After birth, transplacental antibodies in children in protective values were observed in 52.3–68.9% of cases, did not differ from the control group, and disappear after 6 months. Respiratory infections during the first 6 months of life of infants born from mothers vaccinated against influenza registered in 1.8 times less frequently.


Introduction
The reports of Strategic Advisory Group of Experts (SAGE) on Immunization underscore that between 7 and 10% of all hospitalized patients with severe influenza are women in the second and third trimester of pregnancy. The requirements of pregnant women with influenza infection for providing medical care in an intensive care unit are 10 times that of other population groups diagnosed with influenza [1][2][3][4].
Vaccination of pregnant women using the subunit and split influenza vaccines is routinely performed in a number of countries of Europe and America for over 20 years, and the vaccine efficacy reaches 70-85% [5,6]. Clinical studies have shown that vaccination of pregnant women using modern inactivated influenza vaccines neither affect the course of pregnancy and fetal growth nor cause undesirable post-vaccination effects. It was found that vaccination of pregnant women using inactivated influenza vaccines leads to 50-63% reduction of flurelated morbidity among infants up to 6 months of age [7,8].
The WHO Global Advisory Committee on Vaccine Safety indicates that influenza vaccination is a non-alternative approach to safe and effective prevention of influenza in pregnancy [1,9,10]. In Russia, the indications for vaccination of pregnant women using modern vaccines are defined within the National Immunization Program Schedule of Russian Federation (RF) (order of Ministry of Healthcare of Russian Federation №125n of 21 March 2014). Federal clinical guidelines "Influenza vaccination of pregnant women" and manuals for physicians have been published which establish the main vaccination requirements for the primary health care in Russia [11][12][13].
The unfavorable epidemiological situation with influenza that occurred in 2009 has accelerated the development and implementation in healthcare practices of adjuvant-containing pandemic influenza vaccines such as Fluad (containing squalene) and Arepanrix™H1N1 (containing AS03-squalene + α-Tocopherol acetate) which confer enhanced immunogenicity [1,14]. Adjuvants accelerate, change the dynamics of development of the immunity, and increase its level and the duration of persistence of post-vaccination antibodies. With the help of an adjuvant, durable and solid immunity is achieved by administering small doses of antigen and a less number of injections.
In Russia, two adjuvant-containing subunit influenza vaccines have been developed (monovalent (pandemic) and trivalent preparations). These drugs, in contrast to non-adjuvant subunit vaccines against influenza (e.g., Agrippal S1 containing 15 μg strains of influenza viruses type A and B), have 5 μg of both strains of the influenza virus and an adjuvantimmunomodulator polyoxidonium. In clinical trials, immunoadjuvant vaccines demonstrated high efficacy and safety in children aged 6 months and older and in adults. The trivalent adjuvant-based influenza vaccine is used in clinical practice for more than 20 years [15][16][17][18][19][20][21][22][23][24]. In experimental studies, these vaccines showed no teratogenic effect on the developing fetus. Despite extensive use of these vaccines for specific prevention of influenza in Russia, studies on their safety in pregnancy have not been conducted until recently. The information on the effects of adjuvant-containing vaccines on the fetus and post-natal development was missing. The information on vaccine immunogenicity for pregnant women at different gestational age, as well as vaccine ability to confer an adequate passive immunity to a fetus, was insufficient.
The study aimed at determining clinical safety and immunogenicity of "MonoGrippol Plus" and "Grippol ® Plus" vaccines in pregnant women in the second and third trimesters of pregnancy with assessment of fetal condition and condition of infants during the first 6 months of life.

Legal basis of the research
The study was carried out according to the protocol which met the National standard of Russian Federation-GOST P 52379-2005 "Good Clinical Practice" and the international GCP (good clinical practice) standards. Vaccination of pregnant women was carried out with adherence to the ethical norms and guidelines of the WHO and Ministry of Health care of RF.
Women to be vaccinated and followed up were selected strictly in accordance with a case report form that was examined and approved by the Ethics Committee of the Ulyanovsk State University (protocol №35 of 14.01.2010).
The observation of pregnant women before and after vaccination was carried out jointly with an obstetrician-gynecologist in accordance with requirements of the Order of Ministry of Health care and Social Development of RF of 02.10.2009 N 808n "On the approval of the Order of providing obstetric and gynecologic care." Before vaccination, women underwent laboratory testing after they have given the informed consent to participate in the study (Figure 1).
During observation and examination of infants, we also adhered to the ethical requirements applicable to biomedical studies. Development of the order and scope of studied parameters was based on provisions listed in the Order № 370 of Ministry of Healthcare of RF of 28.04.2007.

Randomization
The study was a randomized, placebo-controlled, single-blind, comparative, parallel-group study conducted on pregnant women and infants.
All candidates for study program underwent a preliminary assessment of whether they met the protocol inclusion and exclusion criteria (in accordance with the GMP standards).

2.
Volunteers capable of fulfilling the protocol requirements (i.e., able to fill in the self-observation diary and turn up for the scheduled visits).

3.
Written informed consent of the volunteers to participate in the clinical study.
Exclusion criteria: 1. History of leukemia, oncologic conditions or positive tests for HIV, hepatitis B and C.

2.
Volunteers who had received the immunoglobulin preparations or blood transfusions within the last three months prior to the study.

3.
Long-term (more than 14 days) administration of immunodepressants or other immunomodulating drugs within the last six months prior to the study.

4.
Any confirmed or suspected immunosuppressive or immunodeficiency disorder.

5.
History of chronic alcohol abuse and/or substance abuse.

6.
Presence of respiratory or cardiovascular insufficiency, hepatic or renal impairment revealed during physical examination or by laboratory tests at visit 1.

7.
Severe congenital defects or serious chronic diseases including any clinically significant diseases of lungs, kidneys, cardiovascular system, nervous system, psychiatric diseases or metabolic disorders confirmed by anamnestic data or objective clinical examination.

8.
Presence of acute infectious and/or non-infectious diseases at the time of enrollment in the study.

Duration of observation
A total number of pregnant women vaccinated against influenza during the epidemic seasons of 2009-2010, 2010-2011, and 2011-2012 were 345 subjects. Of those, the number of women and their children participated in an in-depth examination and their assignment to groups and subgroups is presented in Table 1.
The frequency of clinical examination and blood collection for laboratory testing in the post-vaccination period was based on the gestational age at the start of observation. Women vaccinated in the second and third trimesters underwent 7 and 6 examinations, respectively.

Assessment of fetal conditions
Fetometric measurements were carried out using the ultrasound (US) examination during pregnancy weeks 21-22 and 33-35 and included determination and calculation of biparietal diameter (BPD), fronto-occipital size (FOS), head circumference (HC), abdominal circumference (AC), estimated fetal weight (FW) and the femur length/abdominal circumference (FL/AC) ratio. The generally accepted guidelines were followed to evaluate the parameters obtained.

Assessment of infant conditions
Infant observation started from the first hours and days of life (day 2-3) and was conducted jointly with a neonatologist at maternity home. The basic signs of functional and morphological maturity of the newborn (Apgar score), blood work parameters/biochemical profile and antibody levels to influenza virus strains have been analyzed. All newborns at maternity home underwent neurosonography and cardiac sonography. Basic anthropometric measurements included body weight (BW), body length (BL), head circumference (HC), chest circumference (CC), and height-weight index (Ketle 1).
At the age of 3 and 6 months, the main parameters of physical and neuropsychological development and feeding pattern have been recorded.

Vaccines
All vaccines used in the study were subunit inactivated preparations. Development of "MonoGrippol Plus" and "Grippol ® Plus" vaccines (LLC "NPO Petrovax Pharm," Russia) is based on a special technology of coupling of highly purified protective influenza virus antigens with a polymeric, water-soluble, high-molecular weight adjuvant polyoxidonium. This technology enables a threefold reduction of hemagglutinin (HA) of each viral strain (down to 5 μg) in the vaccine compared to the analog subunit, adjuvant-free vaccine "Agrippal S1" ("Novartis Vaccines and Diagnostics," Italy).

Placebo
As a placebo, we used phosphate buffer saline ("GlaxoSmithKline Biologicals") which is used as a diluent for lyophilized vaccines.

Vaccination
Vaccination of pregnant women was performed in the vaccination room with adherence to sanitary and hygiene regulations, with emergency care available at once. Vaccine preparations were injected intramuscularly as a single-dose of 0.5 mL in the upper third of the arm (deltoid muscle).

Evaluation of vaccination safety
After an injection, the woman was observed for 40 min for the adverse reaction(s), which were scored to categorize reactions as described in Table 2. All possible changes in the well-being and health state were recorded in the case report form (CRF) and self-observation diary (SOD) which the women continued to fill in on a daily basis throughout the first month of the follow-up.

Statistical analysis
Statistical analysis of samples which did not follow a normally distributed pattern was carried out using the non-parametric tools or parametric methods when the samples followed a normal distribution. We used the applied software package "Microsoft Excel" with the "AtteStat" application (version 10.10.2.). The differences were considered significant at p < 0.05.

Particulars of the course of pregnancy in women vaccinated with "Grippol ® Plus"
Although many pregnant women had a history of somatic diseases before they entered the study, as a rule, no exacerbation of pre-existing disease has been observed. The most commonly encountered illness was a mild hypochromic anemia (60.5, 70.0, 61.3, and 60.9% of pregnant women in group I-IV, respectively). Markers of chronic urogenital infection were detected fairly often (44.2% in group I, 46.0% in group II, 59.1% in group III, and 41.5% in group IV). Also, the syndrome of vegetative dystonia (20.9% in group I, 38.0% in group II, 20.5% in group III, and 29.3% in group IV) and altered allergic response (16.3% in group I, 20.0% in group II, 18.2% in group III, and 19.5% in group IV) had been observed. All women had approximately the same frequency of cases of threatened miscarriage in the past (53.5% in group I, 48.0% in group II, 47.7% in group III, and 51.2% in group IV). Therefore, clinical condition of pregnant women was comparable among the groups [25].

Clinical course of the post-vaccination period
Evaluation of the clinical course of the post-vaccination period has shown that it was asymptomatic in 58.1% of women from group I, 60.0% from group II, 54.5% from group III, and 60.9% from group IV (p > 0.05). It came under notice that in groups I, II, and III, women vaccinated in the third trimester of pregnancy developed the post-vaccination local and systemic undesirable effects significantly less often than women vaccinated in the second trimester (p < 0.05 to p < 0.01). The local symptoms occurred in the first few days after vaccination included pain, hyperemia, and infiltration at the site of injection. Such reactions occurred more often in pregnant women immunized with trivalent vaccines (group II-8.0%, group III-10.4%) than in women from placebo group (4.9%), (p < 0.05). It was noted that pregnant women from group I developed no or minimal systemic adverse reactions (nausea, fatigability, dizziness or myalgia) where intensity was significantly lower compared to that in women vaccinated with trivalent vaccine, namely group I-6.9% (p < 0.05 versus group II), group II-12.0%, group III-10.4%, and group IV-10.2% [26,27].

Clinical blood analysis
In the late (8-30 days) post-vaccination period, no local post-injection reactions have been reported in either group. Systemic reactions included frequent complaints on increased fatigability and headaches in women from group III (p < 0.05). With regard to other symptoms, the groups did not differ significantly between each other and the placebo group (group I-9.3%, group II-14.0%, group III-12.5%, and group IV-12.2%). All symptoms were of a transient nature and required no medication management [26,27].
Analysis of complete blood count has shown that for the majority of formed elements, cell counts did not differ from normal values in both pre-vaccination and post-vaccination periods.
Occasional differences were mostly related to the particulars of the pregnancy period. Analysis of basic metabolic panel performed in dynamics on day 7 and day 30 of the post-vaccination period in each group also did not reveal significant abnormalities which could reflect changes in the metabolic homeostasis (p > 0.05). Small changes in creatinine level (minimal value in group III at day 30 post-vaccination-58.04 ± 1.57 μmol/L) and alkaline phosphatase (AP) (maximum value in group III at day 30 post-vaccination-86.23 ± 7.84 IU/L) are not remarkable and fall within the average normal values, attesting to normal variability of this parameter [28,29].

Lipid metabolism
Analysis of lipid panel obtained 30 days post-vaccination has shown that in all groups, the parameters of lipid/cholesterol metabolism are not significantly altered, and remain within physiological variations [12,13].

Hormonal profile
Analysis of hormonal profile among vaccinated women has revealed only the intra-group changes in hormone levels which are not so much related to vaccination but rather are due to the gestational age.
Significant differences in prolactin, progesterone, estradiol, and cortisol serum levels were observed in women of different gestational age regardless of whether they received monovalent or trivalent influenza vaccine or placebo (p < 0.05). Therefore, it can be affirmed that, despite certain differences in the composition of influenza vaccines used in the study, there are no hormonal changes which could have influenced the state of the fetoplacental unit [12,13].

Humoral immunity
Serum levels of immunoglobulins measured immediately after vaccination and on day 7 post-vaccination were comparable in pregnant women immunized with different influenza vaccines. At day 30, post-vaccination pregnant women who had received the monovalent influenza vaccine demonstrated higher IgA levels (2.56 ± 0.27 mg/mL) compared to women vaccinated with trivalent preparations (1.61 ± 0.09 mg/mL in group II, 1.34 ± 0.11 mg/mL in group III, and 1.14 ± 0.14 mg/mL in group IV) (p < 0.01 to p < 0.001). Despite the above differences, the antibody levels reflect normal serum IgA variations. Despite the established difference, the IgA content in all comparison groups was recorded within the physiological norm. Levels of IgM and IgG antibodies did not differ significantly between the groups.
Some variations in the IgG subclasses (1,2,3,4) in the early and late post-vaccination periods were found. However, these variations remained within the acceptable range. In pregnant women with a history of allergic diseases, vaccination against influenza had no subsequent effect on serum total IgE levels.

Cytokine profile
It was noted that all pregnant women vaccinated with different vaccine preparations had elevated levels of mitogen-stimulated IL-1α at day 7 post-vaccination. By day 30, concentration of IL-1α remained elevated as compared to placebo control (p < 0.05) although was significantly lower than in vaccinated non-pregnant women (p < 0.01). No changes in the IL-2 and TNFα levels have been observed in vaccinated pregnant women although were also significantly higher than in non-pregnant women (p < 0.01). The IL-1RA values in a spontaneous cytokine production assay were significantly elevated only after vaccination with trivalent preparations by day 7 (group II) and by day 30 (groups II and III) post-vaccination (p < 0.01). At the same time, following mitogen stimulation, no significant changes in the IL-2 concentration have been found in any group. All pregnant women demonstrated significant increase in the IL-IRA and IL-10 following mitogen stimulation regardless of the type of vaccine that reflected the mechanism of physiological control of immune activation.
The IL-4 levels were most stable, with no significant dynamic changes among the groups. The only exception was a subgroup of women immunized with a non-adjuvanted trivalent vaccine in different trimesters of pregnancy. It was noted that by day 7 post-vaccination, a higher level of stimulated IL-4 was found in vaccinated women in the third trimester of pregnancy (6.85 ± 0.11 pg/mL in group III) as compared to pregnant women who had received the adjuvant-containing vaccine during the same period (2.95 ± 0.09 pg/mL in group II) (p < 0.05). Subsequently (on day 30 post-vaccination), such differences between the groups could not be found.
Pregnant women had lower IFNγ levels in the mitogen-stimulated cytokine production assay (881.86 ± 92.93 pg/mL in group I, 784.17 ± 65.03 pg/mL in group II, 854.89 ± 68.71 pg/mL in group III, and 790.30 ± 45.55 pg/mL in group IV) than the non-pregnant women (1419.60 ± 69.45 pg/mL in group V) which reflected a natural background level of physiological immune response (p < 0.05). At the same time during the first 7 days, post-vaccination elevated IFNγ was detected only in pregnant women who had received the polymer-subunit vaccines (6.47 ± 1.68 pg/mL in group I and 5.89 ± 1.08 pg/mL in group II) as compared to group III (3.03 ± 0.39 pg/mL) (p < 0.05). These differences were short-lived, and by day 30, post-vaccination was undetectable [30].
Therefore, the overall picture of cytokine profile in pregnant women had a trend characteristic of physiologic immunosuppression in pregnancy, that is, moderately elevated IL-IRA and IL-10 and the absence in the post-vaccination period of significantly elevated anti-inflammatory cytokines in the mitogen-stimulation cytokine production assay. Nonetheless, the adjuvanted subunit vaccines had certain differences in their ability to influence cytokine secretion and short-term elevation of IFNγ which is most prominent in women in the second trimester of pregnancy that may reflect active involvement of the Th 1 -mediated mechanisms of post-vaccination immunity. Use of non-adjuvanted vaccines leads to immune processes in the early post-vaccination period which are accompanied by increased IL-4 synthesis by blood leukocytes (a sign of Th 2 -mediated activation) especially in women vaccinated in late pregnancy. The indirect evidence in favor of this suggestion is the absence of significance changes in the IFNγ levels in the early and late post-vaccination periods. All found changes of parameters recorded in different groups of vaccinated women remained within an acceptable range of variation. Also, no changes pertaining to destabilization of regulation and functioning of immune system due to influenza vaccination of pregnant women have been found [31].

Fetoplacental complex
Monitoring of fetal development was carried out using a complex of measures, which included analysis of markers of fetoplacental complex and ultrasound fetometry. In all groups of women in the early and late post-vaccination period, no changes in the basic parameters of embryo/fetogenesis (AFP, hCG, TBG) have been found ( Table 3). Changes in the above parameters did not depend on the type of influenza vaccine used and corresponded to the gestational age (second and third trimesters of pregnancy  groups (including placebo control) of women vaccinated in the second trimester of pregnancy was significantly lower than in the third trimester (p < 0.05 to p < 0.01). A direct relationship was found between the TBG and AFP concentrations (r = 0.60; p < 0.05) with TBG levels rising as pregnancy progresses (p < 0.001). The hCG levels were dropping during the follow-up and inversely correlated with the TBG levels (r= -0.50; p < 0.01). All serum markers had no deviations from the reference values and reflected physiological changes in pregnancy [32].

Fetometry
Fetometry performed in the second (21-22 weeks) and third (33-35 `) trimester of pregnancy failed to reveal differences among the groups of pregnant women ( Table 4).
Therefore, study results indicate that vaccination of pregnant women using the adjuvantcontaining influenza vaccines "MonoGrippol Plus" and "Grippol ® Plus" has no effect on the intrauterine fetal development. Changes of the basic parameters of fetoplacental unit are comparable between the groups and reflect physiological changes during fetal growth.

Pregnancy outcomes in women vaccinated with "MonoGrippol Plus" and "Grippol ® Plus" influenza vaccines
In the majority of cases (85.4-90.7%), pregnancy resulted in physiologic birth ( Table 5). In a fraction of women, their pregnancy terminated prematurely with the birth of preterm babies (between 2.0 and 8.3%) which corresponds to the preterm birth rate in the Ulyanovsk region of Russia (3.7-5.8%) where the study was taking place. Such outcome was due to the obstetric pathology which was unrelated to prior vaccination. Also, cases of birth of babies with perinatally acquired neurological impairment were mostly associated with gestational immaturity (7.3-10.4%). A fraction of babies had the intrauterine infection-like syndrome (2.0-6.3%) and developmental abnormalities and defects in 2.0-4.9% of cases (3.8-5.9% across the Ulyanovsk region) [33]. Owing to the above abnormalities, such babies were excluded from further study.

Apgar scale
The early neonatal period of infants born to mothers vaccinated during pregnancy had a comparable dynamics between the groups. It was shown that, immediately after birth, the number of babies with Apgar score of 8-9 points was similar between the groups (group I-92.1%, group II-87.5%, group III-80.9%, and group IV-94.3%) which attests to the overall good functional maturity. The period of adaptation in newborns passed without complications [34,35].

Feeding
The feeding of infants born to mothers vaccinated with different influenza vaccines did not differ significantly between the groups. The highest number of nursing mothers (100%) during the neonatal period was observed in groups I and IV and was somewhat lower in groups Therefore, vaccination of women with subunit adjuvanted influenza vaccines during pregnancy has no further impact on lactation and duration of breastfeeding.

Body weight and length
Parameters of physical development of infants of the first 6 months of life from different groups were generally comparable. Body weight and body length at different time points were within the percentile rank (25-50-75 Note: p > 0.05 for differences between groups. Therefore, our results attest to the sufficiency of basic criteria for infant development and reflect the population maturity in terms of their physical development regardless of vaccination of their mothers during pregnancy with different influenza subunit vaccines.

Neuropsychological development
Parameters of neuropsychological development (NPD) of children born to vaccinated mothers did not differ significantly from those of the placebo group. Overall, no changes of NPD have been observed in 81.6% of group I infants in their first 6 months of life who had been born to mothers vaccinated during pregnancy with a monovalent influenza vaccine. In other clinical groups, this parameter was 83.3% (group II), 78.6% (group III), and 77.1% (group IV) (p > 0.05).
Within the structure of occasional NPD disorders, there were conditions which number did not exceed the average statistical rate of neurological pathology in a given pediatric age group.
It was noted that infants born to women vaccinated during pregnancy with trivalent influenza vaccines were 1.8-times less likely to develop non-influenza respiratory infections within the first 6 months of life as compared to infants from placebo control group (Figure 2).

Immunogenicity of adjuvanted influenza vaccine "Grippol ® Plus" in pregnant women vaccinated during different trimesters of pregnancy
In this study, the level of post-vaccination antibodies to influenza virus was evaluated only in a group of pregnant women and non-pregnant women vaccinated with a trivalent adjuvanted influenza vaccine with the aim of revealing the features of the effect of pregnancy on the synthesis of antibodies. Since it has been already proven that the introduction of subunit unadjuvanted vaccines in pregnant women is accompanied by the formation of antibodies to the influenza virus in values not differing from those in non-pregnant ones, it seemed to us interesting to investigate the interaction of the immunoadjuvant preparation with the transiently altered immune status of the pregnant woman [36]. It was found that a part of women before immunization had a seroprotective (≥1:40) baseline antibody level to vaccine strains of influenza virus ( Table 6). In all examined women, the antibody titer was higher to influenza virus B (22.2% of women in the second trimester, 26.1% in the third trimester and 25.7% in non-pregnant women). This is probably due to the duration of its circulation in the population and the formation of natural immunity. It should be noted that none of the participants in the study was vaccinated before and did not confirm an acute illness caused by the influenza virus. One month after vaccination, women of all groups demonstrated a significant rise in antibody titer that fully met one of the CPMP criteria. In the post-vaccination period, the antiviral antibody titer gradually declined reaching a significant difference against baseline by 3 months postpartum in women vaccinated in the second and third trimesters. The observed difference referred only to viral strain A/H1N1/v (p < 0.05). It is possible that the loss of antibodies to a pandemic strain is associated with the peculiarities of the formation of immunity after its first administration. Other authors have shown that specific antibodies to this strain in the post-vaccination period can be synthesized at lower values and therefore be accompanied by their faster loss. At 6 and 12 months post-vaccination, there was a marked regression of seroprotection level with regard to antibody titer against strains A/H1N1/v, A/H3N2/and B in women vaccinated during pregnancy in the second and third trimesters (p < 0.01). Such trend was also traced in a group of non-pregnant women; however, the changes were less remarkable, with a fairly significant fraction of subjects having a high level of protective antibodies. Similar dynamics of post-vaccination antibodies were noted in pregnant women vaccinated with subunit non-adjuvanted influenza vaccine [37].
The rate of development and intensity of protective immunity include the level and factor of seroconversion across all influenza virus strains. Those were compared between the groups, and it was found that their values met the CPMP criteria ( Table 7). The majority of data obtained did not differ between the groups. One exception was the seroconversion factor for strain B in pregnant women vaccinated in the third trimester of pregnancy (5.1) when it was compared with the matching parameter in the non-pregnant women group (6.9) (p < 0.05).
The dynamics of influenza antibody (AB) titers based on the geometric mean titer (GMT) reflect the decline of antibody level with time in the post-vaccination period ( Table 8). It was noted that at one month post-vaccination, the value of GMT AB to A/H1N1/v strain in women vaccinated in the second trimester of pregnancy was significantly lower (49.12 ± 0.29) compared to subjects vaccinated in the third trimester of pregnancy (60.99 ± 0.25) (p < 0.05). During all subsequent periods, this parameter showed no differences with regard to the trimester of pregnancy.
Pregnant women vaccinated in the third trimester of pregnancy showed at 3 months postvaccination and throughout the follow-up period lower GMT AB titers to strain B compared to the non-pregnant women (p < 0.05 to p < 0.01). Similar trend was traced in women of the same group with regard to all influenza virus strains at 6 months postpartum compared to the non-pregnant women (p < 0.05 to p < 0.01).
Therefore, the post-vaccination immune response in women vaccinated with a trivalent adjuvanted influenza vaccine at different times of pregnancy, during the first month, did not differ  *: p < 0.01-the intra-group difference for the second/third trimesters of pregnancy; non-pregnant in-between pre-vaccination and 1 month post-vaccination. ∇: p < 0.05; ∇∇: p < 0.01-the intra-group difference for the second/third trimesters of pregnancy; non-pregnant in-between 1 month and 12 (9) months post-vaccination.
×: p < 0.05; ××: p < 0.01-difference between the second/third trimesters of pregnancy group and non-pregnant group. Table 6. Seroprotection level in pregnant women vaccinated with "Grippol ® Plus," allowing for trimester of pregnancy. Note: *p < 0.05 for difference between the third trimester group and non-pregnant group. Table 7. Seroconversion level and seroconversion factor in pregnant women vaccinated with "Grippol ® Plus," allowing for the trimester.

Note:
Time period elapsed since the moment of vaccination of women in the third trimester of pregnancy is given in brackets.
×: p < 0.05; ××: p < 0.01-for difference between the second trimester group, third trimester group and non-pregnant group. Table 8. Geometric mean antibody titer in pregnant women vaccinated with "Grippol ® Plus," allowing for the trimester.
from that in vaccinated non-pregnant women and fully met the CPMP criteria. The level of antibodies to strain A/H1N1/v following administration of a trivalent vaccine was nearly the same as with vaccination of pregnant women with the monovalent, subunit, adjuvanted vaccine [38,39] With time the postpartum women demonstrated a more pronounced reduction of seroprotection level, especially against the A/H1N1/v strain. After 6 months postpartum, the rate of regression of seroprotection level in subgroups of vaccinated pregnant women (taking into account the gestational age) has increased 1.6-1.7-fold (A/H1N1/v), 1.4-fold (A/H3N2) and 1.5-to 1.6-fold (B), whereas in the non-pregnant women group same parameter was 1.2-fold (A/H1N1/v), 1.2-fold (A/H3N2) and 1.4-fold (B), respectively. This trend was in line with dynamic reduction of the MGT AB values during the last months of the follow-up [38]. Consequently, the existing physiological immunological changes in the immune system during pregnancy may affect the formation and preservation of post-vaccination antibodies to strains of influenza virus when using subunit immunoadjuvant vaccines. However, this assumption should be confirmed by new data research.

Immunologic effectiveness of vaccination of pregnant women using "Grippol ® Plus" influenza vaccine in mother-infant pairs
Analysis of transplacental immunity in the first months of life of infants born to women vaccinated during pregnancy with "Grippol ® Plus" vaccine has shown that the level of seroprotection against influenza virus strains significantly differed only in the mother-infant pairs from the group of subjects vaccinated in the second trimester of pregnancy (p < 0.05), while no differences in the number of seroprotected infants have been found (p > 0.05) ( Table 9). At 3 months after birth all infants, regardless of the time of their mothers' vaccination, demonstrated a significant reduction of protective titers of transplacental antibodies to vaccine strains of influenza virus as compared to their antibody titers obtained at birth and antibody titers in their mothers (p < 0.01). Further onwards, protective antibodies to vaccine strains of influenza virus completely vanished, and among the 6-month infants, the titer dropped to zero in both groups [40]. It should be noted that by 3 months of life, the rate of regression of antibody titer was higher in the subgroup of infants born to mothers vaccinated in late pregnancy, namely 2.8-fold higher for A/H1N1/v, 2.6-fold higher for A/H3N2/, and 4.0-fold higher for B strain.
Therefore, 52.3-61.9% of babies born to women vaccinated during pregnancy with "Grippol ® Plus" vaccine had protective antibody levels against vaccine influenza strains at the time of their birth. This level of protection significantly declined with time and by 3 months of life remained at a protective level in only 14.2-24.0% of infants. At the age of 6 months, protective titers of maternal antibodies completely vanished in all infants. Infants born to women vaccinated in the second trimester of pregnancy had higher activity of protective antibodies and lower rate of reduction of seroprotection level which attests to a better preservation of the post-vaccination transplacental immunity. Thus, the advantage of vaccination of pregnant women with the use of immunoadjuvant subunit vaccine in the II trimester of gestation was revealed. ▲: p < 0.05-for differences between infants from different observation groups. ×: p < 0.05; ××: p < 0.01-for differences between "mother-infant" groups. ⊗: p < 0.05; ⊗⊗: p < 0.01-for infants' intra-group differences versus at birth data. Table 9. State of transplacental post-vaccination immunity in "mother-infant" pairs following vaccination against influenza using "Grippol ® Plus" vaccine. and 48.1-65.4% (6 months) in their mothers. This observation provides substantiation that vaccination against influenza in high-risk infants shall start at the age of 6 months.

12.
The results obtained allow us to recommend the "Grippol ® Plus" vaccine for use in healthcare practice for specific prevention of seasonal influenza in pregnant women and their offspring up to age 6 months inclusive, using a single-dose vaccination schedule.